EXSC 541 LU Power Training Performance Essay.pdf

*ARTICLE PDF ATTACHED

5 page critical review on the provided speed and agility articles. At least 3 additional references of the

original research article must be cited in current APA format within the review.

Include at least 3 additional references to defend your analysis. Reference articles as a PDF with your

assignment

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ESSAY FORMAT

A. Background (Section One: Reason for article choice AND importance)

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1. Reason for Article Selection and Importance/Relationship to Speed and Agility for Elite Athletes

2. Introduction

3. Methods

1. Consider the strength and weaknesses of the research study’s design.

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B. Summary (Section Two: Significance)

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1. Results

2. Discussion

1. Consider the strength and weaknesses of the authors’ discussion section.

3. Conclusion

Influence of Strength on Magnitude and

Mechanisms of Adaptation to Power Training

PRUE CORMIE

, MICHAEL R. MCGUIGAN

2,3

, and ROBERT U. NEWTON

School of Exercise, Biomedical and Health Sciences, Edith Cowan University, Perth, AUSTRALIA;

New Zealand Academy

of Sport North Island, Auckland, NEW ZEALAND; and

Institute of Sport and Recreation Research New Zealand,

Auckland University of Technology, Auckland, NEW ZEALAND

ABSTRACT

CORMIE, P., M. R. MCGUIGAN, and R. U. NEWTON. Influence of Strength on Magnitude and Mechanisms of Adaptation to

Power Training. Med. Sci. Sports Exerc., Vol. 42, No. 8, pp. 1566–1581, 2010. Purpose: To determine whether the magnitude of

performance improvements and the mechanisms driving adaptation to ballistic power training differ between strong and weak indi-

viduals. Methods: Twenty-four men were divided into three groups on the basis of their strength level: stronger (n= 8, one-repetition

maximum-to-body mass ratio (1RM/BM) = 1.97 T0.08), weaker (n= 8, 1RM/BM = 1.32 T0.14), or control (n= 8, 1RM/BM = 1.37 T

0.13). The stronger and weaker groups trained three times per week for 10 wk. During these sessions, subjects performed maximal-

effort jump squats with 0%–30% 1RM. The impact of training on athletic performance was assessed using a 2-d testing battery

that involved evaluation of jump and sprint performance as well as measures of the force–velocity relationship, jumping mechanics,

muscle architecture, and neural drive. Results: Both experimental groups showed significant (Pe0.05) improvements in jump

(stronger: peak power = 10.0 T5.2 WIkg

, jump height = 0.07 T0.04 m; weaker: peak power = 9.1 T2.3 WIkg

, jump height =

0.06 T0.04 m) and sprint performance after training (stronger: 40-m time = j2.2% T2.0%; weaker: 40-m time = j3.6% T2.3%).

Effect size analyses revealed a tendency toward practically relevant differences existing between stronger and weaker individuals in the

magnitude of improvements in jump performance (effect size: stronger: peak power = 1.55, jump height = 1.46; weaker: peak power =

1.03, jump height = 0.95) and especially after 5 wk of training (effect size: stronger: peak power = 1.60, jump height = 1.59; weaker:

peak power = 0.95, jump height = 0.61). The mechanisms driving these improvements included significant (Pe0.05) changes in the

force–velocity relationship, jump mechanics, and neural activation, with no changes to muscle architecture observed. Conclusions: The

magnitude of improvements after ballistic power training was not significantly influenced by strength level. However, the training had a

tendency toward eliciting a more pronounced effect on jump performance in the stronger group. The neuromuscular and biomechanical

mechanisms driving performance improvements were very similar for both strong and weak individuals. Key Words: BALLISTIC,

JUMP, SQUAT, SPRINT, NEUROMUSCULAR ADAPTATIONS

After strength training, the magnitude of improve-

ments in strength and the mechanisms driving

those adaptations differ as the strength level of

the athlete improves (14,17,33,37). Specifically, initial im-

provements in strength are much greater and predomi-

nately driven by neural adaptations (although early phase

muscular adaptations also occur), whereas further increases

in strength are progressively harder to achieve and morphol-

ogical adaptations in the muscle become more important

(14,17,33,37). Thus, training programs geared at signifi-

cantly improving strength in individuals with an existing

high level of strength require a much more sophisticated de-

sign (i.e., greater specificity and variation) (17,33). Although

the factors contributing to maximal strength and the appli-

cation of strength training are well understood, much less is

known concerning the adaptations to and utilization of bal-

listic power training. In particular, the influence of strength

level on both the magnitude of improvement and the mech-

anisms driving adaptation after ballistic power training is not

known. Such knowledge is vital to the development of train-

ing programs that most effectively improve maximal power

production and athletic performance in athletes with a wide

variety of training backgrounds.

Cross-sectional comparisons have revealed that individ-

uals with higher strength levels have markedly superior

power production capabilities than those with a low level of

strength (2,7,10,24,34,35). For example, significant differ-

ences in power output and/or jump height between in-

dividuals with significantly different strength levels have

been reported in comparisons of well-trained athletes and

relatively untrained controls (10,24,35), athletes competi-

tive in power-type sports (i.e., volleyball) and endurance

events (7), rugby league players involved in national versus

Address for correspondence: Prue Cormie, Ph.D., School of Exercise,

Biomedical and Health Sciences, Edith Cowan University, 270 Joondalup

Dr. Joondalup, WA 6027, Australia; E-mail: [email protected].

Submitted for publication September 2009.

Accepted for publication December 2009.

0195-9131/10/4208-1566/0

MEDICINE & SCIENCE IN SPORTS & EXERCISE

DOI: 10.1249/MSS.0b013e3181cf818d

1566

APPLIED SCIENCES

state competitions (2), as well as the strongest and weakest

in a pool of resistance-trained men with various training

backgrounds (34). These findings are supported by reports

of a significant and positive relationship existing between

maximal strength and maximal power production (1,30,34).

While skeletal muscle mechanics that cause the force–

velocity relationship dictate that maximal strength plays a

role in muscular power, the development of muscular power

is influenced by a multitude of factors in addition to maximal

strength (27). For example, McBride et al. (24) observed that

despite no differences in the maximal strength of national-

level weightlifters and powerlifters (Smith machine squat

one-repetition maximum-to-body mass ratio (1RM/BM) =

2.86 T0.15 and 2.88 T0.14, respectively), weightlifters

generated significantly greater power output during an un-

loaded countermovement jump (CMJ; weightlifters = 63.0 T

2.7 WIkg

, powerlifters = 56.9 T2.5 WIkg

). Such obser-

vations were attributed to the various training protocols com-

monly used by these athletes (i.e., high force–high velocity

training vs high force–low velocity training common to

weightlifting and powerlifting, respectively) (24). Similar

results were observed in comparisons between well-trained

power athletes and recreational bodybuilders with similar

strength levels (35). Therefore, although the development

of maximal muscular power required for the successful

performance of many athletic movements is influenced by

a multitude of factors (27), stronger individuals have con-

sistently been shown to display superior power output

than individuals with a significantly lower strength level

(2,7,10,24,34,35).

This raises the question of what mechanisms contribute to

the improved power production capabilities of stronger in-

dividuals. Stronger individuals possess neuromuscular char-

acteristics that form the basis for superior maximal power

production and ultimately contribute to enhanced athletic

performance. Specifically, an individual with a substantially

greater level of strength (i.e., strong vs weak person) would

have larger whole-muscle cross sectional area (CSA), a re-

sult of greater myofibrillar CSA of both Type I and Type II

fibers, with more pronounced hypertrophy of Type II fibers

evident (18,22). Pennation angle may also be greater and

possibly even fascicle length (3,4,21). In addition, effective-

ness of neural drive (i.e., recruitment, rate of onset, firing

frequency) as well as intermuscular coordination would be far

superior in the significantly stronger individual (15,25,33).

These neuromuscular characteristics would result in a shift

in the force–velocity relationship so that the force of muscle

contraction would be greater for any given velocity of short-

ening (19). As a consequence, the ability to generate maximal

muscular power, and therefore perform athletic movements,

would be superior in a considerably stronger individual com-

pared with a weaker person (19,23,38).

Despite the advantage of strength for maximal power

production and athletic performance, little is known re-

garding whether neuromuscular characteristics of stronger

individuals allow for superior adaptation to ballistic power

training. More importantly, it is not known if the mech-

anisms driving adaptations to ballistic power training are

influenced by strength level. To the authors’ knowledge,

only one intervention study examining the possible influence

of strength on improvements after ballistic power training

exists (37). Wilson et al. (37) compared the improvements

between stronger (n= 4; squat 1RM/BM = 1.99 T0.30) and

weaker men (n= 5; squat 1RM/BM = 1.21 T0.18) after 8 wk

of drop jump training. Neither group significantly improved

either jump and reach height or 20-m sprint time, and no

correlation was found between strength level and the mag-

nitude of the training-induced change in jump and reach

height (r=j0.13, n= 14) (37). Therefore, this previous

study offers little insight into the influence of strength on the

magnitude of improvements in athletic performance after

ballistic power training. Furthermore, the investigation did

not involve measures to examine any mechanistic factors pos-

sibly involved in adaptations to the training. Simulation data

suggest that although jump height is most sensitive to in-

creases in strength-to-BM ratio (31), an increase in strength

does not translate into increased jump height unless control

(i.e., intermuscular coordination—the appropriate magnitude

and timing of activation of agonist, synergist, and antagonist

muscles during a movement) is tuned to the strengthened

muscle properties (6). These data are supported by the con-

cept of a delayed training effect in which it takes consid-

erable time and effort for an increase in strength to transmute

to improved performance in multijoint movements because

intermuscular coordination needs to be adapted to the stronger

motor units/muscles (39). Therefore, it may be speculated

that a stronger individual exposed to ballistic power training

would improve performance by fine tuning the timing and

patterns of neural drive, thus improving intermuscular coor-

dination and movement technique (although muscular adap-

tations at the cellular level such as alterations to protein

expression, calcium release, and uptake as well as relaxation

time may also contribute). It is unknown if the combination

of these potential changes with the underlying neuromuscular

characteristics of the stronger individual would result in a

training response dissimilar to individuals with lower levels

of strength.

In comparisons of individuals not currently involved in

ballistic power training, previous research has demonstrated

that stronger individuals are able to generate superior levels

of maximal power than significantly weaker individuals

(2,7,10,24,34,35). Although the enhanced maximal power

production of stronger individuals in the absence of ballistic

power training is theorized to be due to the underlying neu-

romuscular characteristics of stronger individuals, it is un-

clear if these characteristics improve the ability to adapt to

ballistic power training. Specifically, it is not known whether

ballistic power training elicits performance improvements

in stronger individuals of a greater magnitude than that

which is achievable with weaker subjects. Furthermore, there

is a paucity of research investigating whether strength level

influences the mechanisms responsible for performance

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise

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improvements after ballistic power training. Therefore, the

purpose of this experiment was to determine whether the

magnitude of performance improvements and the mecha-

nisms driving adaptation to ballistic power training differ

between strong and weak individuals.

METHODS

Experimental design. This study used a randomized,

control design and was conducted during a period of 15 wk.

Subjects were divided into two strata on the basis of their

1RM/BM: stronger (1RM/BM 91.55) or weaker (1RM/BM G

1.55). Subjects in the stronger stratum were allocated into

the stronger group. Subjects in the weaker stratum were ran-

domized into one of two groups: weaker group or control

group. The stronger and weaker groups completed a total of

10 wk of ballistic power training, while the control group

maintained their normal level of activity throughout the

duration of the study. Training involved three sessions per

week in which subjects performed maximal-effort jump

squats with 0%–30% 1RM. Subjects completed a 2-d testing

battery before initiating training (baseline), after 5 wk of

training (midtest—stronger and weaker groups only), and

after the completion of 10 wk of training (posttest). Subjects

were adequately familiarized to all testing procedures before

actual assessment. Testing involved evaluation of jump and

sprint performance as well as measures of the force–velocity

relationship, jumping mechanics, muscle architecture, and

neural drive.

Subjects. Subjects were recruited on the basis of their

ability to perform a back squat with proficient technique. A

total of 35 men fulfilled all the testing and training re-

quirements of this investigation. Data from 11 of these men

were removed on the basis of their 1RM/BM to establish

two experimental groups with very distinct differences in

maximal strength (i.e., data from subjects with a 1RM/BM

between 1.55 and 1.85 were not included). The remaining

24 men were allocated into three groups: stronger group

(n= 8, 1RM/BM = 1.97 T0.08), weaker group (n=8,

1RM/BM = 1.32 T0.14), or control group (n= 8, 1RM/BM

= 1.37 T0.13). Subjects’ characteristics throughout the du-

ration of the study are outlined in Table 1. The participants

were notified about the potential risks involved and gave

their written informed consent. This study was approved by

the university’s human research ethics committee.

Training program. The training program consisted of

three sessions per week separated by at least 24 h of rest.

Each training session was initiated via a warm-up consisting

of two sets of six submaximal jump squats with 0% 1RM

(i.e., CMJ with no external load, just the resistance applied

by a carbon fiber pole (mass = 0.4 kg) held across the

shoulders). During sessions 1 and 3 of each week, subjects

performed seven sets of six maximal-effort jump squats

separated by a 3-min recovery. Jump squats were performed

at the load that maximized power output for each subject, as

determined during the baseline testing session. Similar to

previous research, a load consistent with the subject’s BM

(i.e., no external load or 0% 1RM) maximized power output

for each of the participants in this study (11,12). The second

training session of each week included an additional warm-

up set consisting of five submaximal jump squats with 30%

1RM. Subjects then performed five sets of five maximal-

effort jump squats with 30% 1RM separated by a 3-min

recovery. Subjects were encouraged to perform each jump

as rapidly as possible. Intensity was modified for each

session so that an audible beep could be heard by subjects

during jumps that reached 95% of the maximal power

output at that load from their previous training or testing

session. Previous literature has shown significant perfor-

mance improvements after jump squat training with similar

programming parameters at both 0% 1RM (11) and 30%

1RM (25,38). Subjects refrained from any additional lower

body resistance training, plyometrics, or sprint training

throughout the course of the study.

Testing protocol. Subjects rested for 3–5 d between

their previous training session and the midtest testing ses-

sion and for 7–10 d between the final training session and

the posttest testing session to allow full recovery. Subjects

completed two testing sessions separated by at least 3 d of

recovery at the baseline, midtest, and posttest occasions.

Testing session 1 was initiated with an examination of

the vastus lateralis (VL) muscle architecture through

TABLE 1. Subject characteristics of the stronger, weaker, and control groups throughout the 10 wk of training.

BM (kg) Body Fat (%) 1RM/BM 40-m Sprint (s) Jump Height (m)

Stronger group

Baseline 79.1 T12.8 12.2 T3.9‡1.97 T0.08†5.41 T0.25†0.43 T0.03‡

Midtest 79.8 T12.9 12.3 T3.8‡1.93 T0.11‡5.39 T0.20‡0.50 T0.03*

Posttest 79.6 T13.0 12.6 T3.5‡1.88 T0.11†5.29 T0.20†0.50 T0.03*

Weaker group

Baseline 79.9 T14.5 17.3 T3.8 1.32 T0.14 5.90 T0.27 0.38 T0.04

Midtest 79.2 T14.2 17.5 T4.0 1.38 T0.16 5.79 T0.20 0.42 T0.07

Posttest 79.1 T13.8 17.7 T3.6 1.39 T0.17 5.69 T0.22 0.44 T0.06

Control group

Baseline 77.5 T8.1 14.6 T3.5 1.37 T0.13 5.79 T0.19 0.41 T0.04

Posttest 78.4 T8.8 14.7 T3.8 1.35 T0.12 5.79 T0.12 0.40 T0.04

BM, body mass; 1RM:BM, squat one repetition maximum to body mass ratio.

* Significantly (Pe0.05) different from baseline.

†Significant (Pe0.05) difference between stronger group and all other groups.

‡Significant (Pe0.05) difference between stronger group and weaker group.

§ Significantly (Pe0.05) different from control group.

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ultrasonography. Maximal dynamic strength was then as-

sessed using a back squat 1RM to a depth consistent with a

knee angle of at least 90-of flexion assessed using two-

dimensional motion analysis. During a 30-min recovery,

body composition was assessed using dual-energy x-ray

absorptiometry. Maximal isometric strength was then eval-

uated using an isometric squat test performed at a knee angle

of 140-to allow for the determination of maximum force

output at zero velocity (12,32). Adequate recovery was

permitted (10 min) before examination of jump squat per-

formance across a series of intensities: 0% (i.e., no external

load or BM only), 20%, 40%, 60%, and 80% of squat 1RM.

Subjects completed the jump squats in a randomized order,

which was consistent across the three testing occasions for

each individual subject. Kinematic (both linear position

transducer (LPT) and two-dimensional motion analysis),

kinetic, and EMG data were obtained simultaneously through-

out the testing session. After at least 3 d of recovery, sub-

jects completed the second testing session involving a 40-m

sprint test.

Data acquisition and analysis procedures. The

back squat 1RM involved subjects completing a series of

warm-up sets (four to six repetitions at 30% estimated

1RM, three to four repetitions at 50% estimated 1RM, two

to three repetitions at 70% estimated 1RM, and one to two

repetitions at 90% estimated 1RM), each separated by 3 min

of recovery. A series of maximal lift attempts was then

performed until a 1RM was obtained. No more than five

attempts were permitted with each attempt separated by

5 min of recovery. This protocol has been frequently used

throughout the previous literature for the assessment of

maximal dynamic strength (11,12,30). Only trials in which

subjects reached a relative knee angle (i.e., angle between

the midline of the lower leg and the midline of the thigh)

G90-of flexion were considered successful. This depth was

visually monitored during testing and was confirmed by

two-dimensional motion analysis (stronger group: baseline

= 85.7-T4.2-, posttest = 82.5-T6.8-; weaker group:

baseline = 83.4-T2.4-, posttest = 80.4-T4.3-; control

group: baseline = 83.1-T6.2-, posttest = 82.6-T5.4-; rater

reliability: r= 0.95). Although no significant differences in

the depth of the squat 1RM existed among the testing occa-

sions, there is the potential that the slightly deeper depth

obtained during posttesting could have influenced the 1RM

values obtained.

The isometric squat test was performed with subjects

standing on a force plate (9290AD; Kistler Instruments,

Winterthur, Switzerland) in a back squat position pushing

against an immovable rigid bar. The bar was positioned so

that subjects had a knee angle of 140-of flexion to allow

for the determination of maximal force output at zero ve-

locity. Previous research has shown this knee angle to cor-

respond with the highest isometric force output for the squat

compared with a range of other knee angles (32). Subjects

were instructed to perform a rapid, maximal effort to reach

maximal force output as soon as possible and maintain that

force for 3 s. The analog signal from the force plate was

collected for every trial at 1000 Hz using a data acquisition

system including an analog-to-digital card (cDAQ-9172;

National Instruments, North Ryde, NSW, Australia). Cus-

tom programs designed using LabVIEW software (Version

8.2; National Instruments) were used for recording and an-

alyzing the data. The signal was filtered using a fourth-

order, low-pass Butterworth filter with a cutoff frequency

of 50 Hz. From laboratory calibrations, the voltage output

was converted into vertical ground reaction force. Peak

force relative to BM was assessed as the maximal force

output during the 3-s period divided by the individual’s

BM. The test–retest reliability for peak force relative to BM

was r= 0.98.

Performance of a jump squat involved subjects complet-

ing a maximal-effort CMJ while holding a rigid bar across

their shoulders. Subjects held a 0.4-kg carbon fiber pole for

the 0% 1RM jump squat, whereas for all other intensities,

subjects held a 20-kg barbell loaded with the appropriate

weight plates. Participants were instructed to keep constant

downward pressure on the bar throughout the jump and

were encouraged to move the resistance as fast as possible

to achieve maximal power output with each trial. The bar

was not allowed to leave the shoulders of the subject, with

the trial being repeated if this requirement was not met. A

minimum of two trials at each load were completed, with

additional trials performed if both peak power and jump

height were not within 5% of the previous jump squat.

Adequate rest was enforced between all trials (3 min).

All jump squats were performed while the subject was

standing on the force plate with a LPT (PT5A-150; Celesco

Transducer Products, Chatsworth, CA) attached to the bar.

The LPT was attached 10 cm to the left of the center of the

bar to avoid any interference caused by movement of the

head during the jump. The LPT was mounted above the sub-

ject, and the retraction tension of the LPT (equivalent to

8 N) was accounted for in all calculations. Analog signals

from the force plate and LPT were collected for every trial

at 1000 Hz and analyzed using custom programs designed

using LabVIEW software. The signal from the LPT was

filtered using a fourth-order, low-pass Butterworth digital

filter with a cutoff frequency of 10 Hz, and the voltage output

was converted into displacement using laboratory calibra-

tions. The vertical velocity of the movement was deter-

mined using a first-order derivative of the displacement data.

Power output was calculated as the product of the vertical

velocity and the vertical ground reaction force data. Ac-

celeration of the movement was calculated using a second-

order derivative of the displacement data and smoothed

using a fourth-order, low-pass Butterworth digital filter with a

cutoff frequency of 10 Hz. These data collection and analysis

methodology have been validated previously (9), and the test–

retest reliability for all jump variables examined was con-

sistently rQ0.90.

A series of performance variables was assessed during

the jump squats. Peak force, velocity, power, displacement,

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and acceleration were determined as the respective maximal

values achieved during the entire movement. Net impulse

was assessed as the integral of vertical ground reaction

force during the period of application in which force ex-

ceeded that required during stationary standing (i.e., above

body weight). Rate of force development (RFD) was de-

termined between the minimum and maximum force that

occurred throughout the movement. Similarly, rate of power

development (RPD) was also determined between the min-

imum and maximum power that occurred throughout the

movement. Average power output was calculated during the

concentric phase of the movement (i.e., time between min-

imum displacement and takeoff). Instantaneous force and

velocity output at the time at which peak power occurred

was also examined and was termed force at peak power and

velocity at peak power, respectively. These values across

each of the loads examined were used to generate the force–

velocity and force–power relationships for the jump squat.

Velocity at takeoff was defined as the velocity of movement

at the time at which the force output was first zero (i.e.,

when the toes first left the force plate). Furthermore, time

to takeoff was determined as the time between the initiation

of the countermovement (i.e., start of the eccentric phase)

and the point that force was zero (i.e., end of the concentric

phase—takeoff ).

In addition to these instantaneous performance variables,

analyses of parameters throughout the jump movement were

conducted. The power–time and force–time curves from each

individual subject were selected from the beginning of the

eccentric phase (i.e., initiation of countermovement as-

sessed as the time at which a change in velocity first oc-

curred) through to the end of the concentric phase (i.e., at

takeoff when force and power reached zero). The velocity–

time and displacement–time curves were selected from the

beginning of the eccentric phase to peak displacement (i.e.,

zero velocity). Using a custom-designed LabVIEW program,

the number of samples in each individual curve was then

modified to equal 500 samples by changing the time delta

(dt) between samples and resampling the signal (dt = number

of samples in the original signal/500). The sampling fre-

quency of the normalized signals was calculated according to

the following equation:

normalized sampling

frequenzy ðHzÞ¼1 second

no:samples in original signal

no:samples in normalized signal





seconds

per sample



Consequently, the sampling frequency of the modified

signals was then equivalent to 815 T154 Hz for the power–

time and force–time curves and 538 T68 Hz for the velocity–

time and displacement–time curves. This resampling allowed

for each individual’s power, force, velocity, and displace-

ment curves to be expressed during equal periods of time

(i.e., the 500 samples represented the relative time—from

0% to 100%—taken to complete the jump). In other words,

the various data sets were normalized to total movement

time so that data could be pooled. Each sample of the nor-

malized power–, force–, velocity–, and displacement–time

curves was then averaged across subjects within the stronger,

weaker, or control groups, resulting in averaged curves with

high resolution (sampling frequency of 538–815 Hz). This

allowed for power, force, velocity, and displacement through-

out the jump to be compared across baseline, midtest, and

posttest as well as between groups. Intraclass test–retest

reliabilities for power–, force–, velocity–, and displacement–

time curves during the CMJ have consistently been rQ0.94,

rQ0.90, rQ0.89, and rQ0.92, respectively, using this

methodology (10).

Before assessment of sprint performance, subjects per-

formed a warm-up consisting of 5 min of light jogging and

three submaximal 20-m sprints. The sprint test was initiated

from a standing start involving a staggered stance with the

same front–back leg orientation used for every trial through-

out the baseline, midtest, and posttest. Subjects were in-

structed to commence the sprint at will and accelerate as

quickly as possible throughout the 40 m. Three trials were

performed with each separated by a 3-min recovery. A series

of six dual-beam timing gates (Speedlight; Swift Sports,

Lismore, NSW, Australia) was used to record instanta-

neous time at 5, 10, 20, 30, and 40 m (timing was com-

menced when subject passed through a dual-beam timing

gate positioned at their front foot). Flying 5 m was calculated

as the time between the 5- and 10-m gates, whereas flying

15 m was calculated at the time between the 5- and 20-m

gates. Intraclass test–retest reliability for all sprint perfor-

mance variables examined was consistently rQ0.90.

EMG of the VL, vastus medialis (VM), and biceps fe-

moris (BF) was collected on the dominant leg during the

isometric squat and all jump squats. Disposable surface

electrodes (self-adhesive Ag/AgCl snap electrode, 2-cm in-

terelectrode distance, 1-cm circular conductive area; prod-

uct 272; Noraxon USA, Inc., Scottsdale, AZ) were attached

to the skin over the belly of each measured muscle, distal to

the motor point, and parallel to the direction of muscle

fibers. A reference electrode was placed on the patella. The

exact location of the electrodes relative to the anatomical

landmarks was marked on a sheet of tracing paper after the

first testing session to ensure consistent placement in sub-

sequent tests. Each site was shaved, gently abraded, and

cleansed with alcohol before electrode placement to mini-

mize skin impedance. Raw EMG signals were collected at

1000 Hz and amplified (gain = 1000, bandwidth frequency

= 10–1000 Hz, input impedance G5k6; Model 12D-16-OS

Neurodata Amplifier System; Grass Technologies, West

Warwick, RI). The amplified myoelectric signal was col-

lected simultaneously with force plate and LPT data using a

data acquisition system including an analog-to-digital card.

Custom programs designed using LabVIEW software were

used for recording and analyzing the data. The signal was

full-wave–rectified and filtered using a dual-pass, sixth-

order, 10- to 250-Hz band-pass Butterworth filter as well as

a notch filter at 50 Hz. A linear envelope was created using

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a low-pass, fourth-order Butterworth digital filter with a

cutoff frequency of 6 Hz. Maximal voluntary contraction

(MVC) for all muscles were determined by averaging the

integrated EMG signal during a 1-s period of sustained

maximal force output after the initial peak in the force curve

during the isometric squat (intraclass test–retest reliability

consistently rQ0.91). EMG activity during jumping was

analyzed by averaging the integrated EMG signal from the

beginning of the eccentric phase to takeoff. To standardize

for the time taken to complete a jump squat, this value was

then divided by the time to takeoff. The average integrated

EMG (AvgIEMG) was then normalized by expressing it

relative to the MVC. This is similar to methods previously

used when comparing EMG between movements with dif-

ferent time components (29). The rate of rise in AvgIEMG

(expressed as a percentage of MVC per second) was as-

sessed during the 0% 1RM jump squat as the rate of change

between minimum and maximum AvgIEMG throughout

the movement (i.e., from the initiation of the countermove-

ment to takeoff). Intraclass test–retest reliability for all EMG

variables examined was consistently rQ0.80.

In vivo muscle architecture was assessed by B-mode ultra-

sonography recorded using an ultrasound console (SSD-1000;

Aloka Incorporated, Tokyo, Japan) with a 7.5-MHz, 9-cm

linear probe. The same experienced examiner completed all

scans across the baseline, midtesting, and posttesting occa-

sions. Scans were performed on the VL of the dominant leg,

with subjects lying supine and their leg muscles completely

relaxed. To assist with acoustic coupling, water-soluble trans-

mission gel was applied to the transducer. Measurements

were taken at 50% of the thigh length calculated as half the

distance between the centers of the greater trochanter to the

lateral condyle of the femur. Longitudinal images were ob-

tained with the transducer oriented parallel to the muscle

fascicles and perpendicular to the skin. The impact of dis-

crepancies in transducer location and orientation on architec-

tural differences observed between baseline, midtest, and

posttest was minimized using several techniques. The location

and two-dimensional orientation of the transducer relative to

anatomical landmarks were mapped onto a sheet of tracing

paper to ensure that the same site was used across all testing

occasions. In addition, in an effort to minimize any differ-

ences in the three-dimensional orientation of the transducer

between testing occasions, the live, onscreen image during

midtest and posttest was compared with those taken during

baseline testing. This allowed the examiner to match the

unique features of the ultrasound images (i.e., heterogeneities

in the subcutaneous adipose tissue and echoes from inter-

spaces among the fascicles). These procedures have been used

previously in literature, monitoring the impact of training on

muscle architecture (3). Images were digitally recorded, and

the superficial and deep aponeuroses were identified, with

muscle thickness measured as the distance between the apo-

neuroses (3,20). Pennation angle was defined as the angle at

which the fascicles arose from the deep aponeurosis (20).

Intraclass test–retest reliabilities for muscle thickness and

pennation angle were r=0.95andr= 0.90, respectively. In

addition, rater reliabilities for muscle thickness and pennation

angle were r=0.98andr= 0.93, respectively.

Two-dimensional motion analysis was used to evaluate the

movement mechanics during the squat 1RM and jump squats.

A digital video camera (25 Hz; MV830i; Canon Australia

Pty Ltd., North Ryde, NSW, Australia) was positioned 3.1 m

from the subject, perpendicular to the subjects’ sagittal plane.

Dartfish software (Version 4.5 ProSuite; Dartfish, Sydney,

NSW, Australia) was used to analyze the movements. The

video footage was deinterlaced into fields, yielding a 50-Hz

sampling frequency. During the jump squats, the minimum

and maximum knee, hip, and ankle joint angles were assessed

at the transition between the eccentric and concentric phases

(minimum) and the last field before takeoff where the foot is

still in contact with the force plate (maximum). The minimum

knee, hip, and ankle angles were also assessed during the

squat 1RM, representing the joint angles at the transition

between the eccentric and concentric phases. Intraclass test–

retest reliability and rater reliability for all joint angles as-

sessed using motion analysis were consistently rQ0.92 and

rQ0.93, respectively.

FIGURE 1—Change from baseline in peak power, average power, and

maximum jump height during the 0% 1RM jump squat at midtest (A)

and posttest (B). *Significant (Pe0.05) change from baseline.

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Statistical analyses. A general linear model with

repeated-measures ANOVA followed by Bonferroni post

hoc tests was used to examine the impact of training on

performance variables and to determine whether differences

existed between the groups at baseline, at midtest, and at

posttest. t-tests were used for comparisons between var-

iables at baseline and posttest for the control group as well

as for comparison of variables between stronger and weaker

groups at midtest. Statistical significance for all analyses

was defined by Pe0.05, and results were summarized as

means TSD. Estimated effect sizes (ES) of G

= 0.502 and

= 0.300 at observed power levels of 0.976 and 0.626 for

maximum peak power relative to BM after training existed

for stronger and weaker groups, respectively. In addition,

estimated ES of G

= 0.406 and G

= 0.319 at the observed

power levels of 0.821 and 0.644 existed for the compari-

son of maximum peak power relative to BM between the

stronger and weaker groups at baseline and posttest,

respectively. Mean ES were also calculated to examine

and compare the practical significance of the performance

improvements among the experimental groups. Based on the

study of Cohen (8), which suggests ES of 0.2, 0.5, and 0.8 to

represent small, moderate, and large effects, respectively,

practical relevance was defined as an ES Q0.8 for the

TABLE 2. Change in performance variables during the 0% 1RM jump squat (i.e., BM only) from baseline ($).

Stronger Group Weaker Group

0% 1RM Jump Squat $at Midtest $at Posttest $at Midtest $at Posttest

Power

Peak power (WIkg

) 10.3 T5.1* 10.0 T5.2* 7.1 T4.1* 9.1 T2.3*

Average power (WIkg

) 8.6 T4.8* 9.6 T5.2* 7.0 T2.3* 8.5 T1.9*

Rate of power development (WIkg

) 343.0 T164.4*

‡325.2 T132.6* 169.3 T79.4* 236.7 T129.7*

Force

Peak force (NIkg

) 6.3 T5.4* 6.3 T4.7* 3.5 T3.7 4.7 T4.2*

RFD (NIkg

) 76.8 T46.1* 82.3 T58.3* 55.3 T28.8* 94.6 T74.9*

Force at peak power (NIkg

) 2.8 T2.1* 2.1 T1.6* 1.5 T1.8 1.4 T1.9

Net impulse (NIs) 38.9 T22.6* 45.9 T28.9* 48.8 T26.5* 57.4 T20.9*

Velocity

Peak velocity (mIs

) 0.43 T0.29* 0.42 T0.22* 0.34 T0.34* 0.55 T0.26*

Velocity at peak power (mIs

) 0.19 T0.34 0.22 T0.31 0.15 T0.30 0.28 T0.27

Velocity at takeoff (mIs

) 0.43 T0.29* 0.41 T0.24* 0.35 T0.36* 0.60 T0.30*

Acceleration

Peak acceleration (mIs

) 11.7 T6.2*

‡14.6 T6.5*

‡6.4 T2.0* 8.6 T4.7*

Displacement

Peak displacement (m) 0.07 T0.03* 0.07 T0.04* 0.04 T0.05 0.06 T0.04*

Time

Time to takeoff (s) j0.18 T0.07* j0.19 T0.6* j0.27 T0.12* j0.31 T0.13*

‡

* Significant (Pe0.05) change from baseline.

‡Significant (Pe0.05) difference between stronger group and weaker group.

TABLE 3. Comparison of 40-m sprint times among stronger, weaker, and control groups throughout the 10 wk of training.

Sprint Times (s)

5 m 10 m 20 m 30 m 40 m Flying 5 m Flying 15 m

Stronger group

Baseline 1.10 T0.10 1.82 T0.12 3.08 T0.15‡4.25 T0.19†5.41 T0.25†0.72 T0.03†1.98 T0.07†

Midtest 1.08 T0.02‡1.82 T0.04‡3.07 T0.09‡4.23 T0.14‡5.39 T0.20‡0.74 T0.03 1.99 T0.08‡

Posttest 1.02 T0.08§ 1.75 T0.08§ 2.99 T0.11‡4.14 T0.15†5.29 T0.20†0.73 T0.01†1.97 T0.05†

$between baseline

and posttest

j0.08 T0.04*

§j0.07 T0.04* j0.09 T0.08*

§j0.11 T0.11*

§j0.12 T0.11* 0.01 T0.02 j0.01 T0.07

ES 0.86 0.67 0.69 0.65 0.54 0.64 0.18

Weaker group

Baseline 1.17 T0.12 1.95 T0.16 3.32 T0.19 4.60 T0.23 5.90 T0.27 0.78 T0.04 2.14 T0.08

Midtest 1.13 T0.05 1.90 T0.08 3.24 T0.11 4.51 T0.16 5.79 T0.20 0.77 T0.05 2.10 T0.09

Posttest 1.09 T0.05 1.86 T0.07 3.18 T0.12 4.44 T0.17 5.69 T0.22 0.77 T0.02 2.09 T0.08

$between baseline

and posttest

j0.08 T0.09 j0.09 T0.10 j0.13 T0.09*

§j0.16 T0.10*

§j0.22 T0.14*

§j0.01 T0.03 j0.06 T0.03*

ES 0.79 0.69 0.78 0.76 0.82 0.25 0.66

Control group

Baseline 1.13 T0.04 1.88 T0.07 3.25 T0.11 4.52 T0.14 5.79 T0.19 0.75 T0.03 2.12 T0.08

Posttest 1.15 T0.04 1.91 T0.05 3.27 T0.07 4.53 T0.09 5.79 T0.12 0.76 T0.02 2.12 T0.05

$between baseline

and posttest

0.02 T0.05 0.03 T0.06 0.02 T0.08 0.01 T0.08 0.00 T0.10 0.01 T0.02 0.00 T0.04

ES 0.41 0.46 0.24 0.10 0.01 0.40 0.07

Flying 5 m time is calculated as the time between 5 and 10 m.

Flying 15 m time is calculated as the time between 5 and 20 m.

The change in sprint times between baseline and posttests as well as ES of the change is also displayed.

* Significant (Pe0.05) change from baseline to posttest.

†Significant (Pe0.05) difference between stronger group and all other groups.

‡Significant (Pe0.05) difference between stronger group and weaker group.

§ Significantly (Pe0.05) different from control group.

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FIGURE 2—Training-induced changes to the force–velocity and force–power relationships for the jump squat. *Significantly (Pe0.05) different

peak power relative to BM from baseline. +Approaching significantly (Pe0.10) different peak power relative to BM from baseline. xSignificantly

(Pe0.05) different force relative to BM at peak power from baseline. #Significantly (Pe0.05) different velocity at peak power from baseline.

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purpose of this study. A statistical software package (SPSS,

Version 13.0; SPSS, Inc., Chicago, IL) was used to perform

all statistical analyses.

RESULTS

Athletic performance. The stronger group had signif-

icantly greater 1RM/BM than both the weaker and control

groups at all testing occasions and displayed a practically

relevant decrease in 1RM/BM at posttest (ES = 0.93, equiv-

alent to a 7 T7-kg decrease in 1RM). Training resulted in

significant within-group changes in a multitude of jump per-

formance variables between baseline and midtest or be-

tween baseline and posttest sessions for both the stronger

and weaker groups (Fig. 1 and Table 2). However, only the

changes in RPD at midtest and time to takeoff at posttest

significantly differed between the groups (Table 2). Com-

parison of the magnitude of changes in average power, peak

power, and peak displacement between the stronger and

weaker groups is illustrated in Figure 1. Any practically

relevant differences between the groups were more pro-

nounced after 5 wk of training, and they diminished some-

what after 10 wk of training (Fig. 1). The stronger group

completed the 40-m sprint in significantly less time than

both the weaker and control groups across all testing occa-

sions (Table 3). Training resulted in a significant change in

time between baseline and posttest at 5-, 10-, 20-, 30-, and

40-m sprints for the stronger group and at 20-, 30-, and 40-m

sprints for the weaker group (Table 3). These changes

were significantly different from the change in time dis-

played by the control group (which did not significantly

alter sprint time).

Force–velocity relationship. Training-induced

changes to force–velocity and force–power relationships

during the jump squat are displayed in Figure 2. Significant

differences between baseline and posttest were evident for

FIGURE 3—Between-group differences in the force–velocity (A–C) and force–power relationships (D–F) for the jump squat throughout the 10 wk of

training. †Significant (Pe0.05) difference in force or power between stronger and all other groups. ‡Significant (Pe0.05) difference in force or

power between stronger and weaker groups. §Significantly (Pe0.05) different force or power from control group.

TABLE 4. Two-dimensional motion analysis variables assessed during the 0% 1RM jump squat.

Minimum Joint Angle Maximum Joint Angle

0% 1RM Jump Squat Knee Angle (-) Hip Angle (-) Ankle Angle (-) Knee Angle (-) Hip Angle (-) Ankle Angle (-)

Stronger group

Baseline 89.7 T8.5 86.3 T13.8 60.7 T4.4 170.0 T7.1 168.3 T4.9 133.4 T3.9

Midtest 94.8 T6.5 89.4 T10.8 61.8 T4.7 174.5 T5.0 171.9 T8.1 137.5 T5.9

Posttest 94.7 T5.6 91.5 T16.4 59.0 T5.4 176.8 T4.3 174.0 T7.1 140.0 T5.8

Weaker group

Baseline 85.9 T7.1 79.3 T8.8 60.1 T3.0 170.2 T6.4 168.1 T8.1 136.0 T8.2

Midtest 90.1 T9.5 84.8 T18.7 59.9 T3.0 174.8 T4.9 172.8 T7.1 137.4 T6.6

Posttest 89.3 T11.1 88.4 T20.4 57.7 T3.7 171.7 T6.3 172.7 T9.5 138.1 T5.2

Control group

Baseline 89.6 T6.4 78.6 T9.8 67.3 T6.3 174.3 T6.5 172.3 T4.9 141.4 T8.5

Posttest 86.0 T12.2 78.0 T6.9 66.3 T3.5 172.3 T9.3 170.9 T12.2 137.3 T7.0

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both stronger and weaker groups (Fig. 2). Between-group

comparisons of the force–velocity and force–power rela-

tionships during jump squats revealed several differences

(Fig. 3).

Jump mechanics. No significant differences existed

between the groups in either the minimum or the maximum

joint angles during the 0% 1RM jump squat at baseline,

midtest, or posttest (Table 4). Furthermore, training did not

FIGURE 4—Comparison of the power–time curve during the 0% 1RM jump squat among the stronger, weaker, and control groups at baseline (A),

midtest (B), and posttest (C). Normalized time represents the time from initiation of countermovement to takeoff in A,B, and C.‡Significant

(Pe0.05) difference between stronger and weaker groups. 6Significant (Pe0.05) difference between stronger and control groups. §Significant

(Pe0.05) difference between control and both stronger and weaker groups.

FIGURE 5—Training-induced changes to the power–time (A), force–time (B), velocity–time (C), and displacement–time (D) curves for the 0% 1RM

jump squat in the stronger group. Normalized time represents the time from initiation of countermovement to takeoff for power and force (Aand B)

and from initiation of countermovement to peak displacement for velocity and displacement (Cand D). *Significant (Pe0.05) difference between

baseline and both midtest and posttest. xSignificant (Pe0.05) difference between baseline and posttest.

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result in any changes to the joint angles at either the tran-

sition between eccentric and concentric phases or the take-

off during the 0% 1RM jump squat (Table 4). Investigation

of the power–time curve throughout the jump revealed sig-

nificant differences between the groups (Fig. 4). At base-

line, significant differences in power between stronger and

weaker groups existed from 76.0% to 92.8% of normalized

time (Fig. 4A). Similar differences between the stronger

and weaker groups were maintained throughout midtest

(80.4%–91.0% normalized time; Fig. 4B) as well as posttest

(73.8%–90.2% normalized time; Fig. 4C). After training,

significant differences were evident between the control

group and both the stronger and weaker training groups

throughout the power–time curve at posttest (4.4%–23.4%,

40.8%–52.8%, and 69.4%–89.2% normalized time). In addi-

tion, differences existed between the control and stronger

groups from 53.0% to 60.8% normalized time (Fig. 4C).

Several significant training-induced changes were also ob-

served between baseline and both midtesting and posttesting

sessions (Figs. 5 and 6). For the stronger group, significant

differences between baseline and both midtest and posttest

existed during the following phases: (a) power: 6.6%–22.8%,

42.2%–55.6%, and 74.2%–91.2% of normalized time; (b)

force: 9.6%–28.4% and 48.8%–62.8% of normalized time;

ized time; and (d) displacement: 57.4%–89.4% of normal-

ized time. Significant differences between baseline and

posttest also existed during the following phases: (a) power:

70.6%–74.0% of normalized time; (b) force: 28.6%–32.4%,

43.0%–48.6%, and 63.0%–87.0% of normalized time; (c)

velocity: 36.0%–39.4% of normalized time; and (d) dis-

placement: 46.2%–57.2% and 89.6%–100.0% of normalized

time (Fig. 5). For the weaker group, significant differences

between baseline and both midtest and posttest existed dur-

ing the following phases: (a) power: 3.6%–11.4%, 35.4%–

52.6%, and 64.8%–89.6% of normalized time; (b) force:

0.0%–23.2% and 38.0%–70.0% of normalized time; (c)

velocity: 9.4%–32.0%, 39.2%–68.0%, and 75.0%–91.8% of

normalized time; and (d) displacement: 58.0%–88.2% or

normalized time. Significant differences between baseline and

posttest also existed during the following phases: (a) power:

11.6%–18.2% of normalized time, (b) force: 70.2%–79.2%

FIGURE 6—Training-induced changes to the power–time (A), force–time (B), velocity–time (C), and displacement–time (D) curves for the 0% 1RM

jump squat in the weaker group. Normalized time represents the time from initiation of countermovement to takeoff for power and force (Aand B)

and from initiation of countermovement to peak displacement for velocity and displacement (Cand D). *Significant (Pe0.05) difference between

baseline and both midtest and posttest. xSignificant (Pe0.05) difference between baseline and posttest.

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of normalized time, and (c) displacement: 88.4%–92.4% of

normalized time (Fig. 6).

Neuromuscular characteristics. No between- or

within-group differences existed for muscle thickness, pen-

nation angle, or lean mass of the leg across each of the

testing occasions (Table 5). However, percent change in

pennation angle from baseline to posttest was significant

(9.9%, P= 0.05) for the weaker group and approaching sig-

nificance (6.5%, P= 0.07) for the stronger group. At base-

line, no between-group differences were evident in any of

the EMG variables assessed (Table 5). After training, no

significant between- or within-group differences existed in

AvgIEMG during the isometric squat (i.e., MVC) or the 0%

1RM jump squat. However, the stronger power training

group significantly increased the rate of rise in AvgIEMG

during the 0% 1RM jump squat at posttest in both VM and

VL (Table 5). Furthermore, the rate of rise in AvgIEMG of

the VM and VL in the stronger power training group was

significantly greater than that in the control group at post-

test. The weaker power training group also displayed a

significant increase in the rate of rise in AvgIEMG of the

VM during the 0% 1RM jump squat at both midtest and

posttest (Table 5). No within-group changes in any EMG

variables were observed for the control group throughout

the study.

DISCUSSION

This investigation revealed that the ability to adapt to

ballistic power training is quite similar for both strong and

weak individuals. Despite trends toward superior improve-

ments in maximal power production and athletic perfor-

mance in stronger individuals (supported by much greater

mean ES, especially after 5 wk of training), the magnitude of

improvements did not significantly differ between stronger

and weaker groups (Fig. 1 and Tables 2 and 3). Further-

more, the mechanisms driving these adaptations were sim-

ilar for both groups (Figs. 2–4 and Tables 4 and 5).

Athletic performance. The current results indicate

that the experimental training effectively improved athletic

performance in both stronger and weaker individuals. Jump

height and maximal power output during a CMJ were

significantly enhanced after both 5 and 10 wk of training

(Fig. 1 and Table 2). In addition, a range of jump perfor-

mance measures (i.e., net impulse, movement velocity,

RFD, and RPD) also showed significant improvement in

both experimental groups. These observations are similar to

previous research involving ballistic power training on ho-

mogeneous groups of subjects with relatively low (11,38) or

moderate strength levels (25,28). Comparisons of the mag-

nitude of these improvements in jump performance between

the experimental groups showed no statistically significant

differences (Fig. 1 and Table 2). However, ES analyses

revealed that practically relevant differences existed between

stronger and weaker individuals in the magnitude of im-

provements in jump performance after ballistic power train-

ing. Any practical differences between the groups were more

pronounced after 5 wk of training and diminished somewhat

after 10 wk of training. For example, after 5 wk of training,

the ES of improvements in CMJ peak power and jump

height were 1.60 and 1.59, respectively, for the stronger

group compared with 0.95 and 0.61 for the weaker group.

At the completion of 10 wk of training, ES were 1.55 and

1.46 for the stronger group and 1.03 and 0.95 for the weaker

group. Thus, despite the lack of statistically significant

differences between the experimental groups, ballistic power

training had a tendency to elicit a more pronounced effect on

the magnitude of improvements of the stronger group, and

TABLE 5. Neuromuscular factors assessed throughout the 10-wk training program.

Stronger Group Weaker Group Control Group

Baseline Midtest Posttest Baseline Midtest Posttest Baseline Posttest

Muscle architecture

Muscle thickness (cm) 3.29 T0.38 3.30 T0.38 3.26 T0.37 3.02 T0.45 3.07 T0.46 3.08 T0.48 3.06 T0.30 3.06 T0.29

Pennation angle (-) 19.80 T2.23 20.76 T2.15 21.03 T2.14 19.20 T1.92 20.15 T1.59 20.98 T1.71 20.44 T3.99 20.59 T4.17

Lean mass of the leg (kg) 11.35 T1.62 11.56 T1.69 11.50 T1.62 10.82 T1.54 10.76 T1.50 10.66 T1.55 11.28 T1.61 11.30 T1.49

Muscle activation

AvgIEMG IS (MVC)

VM (KV) 81.9 T32.8 64.4 T24.6 68.0 T30.5 69.0 T28.9 82.3 T44.7 81.7 T28.2 79.6 T17.1 78.5 T18.2

VL (KV) 69.5 T22.7 66.6 T12.8 58.8 T26.9 73.3 T39.0 72.6 T42.8 71.2 T28.3 71.3 T11.9 70.0 T23.1

BF (KV) 13.9 T5.9 14.2 T11.9 13.8 T6.7 18.6 T14.2 14.3 T9.4 14.8 T10.2 20.3 T11.1 20.9 T12.0

AvgIEMG 0% 1RM JS

VM (% MVCIs

) 114.5 T24.2 145.2 T25.8 148.0 T45.3 108.8 T29.0 121.9 T52.3 115.3 T19.0 81.3 T13.6 84.6 T23.2

VL (% MVCIs

) 115.2 T28.3 122.3 T20.0 135.0 T28.5 118.8 T46.8 130.1 T59.3 106.0 T25.7 107.1 T30.0 109.7 T28.5

BF (% MVCIs

) 125.6 T21.8 195.5 T102.0 200.9 T112.5 100.8 T58.8 181.7 T73.8 174.4 T90.3 103.0 T53.2 104.9 T76.8

Rate of EMG rise 0% 1RM JS

VM (% MVCIs

) 503.5 T148.9 903.7 T380.9 1057.3 T492.0*

§ 352.3 T174.4 725.1 T305.9* 672.5 T263.8* 373.0 T209.7 386.7 T227.4

VL (% MVCIs

) 479.2 T210.2 634.1 T188.6 971.4 T338.6*

§ 389.6 T187.4 866.0 T672.0 654.1 T283.0 403.6 T209.5 392.9 T146.8

BF (% MVCIs

) 922.6 T413.9 1893.5 T1170.1 1759.3 T941.1 714.5 T551.9 1180.0 T619.0 1359.0 T984.6 449.7 T189.1 477.2 T250.8

AvgIEMG, average integrated EMG; IS, isometric squat; JS, jump squat.

Average integrated EMG (AvgIEMG) was assessed during an isometric squat MVC and during the 0% 1RM jump squat. Rate of rise in AvgIEMG was also assessed during the 0% 1RM

jump squat. Muscle thickness and pennation angle were assessed through ultrasound images. Lean mass of the leg represents the average lean muscle mass of right and left legs as

assessed through dual-energy x-ray absorptiometry scans.

* Significant (Pe0.05) change from baseline.

§ Significantly (Pe0.05) different from control group.

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this could hold great practical relevance. Furthermore, these

results suggest that stronger subjects also display a tendency

toward more rapid improvements in performance after bal-

listic power training than weaker individuals, which is also

of considerable practical importance.

Ballistic power training also resulted in enhanced sprint

performance for both stronger and weaker groups (Table 3).

The change in time between baseline and posttest was sig-

nificant for both groups at 20, 30, and 40 m during a 40-m

sprint. In addition, the stronger group also displayed a sig-

nificant reduction in time at the 5- and 10-m marks. The

improvements in sprint performance for both groups were

of a practically relevant magnitude, representing a 7.3% and

2.2% improvement in 5- and 40-m time (ES = 0.86 and

0.54) in the stronger group and a 6.8% and 3.7% im-

provement in 5- and 40-m time (ES = 0.79 and 0.82) in the

weaker group. Although the additional significant improve-

ments of the stronger group (i.e., at 5 and 10 m) may in-

dicate a greater adaptability to the training stimulus, there

were no significant differences in the magnitude of per-

formance enhancements between the stronger and weaker

groups. Furthermore, the mean ES of the improvements in

sprint performance were quite similar between the experi-

mental groups (Table 3). Previous research has reported

improvements in sprint performance approaching signifi-

cance after ballistic power training (38), but such trends

have not been consistently observed (25,37). Thus, the cur-

rent findings are of great importance because this study

provides the strongest evidence to date that sprint perfor-

mance can be improved by ballistic power training in the

form of vertical jumping. The successful transfer of jump

squat training to sprint performance in the current study is

theorized to be associated with the efficacy of the program

design (i.e., highly sports-specific movements, frequency of

training, effective load, repetition, set, and interset recovery

parameters).

The ability of the current study to elucidate whether the

magnitude of performance improvements after ballistic

power training is influenced by strength level was limited

by several factors. First, the principle of diminished returns

dictates that initial improvements in muscular function are

easily invoked and further improvements are progressively

harder to achieve (37). Thus, the training programs of in-

dividuals with significantly greater strength levels (and more

experienced training backgrounds) typically need to contain

added variability, compared with weaker, inexperienced in-

dividuals (27,37). However, the nature of the research ques-

tions addressed by this study meant that the stronger and

weaker groups completed the same training program during

a period of 10 wk. Although the training program was de-

signed to contain a great deal of specificity to common athletic

movements, the experimental training lacked the level of

variability required to maximize improvements in experienced

athletes. Consequently, the ability of this intervention to

maximize the adaptations of the stronger group may have been

affected. Second, the current observations may have been

confounded by the stronger groups’ cessation of strength

training for the duration of the study (i.e., total of 15 wk). To

examine the specific mechanisms driving adaptation to the

ballistic power training intervention, all subjects were in-

structed to refrain from any lower body resistance training

(or sprint training) outside the scope of the current study.

Although this posed no impact on the strength level of the

weaker group (i.e., not involved in any such training before

commencing the experiment), the stronger group displayed a

statistically nonsignificant but practically relevant decrease

in maximal strength (4.6% decrease in 1RM/BM; ES = 0.91).

The neuromuscular changes associated with detraining peri-

ods of a similar time course (i.e., decreased neural drive and

CSA [15]) are theorized to negatively impact the ability of

the stronger group to adapt to the experimental training.

Thus, the cessation of strength training is theorized to have

negatively affected the ability of the stronger group to adapt

to the ballistic power training. It is imperative, therefore, that

future research is conducted that incorporates strength

maintenance sessions during the training intervention to elu-

cidate further the influence of strength on the magnitude

of improvements in athletic performance. Cognizant of

these limitations, the fact that the stronger group showed

similar performance improvements as the weaker group to

the ballistic power training intervention is of great practical

importance.

Mechanisms responsible for improved performance.

Monitoring the training-induced changes to the force–velocity

and force–power relationships during a sports-specific move-

ment such as the jump squat offers some indication of the

mechanisms driving adaptations in the stronger and weaker

groups. The ability of such an applied in vivo measure of

the force–velocity relationship to delineate exact changes to

muscle mechanics after training is complicated by a range of

factors including mixed fiber composition, muscle architec-

tural characteristics, anatomical joint configuration, levels of

neural activation, as well as the complex nature of the jump

squat movement (23). Despite these limitations, examina-

tion of the force–velocity relationship in sports-specific

movements quantifies the ability of the intact neuromus-

cular system to function under various loading conditions, in-

formation essential to understanding muscular function

during dynamic athletic movements. Data from this investi-

gation revealed that strength level did somewhat influence

the training-induced changes to the jump squat force–velocity

and force–power relationships (Figs. 2 and 3). Specifically,

the stronger group showed a more velocity-specific response

to the training stimulus, displaying the greatest improvements

under the lightest loading conditions (i.e., high-velocity, low-

force portion of the force–velocity relationship). In contrast,

ballistic power training with 0%–30% 1RM resulted in im-

provements in velocity and power throughout a range of

loading conditions for the weaker subjects (i.e., similar im-

provements at both high-velocity and high-force portions of

the force–velocity relationship). These changes caused the

slope of the force–velocity relationship to increase in the

http://www.acsm-msse.org1578 Official Journal of the American College of Sports Medicine

APPLIED SCIENCES

stronger group (aided by a slight decrease in F

max

) but not in

the weaker group. Thus, the current data support the theory

of velocity specificity (13,19) and the notion that relatively

weak or inexperienced subjects display relatively nonspecific

adaptations to training when compared with stronger, more

experienced athletes. Although the stronger group seemed

to display a more velocity-specific response, these apparent

dissimilarities did not translate into major changes in the sig-

nificant between-group differences that were evident before

training (Fig. 3). However, a significant between-group

difference in force at peak power during the lightest load

examined (i.e., 0% 1RM) that did not exist at baseline was

observed at midtest and at posttest. In addition, between-

group differences in power at 20% and 40% 1RM loads that

existed before training were no longer present at posttest.

These training-induced changes to the significant differ-

ences between stronger and weaker groups also indicate that

stronger individuals displayed adaptations with greater

velocity specificity.

Another potential mechanism driving the observed im-

provements in athletic performance was changes to move-

ment mechanics during jumping. Although no significant

changes were observed in the minimum and maximum hip,

knee, and ankle angles during the 0% 1RM jump squat

(Table 4), significant training-induced changes in displace-

ment, velocity, force, and power were evident throughout

the 0% 1RM jump squat for both groups (Figs. 4–6). These

changes are theorized to have led to an optimization of

stretch-shorten cycle (SSC) function, which contributed to

the enhanced jump performance. Arguably, the primary

mechanism driving the enhancement of performance during

SSC movements is the attainment of a greater level of force

at the beginning of the concentric phase in comparison to

concentric-only movements (5). Both stronger and weaker

groups displayed significant increases in force during the

eccentric phase and early in the concentric phase of the 0%

1RM jump squat after training (Figs. 5 and 6). This increase

was generated by the improved acceleration of the BM

during the eccentric phase and, similar to comparisons be-

tween SSC and concentric-only movements, resulted in

greater net impulse, velocity of movement, power output,

and, ultimately, enhanced jump height after training. The

observed changes are hypothesized to be due to the slight

modifications in jumping mechanics (i.e., a marginally

shorter but faster countermovement) and a significant de-

crease in time to takeoff (Tables 3 and 4). Very little pre-

vious research exists examining the impact of training on

performance variables throughout the entire movement

however, similar results have been observed after an

analogous ballistic power training intervention involving

relatively untrained men (10). Comparisons between the

stronger and the weaker groups revealed no new significant

between-group differences after training in joint angles of

the hip, knee, and ankle (Table 4) or power output

throughout the 0% 1RM jump squat (Fig. 4). Thus, changes

to jump mechanics common to both stronger and weaker

individuals after ballistic power training involving jump

squats are theorized to have contributed to improvements in

jump performance.

This investigation revealed that stronger and weaker

subjects displayed similar adaptations in muscle architecture

after ballistic power training (Table 5). As expected, the

training intervention did not elicit any significant changes to

muscle thickness (indicative of whole-muscle CSA [21]) or

the lean mass of the leg for either experimental group. The

relatively light loads used during ballistic power training

(i.e., 0%–30% 1RM) were too small to elicit the necessary

mechanical stimulus required to initiate a significant hyper-

trophic response (16,17). In addition, ballistic power train-

ing did not elicit any significant differences in the pennation

angle of either the stronger or the weaker group. Changes in

pennation angle have been reported previously, with

increases observed after heavy resistance training (4,21),

although not consistently (3), and decreases in response to

sprint training (4). These changes are believed to have a

positive impact on the force- and velocity-generating ca-

pacity of muscle, respectively. However, the potential im-

pact of ballistic power training on changes to pennation

angle has not been examined previously. On the basis of

the current data, ballistic power training did not prompt

structural changes to the muscle (i.e., muscle thickness or

pennation angle). Furthermore, the initial strength level of

the subject did not impact the type of adaptations in muscle

architecture. Muscular adaptations at an intracellular level

(i.e., alterations to anaerobic and aerobic enzymes, muscle

substrates, and/or protein expression) and/or connective tis-

sue remodeling may have contributed to the observed per-

formance improvements (14). However, the potential for

such adaptations cannot be established or rejected because

these mechanisms were not assessed in the current study.

Neural adaptations in response to ballistic power train-

ing were observed, with significant changes evident in the

neural activation patterns of subjects regardless of their ini-

tial strength level. Changes in EMG (indicative of alter-

ations in motor unit recruitment, firing frequency, and/or

synchronization) have been previously reported with im-

provements in performance after ballistic power training

(16,25,36). However, this is one of the first experiments to

show training-induced changes in EMG during complex,

multijoint, sports-specific movements. The current data in-

dicate that ballistic power training resulted in significant

increases in the rate of EMG rise during dynamic athletic

performance (i.e., 0% 1RM jump squat) in both stronger

and weaker groups. Thus, it is theorized that the ballistic

power training enhanced intermuscular coordination by op-

timizing the magnitude and timing of muscle activation.

Similarly, Ha¨kkinen et al. (16) observed ballistic power

training (jump squats with 0%–60% 1RM) to result in a

38% increase in the rate of EMG rise during an isometric

knee extension, which was reported to contribute to im-

proved performance (a 24% improvement in isometric

RFD). Although the current study cannot delineate whether

INFLUENCE OF STRENGTH ON POWER Medicine & Science in Sports & Exercise

1579

APPLIED SCIENCES

the observed changes in EMG were brought about through

alterations in motor unit recruitment, firing frequency, and/or

synchronization, previous research involving intramus-

cular EMG may offer some insight. During ballistic con-

tractions, motor units have been reported to begin firing at

very high frequencies (even in excess of those required to

achieve maximal force) followed by a rapid decline (40).

The high initial firing frequency is believed to result in in-

creased RFD, even if only maintained for a very short

period (26). van Cutsem et al. (36) reported the peak firing

frequency at the onset of ballistic contraction to increase

after ballistic power training. Furthermore, these higher

firing frequencies were maintained for longer throughout

the contraction after training. In addition, a training-induced

increase in the percentage of doublet discharges (i.e., a

motor unit firing two consecutive discharges in a e5-ms

interval) at the onset of a ballistic contraction was also

reported (5.2% of motor units displayed doublet discharges

before training, and this increased to 37.2% after 12 wk of

ballistic power training). These training-induced changes

were reported to contribute to an 82.3% increase in the RFD

and a 15.9% improvement in time to peak force during bal-

listic contractions (36). Therefore, the ballistic power train-

ing intervention of the current study may have induced

adaptations to the pattern of motor unit firing frequency that

subsequently enhanced RFD capabilities and contributed to

enhanced athletic performance. It is important to note that

these mechanisms of adaptation as well as the performance

improvements observed are specific to the movement pattern

and loading parameters used in the current study as power

produced by muscle varies according to both the nature of

the movement and the loading parameters used (12).

In conclusion, the ballistic power training program used

was very effective at enhancing athletic performance, with

both groups showing significant improvements in maximal

power, jump height, movement velocity, and sprint perfor-

mance. The magnitude of improvements in athletic perfor-

mance after ballistic power training did not significantly

differ between strong and weak subjects. However, ES anal-

yses revealed that the training had a tendency toward pro-

ducing a more pronounced effect on jump performance in

the stronger group (especially after only 5 wk of training)

that is believed to hold great practical relevance. Further-

more, this conclusion is strengthened by the confounding

influences of the principle of diminished returns as well as

the stronger groups’ cessation of strength training. Therefore,

because stronger individuals display superior performance

before ballistic power training and have a tendency for

greater improvements after such training, it would be ad-

vantageous for individuals to establish a solid foundation of

strength before focusing on ballistic power training. The

mechanisms driving the performance improvements were

very similar for both the stronger and the weaker groups.

Ballistic power training involving sports-specific movements

increased the rate of EMG rise during jumping, which,

coupled with slight technique modifications believed to be

specific to this training stimulus, led to an improvement in

SSC function. As a result, subjects were able to achieve

greater force and more optimally timed force application

resulting in higher acceleration and movement velocity in

shorter periods. Thus, athletic performance was improved

through enhanced magnitude and RFD, translating to higher

velocity and power production capabilities. Not only do

these findings provide a deeper understanding of the mech-

anistic factors driving improvements in performance after

ballistic power training but they also reveal that the neuro-

muscular and biomechanical adaptations to such training

are not influenced by strength level. These findings have

several implications for the design of ballistic power training

programs that effectively improve athletic performance. The

use of ballistic jump squats with very light loads (i.e., 0%–

30% 1RM) is sufficient to induce significant improvements

in jump and sprint performance of both strong and weak

athletes. However, ballistic power training programs of

stronger athletes need to contain considerably more variabil-

ity than programs of weaker athletes for continued perfor-

mance improvements beyond 5 wk of training. Finally, the

incorporation of strength maintenance sessions throughout

a power training phase is vital because decrements in max-

imal strength (and the ensuing neuromuscular alterations [15])

are theorized to negatively affect the ability of stronger ath-

letes to adapt to ballistic power training.

Funding from the National Strength and Conditioning Association

was received for this work.

Results of the present study do not constitute endorsement by

the American College of Sports Medicine.

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OUTLINE FOR ARTICLE CRITIQUE: TRAINING EXERCISES FOR IMPROVING

SPEED AND AGILITY IN ELITE ATHLETES

 Introduction

 The article by was chosen for analysis because it discusses training exercises that improve

both speed and agility – the capability to move fast and with less effort, for elite athletes.

 The article empirically investigates the adaptation to and use of ballistic power training.

 Ballistic training enables athletes to accelerate a load faster with no deceleration stage,

enabling them to build both power and speed-strength.

 Methodology

 The study design’s procedures are adequate.

 The study design used in the experiment by is the randomized controlled trial (RCT)

undertaken for fifteen weeks.

 The RCT is an intervention study design in which subjects are randomly assigned to two

clusters – the experimental and control groups.

 When the study is conducted, the anticipated difference between the two clusters in the RCT

is the outcome variable under study.

 Results

 The two experimental groups revealed significant enhancements in jump and sprint

performance following the power training.

 Effect size (ES) assessment showed a propensity toward basically relevant variations between

stronger and weaker persons in the improvement degrees in jump performance, mostly after

five weeks’ training.

 The mechanisms responsible for these enhancements entailed significant changes in the neural

activation, jump mechanics, and force-velocity relationship.

 Discussion

 The study offers clear insights into the meaning of the results.

 It explains that the findings show that the trial training efficaciously enhanced athletic

performance in both groups – experimental and control.

 Additionally, the study embeds the results from the perspective of previous studies.

 The study found that the ballistic power training trial training enhanced athletic performance

in both groups – experimental and control.

 Conclusion

 The article’s purpose was to determine if strength level influences the performance

improvement magnitude and the mechanisms controlling adaptation to ballistic power

training.

 Findings showed that strength level did not significantly impact performance improvement

magnitude.

 The study has effectively outlined the RCT study design and met the research ethics

requirements.

 The study’s discussion clearly interprets findings and relates them to previous studies, and

indicates the setbacks.

 References

Running head: ARTICLE CRITIQUE 1

ARTICLE CRITIQUE: TRAINING EXERCISES FOR IMPROVING SPEED AND AGILITY

IN ELITE ATHLETES

Student’s Name:

Institution affiliation:

Date:

ARTICLE CRITIQUE 2

ARTICLE CRITIQUE: TRAINING EXERCISES FOR IMPROVING SPEED AND AGILITY

IN ELITE ATHLETES

Introduction

The article by Cormie et al. (2010) was chosen for analysis because it discusses training

exercises that improve both speed and agility – the capability to move fast and with less effort, for

elite athletes. The article empirically investigates the adaptation to and use of ballistic power

training. Ballistic training enables athletes to accelerate a load faster with no deceleration stage,

enabling them to build both power and speed-strength. This article is vital to speed and agility for

elite athletes because it explores the effect of strength level on enhancement extent and what drives

adaptation following ballistic power training. Understanding this effect is critical to the design of

training exercises that most effectively boost maximal power generation and athlete performance

in athletes with different training backgrounds. Therefore, the article aimed to examine if the level

of performance enhancements and the mechanisms thrusting adaptation to ballistic power training

varies between strong and weak persons (Cormie et al., 2010).

Methodology

The article has salient strengths and weaknesses, as discussed below.

Strengths

The study design’s procedures are adequate. The study design used in the experiment by

Cormie et al. (2010) is the randomized controlled trial (RCT) undertaken for 15 weeks. The RCT

is an intervention study design in which subjects are randomly assigned to two clusters – the

experimental and control groups (Saxena et al., 2012). When the study is conducted, the

anticipated difference between the two clusters in the RCT is the outcome variable under study.

The trial group receives the treatment under assessment, whereas the control group receives an

ARTICLE CRITIQUE 3

alternate intervention or no intervention (Saxena et al., 2012). Investigators monitor under the

experimental conditions to find the trial treatment's efficacy, and effectiveness is evaluated by

comparing it to the control group.

The study was effective in undertaking the necessary steps for the RCT study design. The

RCT requires that subjects are allotted to two or more groups. The investigators divided

participants into two clusters based on their one-repetition-to-body mass ratio (1RM/BM) –

stronger (1BM/BM9 1.55) and weaker (1RM/BMG 1.55). They assigned participants in the

stronger cluster into the stronger group and randomized those in the weaker cluster into one of the

two groups, including the weaker and the control groups.

This study’s methodology also correctly introduced an intervention for different treatments

of the control and experimental groups. In particular, the stronger and weaker sets undertook a 10-

week ballistic training, whereas the control set continued their regular activity level over the study

duration. During the training, three sessions were undertaken every week. Participants did

maximal-effort jump squats having 0-30 percent 1RM. They undertook a 2-d testing battery prior

to commencing training. They did so again after five weeks of training (mid-test), specifically

stronger and weaker sets only, and after ten weeks of evaluation (post-test). The investigators

compared the subjects based on a measured response – the assessment of jump and spring

undertakings and evaluation of the force-velocity connection, jumping mechanics, neural drive,

and muscle architecture.

The study also fulfilled the ethical requirements of studies utilizing human subjects.

According to Resnik (2018), research involving human subjects ethically requires that the likely

participants or those legally permitted to represent the subjects be given the details that a

reasonable individual needs to make an informed judgment whether to take part and a chance to

ARTICLE CRITIQUE 4

assess that information. Cormie et al. (2010) fulfilled this ethical standard by providing their

potential participants with adequate information regarding the study to facilitate their decision-

making whether to participate. Subsequently, Cormie et al. (2010) received the participants’

written informed consent. The study was also authorized by the university’s human research ethics

committee.

Research ethics also requires studies utilizing human participants to minimize risks while

maximizing benefits. Fakruddin et al. (2013) noted that studies with human subjects should protect

subjects from harm and not use unauthorized interventions. Investigators must notify participants

about specific risks linked with specific interventions. They should offer complete details to study

participants regarding the process and possible outcome of randomized trials (Fakruddin et al.,

2013). The expectation of sufficient study design, desirable benefit-harm ratio and equitably free

and adequately informed consent are requisite conditions for an ethical and acceptable human

investigation (Fakruddin et al., 2013). In this regard, Cormie et al. (2010) informed participants

concerning the probable risks involved in the experiment. They also minimized the risks by

recruiting participants based on their capacity to undertake a back squat with adept technique. This

measure enabled the study to obtain 35 men who met all the study’s assessment and training

expectations. The investigators minimized risks further by sufficiently acquainting the participants

with all testing protocols before actual evaluation.

Weaknesses

The article does not explain the sampling method used to obtain the study subjects. The

authors have also not explained the steps they took to obtain the sample. They have only stated

that they recruited participants based on their capability to do a back squat with expertise. They

have not defined the population to which the participants in the experiment belong.

ARTICLE CRITIQUE 5

Results generalizability is difficult since the study did not state the sampling technique

used, the steps undertaken to obtain the sample, or the sample population (target population).

Taherdoost (2016) states that sampling can be utilized to infer about population or generalize

results. However, due to the lack of stating the sampling technique, reasons for its choice, and

necessary steps, it is difficult to determine the population to which the study of can infer.

Results

The two experimental groups revealed significant (stronger and weaker: p ≤ 0.05)

enhancements in jump and sprint performance following the power training. Effect size (ES)

assessment showed a propensity toward basically relevant variations between stronger and weaker

persons in the improvement degrees in jump performance, mostly after five weeks’ training. The

mechanisms responsible for these enhancements entailed significant (p ≤ 0.05) changes in the

neural activation, jump mechanics, and force-velocity relationship. However, there were no

observable changes in muscle architecture. Therefore, strength level did not significantly impact

improvement magnitudes following ballistic power training. However, the training tended to

produce a significant influence on jump performance in the stronger segment. The mechanisms

influencing performance improvements did not differ between strong and weak persons.

Discussion

The strengths and weaknesses of the article’s discussion include the following:

Strengths

The study offers clear results interpretations. It explains that the findings show that the

trial training efficaciously enhanced athletic performance in both groups – experimental and

control. Additionally, the study embeds the results from the perspective of previous studies. The

study found that the ballistic power training enhanced athletic performance in both groups –

ARTICLE CRITIQUE 6

experimental and control. Hence, strength level did not significantly affect improvement levels.

Contrary, McBride et al. (2002) and Wilson et al. (1997) found a significant link between strength

level and performance improvement magnitude after power training. The authors also related their

findings to previous studies. For instance, like this study’s findings, Wilson et al. (1993) reported

significant sprint performance improvements following ballistic power training.

The article indicates the study limitations. Cormie et al. (2010) explain that their current

study’s capacity to determine if the strength level impacted the performance improvement level

following ballistic power training was restricted by factors such as diminishing returns.

Diminishing return is a principle stipulating that preliminary enhancements in muscular activity

are achieved easily, but more improvements are difficult to attain (Wilson et al., 1997).

Weakness

The study does not indicate the degree to which the results can be generalized, although

the discussion presents all the associations illustrated by the results. This issue could be because

the population was not identified earlier in the methodology section. Hence, there is no population

to which to generalize the results.

Conclusion

The article’s purpose was to determine if strength level influences the performance

improvement magnitude and the mechanisms controlling adaptation to ballistic power training.

Findings showed that strength level did not significantly impact performance improvement

magnitude. The study has effectively outlined the RCT study design and met the research ethics

requirements. The study’s discussion clearly interprets findings and relates them to previous

studies, and indicates the setbacks. However, the article does not state its results' generalizability

ARTICLE CRITIQUE 7

due to unstated population and sampling technique. This article generally offers critical insights

into training programs that enhance athletes' speed and agility.

ARTICLE CRITIQUE 8

References

Cormie, P., Mcguigan, M., & Newton, R. (2010). Influence of Strength on Magnitude and

Mechanisms of Adaptation to Power Training. Medicine & Science in Sports &

Exercise, 42(8), 1566-1581. https://doi.org/10.1249/mss.0b013e3181cf818d

Fakruddin, M., Mannan, K., Chowdhury, A., Mazumdar, R., Hossain, M., & Afroz, H. (2013).

Research involving Human Subjects - Ethical Perspective. Bangladesh Journal of

Bioethics, 4(2), 41-48. https://doi.org/10.3329/bioethics.v4i2.16375

McBride, J., Triplett-Mcbride, T., Davie, A., & Newton, R. (2002). The Effect of Heavy- Vs.

Light-Load Jump Squats on the Development of Strength, Power, and Speed. The Journal

of Strength and Conditioning Research, 16(1), 75-82. https://doi.org/10.1519/1533-

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Randomised Controlled Trial? Indian Journal of Medical Specialities, 3(2), 198-202.

https://doi.org/10.7713/ijms.2012.0055

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https://doi.org/10.2139/ssrn.3205035

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Wilson, G., Wilson, R., Murphy, A., & Humphries, B. (1993). The optimal training load for the

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