Quality Work, A++, NO PLAIGARISM, ON TIME

wk5Effects_of_military_basic_trai.pdf

Home >Science homework help >Quality Work, A++, NO PLAIGARISM, ON TIME

Biomedical Human Kinetics, 11, 167–174, 2019 DOI: 10.2478/bhk-2019-0023

Original Paper

Effects of military basic training on VO2max, body composition, muscle strength and neural responses in conscripts of different aerobic condition Kristiina Salo, Jarmo M. Piirainen, Minna M. Tanskanen-Tervo, Heikki Kyröläinen, Jukka Huovinen, Vesa Linnamo

Neuromuscular Research Center, Biology of Physical Activity, Faculty of Sport and Health Sciences, University of Jyväskylä, Jyväskylä, Finland

Summary

Study aim: The purpose of this study was to evaluate neuromuscular adaptations in conscripts with different fitness levels (VO2max) during 8 weeks of military basic training (BT). Material and methods: Twenty-four male conscripts (18–21 years) were divided into two groups (Good Fitness [GF] and Low fitness [LF]) based on their VO2max at the beginning of BT. Body mass (BM), fat free mass (FFM) and Fat% were measured after 2, 4, and 7 weeks of training. VO2max, maximal isometric leg press force (MVC), H-reflex (Hmax/Mmax) at rest and V-wave (V/Mmax) during maximal isometric plantarflexion were measured from the soleus muscle at the beginning, after 5, and after 8 weeks of training. Results: FFM decreased significantly in LF after 7 weeks of training (–3.0 ± 1.7%, p < 0.001), which was not observed in GF. Both GF (6.9 ± 4.6%, p < 0.01) and LF (5.7 ± 4.6%, p < 0.01) showed improved VO2max after 5 weeks, with no changes during the last 3 weeks. A main effect of training was observed in decreased leg press MVC (–7.3 ± 9.3%, F = 4.899, p < 0.05), with no between-group differences. V-wave was significantly lower in LF during 5 (–37.9%, p < 0.05) and 8 (–44.9%, p < 0.05) weeks. Conclusion: Poor development of the neuromuscular system during BT suggests that explosive and/or maximal strength train- ing should be added to the BT protocol for all conscripts regardless of fitness level. In addition, individualized training periodi- zation should be considered to optimize the training load.

Keywords: Aerobic fitness – Neuromuscular adaptation – Military training

Introduction

It has previously been shown that physical fitness of the younger population has decreased during the past decades while their body mass has increased significant- ly [10, 11, 26]. For conscripts of poor physical condition, the basic training (BT) period may be too strenuous and possibly lead to musculoskeletal injuries and drop outs. On the other hand, for high fitness conscripts training may be insufficient, thus leading to poor physical devel- opment. The main purpose of military BT is to prepare conscripts both physically and mentally for specialized military training. Furthermore, it aims to improve sol- diers’ aerobic fitness and neuromuscular performance so that conscripts are able to cope with demanding military tasks [24].

Aerobic and strength capacity can be improved with endurance and strength training; however, neuromuscular adaptation to these training types differs. An explosive and/or maximal strength training regimen can improve neural activity and 3–5 weeks of training has been shown to improve strength capacity [17]. In endurance training, aerobic fitness will generally only be accompanied by mi- nor improvements in strength capacity [4]. In addition to changes in muscle performance, motor control adaptations at spinal and supraspinal levels may also have significant training type related differences. The Hoffmann-reflex (H-reflex) method has been used as an indicator of spi- nal level excitability [18]. However, the H-reflex response is highly influenced by other sources such as excitability of the muscle spindle, inhibitory activation from the Ib afferents, inhibitory sources in motoneuron level (recur- rent inhibition) and especially presynaptic inhibition [16].

Author’s address Jarmo Piirainen, Neuromuscular Research Center, Biology of Physical Activity, Faculty of Sport and Health Sciences, University of Jyväskylä, Snowpolis, Kidekuja 2, 88610 Vuokatti, Finland [email protected]

K. Salo et al.168

Therefore the H-reflex is more like the oligo-synaptic than the monosynaptic response. Because H-reflex measures mostly slow-twitch muscle fibres [3], it seems to be higher in endurance than strength trained athletes [12].

A V-wave response, which is an electrophysiologi- cal variation of the H-reflex [31], is measured during maximal force production. It indicates neural drive ac- tivity from the supraspinal sources, although changes in spinal level activity may also have effects on it. It has been shown during both long [1] and short [32] train- ing periods that heavy resistance training will increase V-wave responses, with minor or no changes in the H-reflex response, showing improvements in supraspinal activation. Thus, neural adaptation is highly dependent on the type of training. However, combining these train- ing forms may lead to improvements in both endurance and strength performances. Several studies have recently shown that it is possible to improve muscular strength and power by combining endurance and strength training without losing aerobic fitness [14, 15]. This is also true in the military environment, where it has been reported that the addition of strength training during the BT period did not interfere with the development of improved aerobic fitness [24, 25]. Despite the background or physical fit- ness of the individuals, the training load during military training is quite similar for all conscripts, which may lead to different training adaptations.

To the best of our knowledge, no previous studies have examined the effects of military BT on neuromuscular function in conscripts with different levels of maximal aerobic fitness (VO2max). In general, a high amount of aer- obic training [33] during BT might lead to minor develop- ment in neuromuscular performance. On the other hand,

in low fitness level conscripts, BT may be too strenuous, which may even result in a decrease in their neuromus- cular performance. Therefore, the purpose of the present study was to evaluate neuromuscular adaptations and their relationship to VO2max during 8 weeks of BT. VO2max was also used to divide subjects into different fitness groups and to evaluate aerobic adaptations during BT. Force production was measured during maximal isometric leg press. Neural adaptations were measured using H-reflex (spinal) and V-wave (supraspinal) tests, which were meas- ured from the soleus muscle. In addition, body composi- tion was measured to analyze possible anthropometric ad- aptations. All methods have been described in more detail in the methods. It was hypothesized that force production would not be improved because of the high amount of en- durance training included in BT. This should also be ob- served in unaltered V-wave responses indicating a lack of supraspinal activity improvements. Spinal level excitabil- ity ( H-reflex) may be improved due to endurance-oriented training.

Materials and methods

Subjects Twenty-four healthy male conscripts (18–21 years)

participated in the study. They were divided into two groups (Good Fitness; GF, N = 12, body mass 68 ± 7 kg, height 1.74 ± 0.09 m and Low Fitness; LF, N = 12, body mass 88 ± 17 kg, height 1.80 + 0.08 m) based on their VO2max values at the beginning of BT as follows: Good Fitness >45.0 (59–45) ml/kg/min and Low Fitness <45.0 (44–29) ml/kg/min (Fig. 1). The level of 45 ml/kg/min was

30.00

40.00

50.00

Good Fitness

V O

2m ax

[m l/m

in /k

Low Fitness

60.00

65.00

25.00

35.00

45.00

55.00

Fig. 1. Individual maximal oxygen uptakes (ml/min/kg) at the beginning of BT (thick line indicates mean value of groups)

Neuromuscular adaptations in military basic training 169

chosen to divide subjects equally into good fitness (supe- rior, excellent, good) and low fitness (fair, poor, very poor) groups [27]. All subjects were informed and fully aware of the experimental protocol and possible risks and they gave their written consent to participate in the study. The conscripts participating in this study were a part of a larger group of conscripts described previously [28–30]. They were randomly selected in the present study. The study was conducted according to the Declaration of Helsinki, and it was approved by the Finnish Defence Forces and the Ethical Committees of the University of Jyväskylä and the Kainuu Region of Finland.

Basic training period The basic training (BT) period lasted 8 weeks, and was

conducted according to the general physical training guide- lines of the Finnish Defence Forces. All conscripts used the same military training schedule throughout the BT period. The intensity level of day-to-day physical activity was low during the first week but thereafter it increased. Military training consisted of heavy physical activities such as marches, battle drills, and other types of physical sport ex- ercises. In addition, conscripts carried heavy combat train- ing equipment of 15–25 kg as part of their exercises, es- pecially during marches and combat training. The training period also included field exercises done overnight. Along with physical exercises, throughout the eight weeks there were theoretical studies taught in the garrison classroom, materials handling, shooting, gun handling, and general military training, such as closed marching exercises dur- ing the BT period. Besides the guided exercises, conscripts were marching four times a day to their eating place (~350 meters in one direction), which increased the total amount of marching up to 5–7 km each day of BT. The BT period also included 4 longer (2–8 hours) marching exercises while carrying heavy combat training equipment, daily outdoor exercises, as well as transitions between venues frequently performed by marching, which increased the amount of endurance activity significantly throughout the training period [24, 25]. Tanskanen et al. [29] reported that in the beginning of the BT the duration of training was ap- proximately 2 hours and 21 minutes per day, increasing to 3–4 hours per day during BT weeks 4–7.

Measurements Aerobic fitness and neuromuscular measurements

were performed at the beginning (1 week), in the middle (5 weeks) and at the end (8 weeks) of BT. The protocol consisted of a maximal aerobic fitness test (VO2max) and neuromuscular measurements (maximal isometric leg press force [MVC], H-reflex and V-wave). Body composi- tion was also measured at the beginning (2 weeks), in the middle (4 weeks) and at the end (7 weeks) of BT. Subject were familiarized with all measurements before the tests. The experimental protocol is presented in Table 1 and all methods are described in more detail in the methods sec- tion.

Body composition Body composition measurements (body mass [BM],

fat free mass [FFM], and percentage of body fat [F%]) were performed at the same time points as the marching tests using 8-point bioelectrical impedance (Inbody720, Biospace Co. Ltd, Seoul, Korea) [2]. For each subject, the measurements were performed at the same time between 6 a.m. and 7 a.m. after an overnight fast and after voiding, with no exercise for 12 hours before the test. The physical activities in the daily programme were planned to be of a low intensity on the day preceding each measurement and fluid status was estimated to be in balance based on the dietary records of the subjects. The subjects were bare- foot and wore similar T-shirts and trousers (weight ~200 g) in each measurement. Body height was measured to the nearest 0.5 cm using a wall-mounted stadiometer.

Maximal leg press force (MVC) MVC was measured from both legs with a leg press

(leg extension) dynamometer (University of Jyväskylä, Finland) [9]. The subjects were seated on the bench with 100 deg hip and 107 deg knee angles, keeping their hands on their chest. Firstly, they did a warm-up of 15 submaxi- mal voluntary contractions. After a 1-minute rest, they performed 3 maximal voluntary contractions at a duration of 2–3 s and with 1 min of seated rest between the per- formances. Force production was simultaneously recorded from both legs with a strain-gauge force transducer. The measured signals were amplified and transferred through

Week 1 2 3 4 5 6 7 8 VO2max test X X X Force* and neuromuscular measurements ** X X X Body composition X X X

Table 1. Experimental design during the 8-week military basic training period

* Maximal isometric leg press force (MVC) and rate of force development (RFD); ** H-reflex and V-wave.

K. Salo et al.170

an isolation unit (ME4ISO, MegaElectronics Ltd., Fin- land) and measuring unit (ME6000, Mega Electronics Ltd., Finland) to the MegaWin software (Mega Electron- ics Ltd., Finland) for later analysis. The best performance according to MVC was selected for further analyses.

Electromyography Electromyographic activity was recorded from the so-

leus and muscle using bipolar (20-mm distance) surface electrodes. The electrode was placed according to SENI- AM [6] guidelines. The EMG signals were measured (15- 500 Hz passband, sampling rate 1000 Hz, gain 1000) and recorded using a wireless connection (MegaWin software, biomonitor ME6000; Mega Electronics Ltd, Finland).

H-reflex H-reflex at rest was measured in a standing position

from the soleus muscle [20]. Measurement was done us- ing a fixed bipolar electrode with an anode placed over the patella and cathode over the tibial nerve. The cor- rect position of the fixed electrode was determined with a movable stimulation electrode by analysing the shape of the EMG waves at different stimulation intensities with a criterion of peak-to-peak amplitude being the only pa- rameter changing. A supramaximal stimulus (with 0.2 ms duration at ca. 0.2 Hz stimulation frequency) was given to the tibial nerve with a stimulator (DS7A, Digimeter Ltd, Welwyn Garden City, England) and the EMG measure- ments were collected from the soleus muscle. Maximal M-wave (Mmax), maximal H-reflex (Hmax) responses and Hmax/Mmax ratio were analysed from the averaged results to evaluate the sensitivity of the α-motoneuron pool and/ or synaptic transmission efficiency at the spinal level.

V-wave V-wave response was measured from the soleus mus-

cle with H-reflex recordings while doing a maximal iso- metric plantarflexion at the hip angle of 110 deg, knee an- gle 180 deg, and ankle angle 90 deg [20]. This knee angle was used to ensure that the subject was mainly using the soleus muscle during the movement. The subject did 5 maximal plantarflexion repetitions with one min intervals. A supramaximal stimulus was given to the tibial nerve af- ter one second from the start of the contraction. Maximal M-wave, V-wave and V/Mmax ratio were analysed after calculating their average measures.

VO2max Each subject performed the test at the same time of

day, between 8:30 a.m. and 5:00 p.m., after consumption of a similar diet at the same time of day before each test. The test started with a warm up of 3 minutes of walking at 4.6 km/h, which was followed by walking or jogging at 6.3 km/h (1 degree slope). After the warm up, the exercise

intensity was increased every three minutes to induce an increase of 6 ml/kg/min in the theoretical VO2max demand of running [23]. The initial running speed was increased by a mean of 1.2 km/h (range 0.6–1.4 km/h), and the initial slope (1 deg) was increased by a mean grade of 0.5 deg (range 0.0–1.0 deg) every three minutes. The test was continued until exhaustion. The collected data con- sidered heart rate (Polar810i; Polar Electro Oy, Kempele, Finland), pulmonary ventilation and respiratory gas ex- change using the breath-by-breath method (Jaeger Oxy- gen Pro; Viasys Healthcare GmbH, Hoechberg, Germa- ny). The mean values were calculated from the measured results at 1 minute intervals for later statistical analysis. The criteria used for determining VO2max were: VO2max and heart rate did not increase despite an increase in the exercise intensity, a respiratory exchange ratio (RER) higher than 1.1, and a post-exercise blood lactate (Lac- tate analyzer; LactatePro, Arkray, Japan) higher than 8 mmol/l (Roitman & Herridge, 2001). All participants sat- isfied these criteria [29].

Statistical analyses Mean values and confidence interval (±95%CI) were

calculated. Two-way repeated measures ANOVA (LSD post hoc) was used to assess the main effects (training; within subjects effects) of measurement interval (VO2max, and neuromuscular performance 1, 5 and 8 weeks, body composition 2, 4 and 7 weeks), fitness (group; between subjects effects) (Good Fitness, Poor Fitness) and interac- tion (training x group). Box’s and Levene’s tests were used to identify normal distribution and if normality was not ob- served, log-transformed values were used. Mauchly’s test of sphericity was used to test the assumption of sphericity. Where this assumption was violated, Greenhouse-Geisser adjustments were used. Where significant main effects or interactions were observed, pairwise comparisons were used to identify the location of differences between meas- urement intervals and training status. Results were consid- ered statistically significant for p-values below 0.05. Data were analyzed using PASW software version 18.0 (SPSS Inc., Chicago, IL, USA).

Results

Body composition At the beginning of BT, significant main effects and in-

teraction were observed in body mass (training; F = 24.657, p < 0.001, group; F = 11.325, p < 0.01, training x group; F = 11.967, p < 0.01,) and FFM (training; F = 8.879, p < 0.01, group; F = 7.534, p < 0.05, training x group; F = 5.376, p < 0.05). BT induced a decrease in body mass (–5.4 ± 1.5%, p < 0.001) and fat free mass (–3.0 ± 1.0%, p < 0.001) in LF, which was not observed in GF (body

Neuromuscular adaptations in military basic training 171

mass –1.2 ± 2.0%; fat free mass –0.3 ± 1.4%). At the end of BT, within group body mass (p < 0.05) and fat free mass (p < 0.05) were still significantly higher in LF than in GF (p < 0.05). In Fat%, no interaction was observed; however, significant main effects (training; F = 12.980, p < 0.001, group; F = 9.440, p < 0.01) were observed. In LF, Fat% decreased during the first 5 weeks (–6.3 ± 2.9%, p < 0.01) and in GF (–6.2 ± 5.6%, p < 0.05) (Table 2).

MVC In MVC (Fig. 2), a significant main effect of training

(F = 4.899, p < 0.05) was observed, but no interaction or between-group differences were observed.

H-reflex and V-wave In H-reflex, no main effects (training or group) or in-

teractions were observed during BT. However, in V-wave, a significant main effect of group (F = 6.007, p < 0.05) was observed (Fig. 3). In LF, normalized V-waves were significantly lower during weeks 5 (–37.9%, p < 0.05) and 8 (–44.9%, p < 0.05).

VO2MAX In VO2max, significant main effects and an interac-

tion were observed (training; F = 42.770, p < 0.001, group; F = 23.230, p < 0.001, training x group; F = 6.760, p < 0.05). Both groups improved their maximal aerobic

GF LF

1 week 4 week 8 week 1 week 4 week 8 week

VO2max [ml/kg/min] 48 ± (2.4) *** 51 ± (2.2) *** ## 51 ± (2.2) ** # 36 ± (3.4) 41 ± (2.9) ### 42 ± (3.0) ###

2 week 5 week 7 week 2 week 5 week 7 week

Body mass [kg] 68 ± (4.5) ** 67 ± (4.6) ** 67 ± (4.4) ** 88 ± (10.1) 85 ± (9.9) ### 83 ± (9.1) ### ¤¤¤

FFM [kg] 57 ± (3.6) * 58 ± (3.9) * 57 ± (3.6) * 69 ± (5.7) 68 ± (6.0) ## 67 ± (5.3) #

Fat% 14 ± (2.5) ** 13 ± (2.5) ** # 14 ± (2.1) * 21 ± (2.9) 19 ± (2.8) ## 19 ± (2.8) ##

Table 2. Aerobic performance and body composition of the conscripts at the beginning, during and after 8 weeks of basic training mean ± (95%CI)

* p < 0.05, ** p < 0.01, *** p < 0.001 between GF and LF; # p < 0.05, ## p < 0.01, ### p<0.001 compared to 1 week; ¤ p < 0.05, ¤¤ p < 0.01, ¤¤¤ p< 0.001 compared to 5 weeks.

500

1000

1500

2000

3000

4000

5000

2500

3500

4500

Good Fitness

1 week 5 week 8 week

Main effect, training; (F = 4.899, p < 0.05)

Le g

pr es

s M

V C

[N ]

Low Fitness

Fig. 2. Leg press MVC at the beginning, after 5 weeks and at the end of BT in the GF and LF conscripts. Significant main effect of training observed between the 1st and 8th week of training (mean, 95%CI)

K. Salo et al.172

performance during the first 5 weeks (GF 6.9 ± 2.7%, p < 0.01; LF 16.7 ± 6.0%, p < 0.01), while changes were not observed between weeks 5 and 8 of BT (GF –0.8% ± 2.4%; LF 1.1% ± 2.5%) (Table 2).

Discussion

The main finding of the present study was that there were no between-group differences in neuromuscular adaptations, measured as neural responses (H-reflex and V-wave) and isometric leg press (MVC), among different aerobic fitness level conscripts during BT. These results confirm the sug- gestion that military BT is mostly endurance type training, which is further supported by the observed improvement in VO2max within both groups during the first five weeks of BT. On the other hand, only LF lost both body mass and FFM during BT, which might thus be one major reason for poor neuromuscular development for the low fitness conscripts. Possible nutrition imbalance and/or overreaching in LF can- not be ruled out either. Development in force production is accompanied by increased voluntary neural activation and/ or an increased muscle cross-sectional area due to hyper- trophy of individual muscle fibres [5]. Thus, maximal leg press force was measured in the current study to evaluate the training adaptations during the BT period in the force production of the large leg muscles that are relevant e.g. for marching and in several combat situations. Interestingly, no between-group difference was observed in MVC during the training and MVC decreased regardless of the aerobic fit- ness level. One reason for insufficient strength development could be the high amount of endurance-based activity in the current military BT [8, 24]. Endurance training has previ- ously been considered to interfere with explosive strength

development by limiting the training induced changes in rapid neural activation [5]. More recently there have been several studies among endurance athletes [14, 15, 21] as well in untrained subjects [13] showing that endurance and strength training programmes can be combined without limitation of strength development. This may suggest that strength development in combined training may not be so strongly related to training history. Therefore, the expla- nation for poor force development seems to be a lack of strength training during BT, which is also supported by un- changed V-wave responses. V-wave measures neural drive from central sources and different forms of strength training have been shown to improve this response [1, 7, 32]. How- ever, it should be mentioned that FFM was decreased in LF, which was not observed in GF. This could be an indicator of negative energy balance, which together with environmen- tal changes and increased physical training leads to higher stress and poorer development of physical performance in prolonged military training [19].

H-reflex responses, which measure motor control at the spinal level, are shown to be dependent on training type. Based on an earlier study, athletes who participate in endurance training have a higher normalised H-reflex response [12], which may be caused by a higher per- centage of slow twitch motor units [3]. However, a 1–2 month training period is an insufficient time period to see changes in muscle fibre types, so some neural adaptation, probably changes in presynaptic inhibition [32] or recipro- cal inhibition, are more likely to be responsible for dif- ferences in the H-reflex response. In the present study, no changes were observed in spinal motor control in either of the groups. This may indicate that if some overreach- ing was present in LF (based on reduced FFM), they were not severely overtrained the end of BT. Raglin et al. [22]

0.00

0.10

0.20

0.30

0.60

0.70

0.40

0.50

Good Fitness

1 week 5 week 8 week

Main effect, group; (F = 6.007, p < 0.05)

V -w

av e

[% M

m ax

]

Low Fitness

Fig. 3. Normalized maximum V-wave (Vmax/Mmax ratio) at the beginning, after 5 weeks and at the end of BT in the GF and LF conscripts. Significant between-group differences were observed during the 5th and 8th week of training (mean, 95%CI)

Neuromuscular adaptations in military basic training 173

observed decreased H-reflex responses in overtrained ath- letes, suggesting that severe fatigue or overtraining will also affect motor control at the spinal level.

Due to technical reasons H-reflex and V-wave were meas- ured from different muscle groups than force production capabilities. Nevertheless, unchanged V-wave and H-reflex responses indicate poor neural adaptation from supraspinal and/or spinal sources. Previously, it has been shown that it is possible to improve neural drive during strength training [1, 32]. This result supports the suggestions that neural strength training (maximal and/or explosive) should be added at the beginning of BT [20, 25]. It should be pointed out that neural responses were similar in both groups and therefore added strength training should be a part of each conscript’s physi- cal training during BT. Previous studies have shown that added strength training during the BT period did not inter- fere with the development of aerobic fitness [24]. Howev- er, it is important to note that the optimal balance between different training responses should be found in a combined strength and endurance type training BT period to prevent overreaching for subjects especially with poor fitness level [5, 25]. It should also be mentioned that a lack of neuromus- cular development may not only lead to higher stress during training, but may also lead higher risks of injuries (e.g. fall- ing accidents and overuse injuries), which increases the risk of dropouts during military service.

Military BT contains a lot of endurance type activities that result in an improvement in aerobic fitness and thus car- diovascular and respiratory performance [24]. The results of the current study support this finding. VO2max results in- creased within both groups during the first five weeks of BT, which was not observed during the rest of the period. Since the VO2max test was conducted with running, which is familiar to all subjects, learning should not play a major role here. One limitation of the study may be that subjects were divided evenly into two groups according to maximal oxygen uptake, which may have caused some overlapping between the groups due to possible small measurement er- ror in VO2max. In order to keep the total number of subjects as high as possible we did not, however, dismiss possible borderline subjects but divided the group equally.

It can be concluded that insufficient development of neural and force responses is a consequence of the lack of strength training during BT. This was the case in all con- scripts regardless of aerobic fitness level before training. Weaker central activity (measured as V-wave) in LF can lead to an overall decrease in performance and increased sensa- tion of the loading and, therefore, result in interruptions from military service. According to the results of the present study, it can be suggested that at the beginning of BT the volume of explosive and/or maximal strength training should be in- creased. This would be recommended for all conscripts but especially those who have poor physical fitness. In addition to added strength training, the volume of endurance training

should be reduced and/or appropriate periodization should be taken into account to avoid possible overreaching.

Conflict of interest: Authors state no conflict of interest.

References

1. Aagaard P., Simonsen E.B., Andersen J.L., Magnus- son P., Dyhre-Poulsen P. (2002) Neural adaptation to re- sistance training: changes in evoked V-wave and H-reflex responses. J. Appl. Physiol., 92: 2309-2318.

2. Anderson L.J., Erceg D.N., Schroeder E.T. (2012) Util- ity of multifrequency bioelectrical impedance compared with dual-energy x-ray absorptiometry for assessment of total and regional body composition varies between men and women. Nutr. Res., (New York, NY) 32: 479-485.

3. Buchthal F. Schmalbruch H. (1970) Contraction times of twitches evoked by H-reflexes. Acta Physiol. Scand., 80: 378-382.

4. Glowacki S.P., Martin S.E., Maurer A., Baek W., Green J.S., Crouse S.F. (2004) Effects of resistance, en- durance, and concurrent exercise on training outcomes in men. Med. Sci. Sports Exerc., 36: 2119-2127.

5. Hakkinen K., Alen M., Kraemer W.J., Gorostiaga E., Izquierdo M., Rusko H., Mikkola J., Hakkinen A., Valkein- en H., Kaarakainen E., Romu S., Erola V., Ahtiainen J., Paavolainen L. (2003) Neuromuscular adaptations during concurrent strength and endurance training versus strength training. Eur. J. Appl. Physiol., 89: 42-52.

6. Hermens H.J., Freriks B., Merletti R., Stegeman D., Blok J., Rau G., Disselhorst-Klug C., Hägg G. (1999) European recommendations for surface electromyogra- phy : results of the SENIAM project. Enschede: Roess- ingh Research and Development.

7. Kinnunen J.V., Piitulainen H., Piirainen J.M. (2019) Neuromuscular adaptations to short-term high-inten- sity interval training in female ice hockey players. J. Strength Cond. Res., 33(2):479-485. DOI: 10.1519/ JSC.0000000000001881.

8. Kraemer W.J., Patton J.F., Gordon S.E., Harman E.A., Deschenes M.R., Reynolds K., Newton R.U., Tri- plett N.T., Dziados J.E. (1995) Compatibility of high-in- tensity strength and endurance training on hormonal and skeletal muscle adaptations. J. Appl. Physiol., (Bethesda, Md : 1985), 78: 976-989.

9. Kyröläinen H. Komi P.V. (1994) Neuromuscular perform- ance of lower limbs during voluntary and reflex activity in power – and endurance-trained athletes. Eur. J. Appl. Physiol. and Occup. Physiol., 69: 233-239.

10. Kyröläinen H., Santtila M., Nindl B.C., Vasankari T. (2010) Physical fitness profiles of young men: associa- tions between physical fitness, obesity and health. Sports Med., (Auckland, NZ), 40: 907-920.

K. Salo et al.174

11. Leyk D., Rohde U., Gorges W., Ridder D., Wunder- lich M., Dinklage C., Sievert A., Ruther T., Essfeld D. (2006) Physical performance, body weight and BMI of young adults in Germany 2000–2004: results of the phys- ical-fitness-test study. Int. J. Sports Med., 27: 642-647.

12. Maffiuletti N.A., Martin A., Babault N., Pensini M., Lucas B., Schieppati M. (2001) Electrical and mechani- cal H(max)-to-M(max) ratio in power – and endurance- trained athletes. J. Appl. Physiol., 90: 3-9.

13. Mikkola J., Rusko H., Izquierdo M., Gorostiaga E.M., Hakkinen K. (2012) Neuromuscular and cardiovascu- lar adaptations during concurrent strength and endur- ance training in untrained men. Int. J. Sports Med., 33: 702-710.

14. Mikkola J., Rusko H., Nummela A., Pollari T., Hakkin- en K. (2007) Concurrent endurance and explosive type strength training improves neuromuscular and anaerobic characteristics in young distance runners. Int. J. Sports Med., 28: 602-611.

15. Mikkola J.S., Rusko H.K., Nummela A.T., Paavolain- en L.M., Hakkinen K. (2007) Concurrent endurance and explosive type strength training increases activation and fast force production of leg extensor muscles in endur- ance athletes. J. Strength Cond. Res., 21: 613-620.

16. Misiaszek J.E. (2003) The H-reflex as a tool in neuro- physiology: its limitations and uses in understanding nervous system function. Muscle Nerve, 28: 144-160.

17. Moritani T. deVries H.A. (1979) Neural factors versus hypertrophy in the time course of muscle strength gain. Am. J. Phys. Med., 58: 115-130.

18. Palmieri R.M., Ingersoll C.D., Hoffman M.A. (2004) The Hoffmann Reflex: Methodologic Considerations and Ap- plications for Use in Sports Medicine and Athletic Train- ing Research. J. Athl. Train., 39: 268-277.

19. Pasiakos S.M. Margolis L.M. (2017) Negative energy balance and loss of body mass and fat-free mass in mili- tary personnel subsisting on combat rations during train- ing and combat operations: a comment on Tassone and Baker. Br. J. Nutr., 117: 894-896.

20. Piirainen J.M., Salmi J.A., Avela J., Linnamo V. (2008) Effect of body composition on the neuromuscular func- tion of Finnish conscripts during an 8-week basic training period. J. Strength Cond. Res., 22: 1916-1925.

21. Psilander N., Frank P., Flockhart M., Sahlin K. (2015) Adding strength to endurance training does not enhance aerobic capacity in cyclists. Scand. J. Med. Sci. Sports, 25: e353-359.

22. Raglin J.S., Koceja D.M., Stager J.M., Harms C.A. (1996) Mood, neuromuscular function, and performance during training in female swimmers. Med. Sci. Sports Exerc., 28: 372-377.

23. Roitman J.L. Herridge M. (2001) ACSM`s resource man- ual for Guidelines for exercise testing and prescription. Lippincott Williams & Wilkins.

24. Santtila M., Häkkinen K., Karavirta L., Kyröläinen H. (2008) Changes in cardiovascular performance during an 8-week military basic training period combined with added endurance or strength training. Mil. Med., 117: 1173-1179.

25. Santtila M., Kyröläinen H., Hakkinen K. (2009) Changes in maximal and explosive strength, electromyography, and muscle thickness of lower and upper extremities in- duced by combined strength and endurance training in soldiers. J. Strength Cond. Res., 23: 1300-1308.

26. Santtila M., Kyröläinen H., Vasankari T., Tiainen S., Palvalin K., Hakkinen A., Hakkinen K. (2006) Physical fitness profiles in young Finnish men during the years 1975–2004. Med. Sci. Sports Exerc., 38: 1990-1994.

27. Shvartz E. Reibold R.C. (1990) Aerobic fitness norms for males and females aged 6 to 75 years: a review. Aviat. Space Environ. Med., 61: 3-11.

28. Tanskanen M., Uusitalo A.L., Hakkinen K., Nissila J., Santtila M., Westerterp K.R., Kyröläinen H. (2009) Aero- bic fitness, energy balance, and body mass index are as- sociated with training load assessed by activity energy expenditure. Scand. J. Med. Sci. Sports, 19: 871-878.

29. Tanskanen M.M., Kyröläinen H., Uusitalo A.L., Huovinen J., Nissila J., Kinnunen H., Atalay M., Hakkinen K. (2011) Serum sex hormone-binding globulin and cortisol concen- trations are associated with overreaching during strenuous military training. J. Strength Cond. Res., 25: 787-797.

30. Tanskanen M.M., Uusitalo A.L., Kinnunen H., Hakkin- en K., Kyröläinen H., Atalay M. (2011) Association of military training with oxidative stress and overreaching. Med. Sci. Sports Exerc., 43: 1552-1560.

31. Upton A.R., McComas A.J., Sica R.E.(1971) Potentia- tion of “late” responses evoked in muscles during effort. J. Neurol. Neurosurg. Psychiatry, 34: 699-711.

32. Vila-Cha C., Falla D., Correia M.V., Farina D. (2012) Changes in H reflex and V wave following short-term en- durance and strength training. J. Appl. Physiol., (1985) 112: 54-63.

33. Williams A.G. (2005) Effects of basic training in the British Army on regular and reserve army personnel. J. Strength Cond. Res., 19: 254-259.

Received 07.05.2019 Accepted 06.11.2019

Acknowledgments

The authors wish to express their gratitude to the assistants who took part in data collection. The study was granted by the Finnish Ministry of Education and Culture and the Scientific Advisory Board for Defence.

Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.