2 QUESTION OPINION

Clin Sports Med 23 (2004) 531–544

Biomechanics and development of the elbow in

the young throwing athlete

Mark R. Hutchinson, MD*, Shawn Wynn, MD

Sports Medicine and Human Performance Center, Department of Orthopaedics,

University of Illinois at Chicago, 270 MSB, M/C 844, 835 South Wolcott, Chicago, IL 60612, USA

Biomechanics is a complex study of function and demands, including struc-

ture, motor power and acceleration, and angular forces and loads. Regarding the

elbow, the study of biomechanics includes: the flexion/extension motion;

pronation/supination motion; motor power and acceleration related to the biceps,

brachialis, triceps, brachioradialis, supinator, and pronator teres muscles; struc-

tural shapes and interactions of the distal humerus/proximal radius/proximal ulna;

and the forces related to a variety of demands, ranging from lifting to throwing.

The biomechanics of throwing is particularly complex, and relies not only on the

function of an isolated segment such as the elbow, but on the performance and

function of an entire kinetic chain of segments, including the foot-ground surface,

hip and core power and rotation, scapular mobility and stability, shoulder motion

and function, and hand and wrist position at ball release. The throwing motion

has been studied in youth, adolescent, and adult pitchers at all levels of com-

petition [1–14]; however, most biomechanical studies have been performed

on the skeletally mature athlete, with closed physes, with years of throwing

experience, and usually involved in competition at some level. There is less

biomechanical information available on the developing child at different stages of

growth, physeal age, and throwing levels.

The complexity of the study of biomechanics is magnified in the skeletally

immature athlete, due to the dynamic changes occurring during the devel-

opmental phases of youth. The forces and torques of throwing with open physes

have been associated with adaptational changes in the growing bone. As long

bones lengthen with physeal growth, moment arms are altered, changing any

force calculation. The maturing neuromuscular system is progressing with in-

0278-5919/04/$ – see front matter D 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.csm.2004.06.005

* Corresponding author.

E-mail address: [email protected] (M.R. Hutchinson).

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544532

creased muscle mass and power. The pace of muscular maturation mirrors, but is

not directly correlated to, advancing maturity of the neurologic system that

controls coordination, proprioception, quickness, and control. Each segment of

the kinetic chain matures at a different pace, requiring the young thrower to make

continuous, subconscious modifications to account for the changes and still

successfully perform the task or demand. Indeed, although young throwers may

develop the normal sequence of throwing—including windup, cocking, accel-

eration, and follow-through—by the age of 8, the coordination of movement and

power develops gradually over the next several years.

This article reviews basic adult elbow kinematics and biomechanics, and

introduces the unique ways in which the skeletally immature and developing

elbow might be affected in comparison. To accomplish this task, the authors also

review normal physical development and growth in children specifically in-

volving the elbow, including adaptive changes that can occur with repetitive

overuse. The discussion of motor skill development in children emphasizes

the developmental phases of throwing to a mature adult throwing mechanic. The

effect of abnormal mechanics in relationship to injury patterns of the skeletally

immature should support the premise that we must be careful to not overthrow

these young athletes and to teach them proper mechanics.

Basic elbow anatomy and kinematics

The elbow is a complex joint that serves as an important link to position the

hand in space and transmit forces along the kinetic chain. The elbow possesses

two degrees of freedom: flexion-extension and supination-pronation. Normally,

elbow flexion ranges from 08, or slight hyperextension, to about 1508 of flexion. Forearm rotation averages from 758degrees of pronation to 858 of supination. The elbow has a relative valgus alignment, called a carrying angle, that measures

about 108 to 158 in men and about 58 greater in women [15,16]. The carrying angle is measured when the elbow is in extension; interestingly, as the elbow is

full-flexed, the arm/forearm angle changes to 258 of varus secondary to the obliquity of the elbow joint [17]. An electromagnetic tracking device that allows

three-dimensional measurement of active elbow motion reveals the amount of

potential varus-valgus laxity with intact ligament restraints that occurs during

elbow flexion averages about 38 [18]. Stability of the elbow joint is accomplished by the congruity of the bony

articulation, the capsule and ligaments, and the muscle tendon units that cross the

joint. The primary stabilizer for varus stability is the articulation of the ulno-

humeral joint. The medial collateral ligament is the primary stabilizer versus

valgus stress, and the most important stabilizer for the throwing motion.

O’Driscoll et al [19] have also documented the importance of the lateral ulnar

collateral ligament to elbow joint stability, and in particular to posterolateral

rotatory instability of the elbow. The flexor-pronator muscle group serves as a

secondary stabilizer to medial elbow instability.

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544 533

Although excessive forces commonly lead to failure in the ligaments and

tendons of mature throwers, children add to the complexity secondary to their

open growth plates and apophyses. The growth plates frequently yield before

failure of ligaments or mature bone. Overuse can lead to adaptational or chronic

pathological changes in the skeletally immature elbow, including medial epi-

condyle apophysitis or avulsion, radial head hypertrophy, or avascular changes in

the capitellum (osteochondritis dissecans). The etiology of adaptational changes

in the elbow are similar to those seen in the shoulder. The rotational adaptation of

humeral head retroversion seen in elite adult throwers has been correlated with

the torque forces placed across the proximal humeral physis in young pitchers

[20]. When evaluating the developing throwing athlete, it is helpful to know the

normal progression of the appearance and closure of the primary and secondary

centers of ossification [21]. Comparative radiographs can also reveal significant

side-to-side differences.

Biomechanics of throwing

Throwing is an integral component of many sports, and has been associated

with more overuse problems about the elbow than any other single mechanism.

The biomechanics of the throwing motion is dependent on a coordinated se-

quence of events along a kinetic chain from the ground reaction force, up the leg

and through the hips and torso (core), across the shoulder through the elbow, and

down to the hand and ball release [22]. Coordinated human motion, energy, and

momentum are transferred and increased through successive body segments,

which allows the ultimate outcome of speed or distance to occur. The coor-

dination of the sequence is important to prevent overload and overuse injuries,

especially in the more terminal segments about the shoulder and elbow [23,24].

The six stages of the pitching motion are well known (Fig. 1) [7,25–28].

Briefly, they include the windup, stride, arm cocking, arm acceleration, arm

deceleration, and follow-through [7,14]. The windup starts with the pitcher on the

mound, and ends when the throwing hand leaves the glove and the front leg

Fig. 1. Four of the six phases of the adult throwing mechanic: wind up, arm cocking, acceleration,

follow-through.

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544534

strides toward home plate. The stride phase ends when the foot contacts the

mound. The cocking phase begins when the front foot contacts the mound, and

ends when the arm is brought into maximum external rotation. The arm

acceleration phase begins when the arm is in maximum external rotation, and

ends right before the ball is released. The arm deceleration phase begins with ball

release, and ends when the shoulder has reached maximal internal rotation.

Follow-through begins with the shoulder in maximal internal rotation, and ends

when the pitcher has reached a stable fielding position [14].

Each of the six phases has a unique biomechanics about the elbow. During the

windup, the forearm is slightly pronated and the elbow is flexed. During the

stride or early cocking phase, the elbow is extended slightly, but very few

significant forces cross the elbow. During late arm cocking, the forearm is fully

pronated and the elbow extended to about 908, and the shoulder is cocked onto external rotation. Valgus forces across the elbow begin to rise in late cocking, but

increase exponentially during acceleration, resulting in loads that approach the

ultimate failure strength of the medial collateral ligament. During acceleration to

ball release, the elbow extends from 908 to 110 8of flexion to 208 to 358 of extension. In late cocking and acceleration, the energy of the forward stride and

the energy from pelvic and upper torso rotation are transferred more distally

along the kinetic change to the upper extremity. In professional pitchers, the hand

can accelerate to 100 miles per hour in the matter of 50 milliseconds. Internal

torque forces on the humerus approach 14,000 in-lbs before ball release [29].

This motion has been described as the fastest in all sports. Shoulder internal

rotation maximum velocity is between 6000 and 75008/s [6,30]. Maximum velocity at the elbow in professional baseball pitchers is about 23008/s; during the football throw it is slightly slower at about 17608/s. The forces to create this

Fig. 2. Valgus forces associated with throwing lead to compressive forces on the lateral aspect of the

elbow and tension forces over the medial aspect of the elbow.

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544 535

powerful acceleration are developed sequentially along the kinetic chain, with

some contribution of triceps contraction during acceleration. The fourth and fifth

phases come after ball release, and involve a quick deceleration and follow-

through. Just after ball release, the biceps and brachialis fire powerfully to avoid

hyperextension of the elbow. The biceps contraction also assists in slowing fore-

arm pronation. If the biceps and triceps are delayed or weak, posterior impinge-

ment occurs.

In the adult, the throwing motion can place extreme forces across the elbow

that, if unbalanced or uncontrolled, can lead to injury. During late cocking and

acceleration, tension forces across the medial elbow jump to about 300 N (Fig. 2)

[31]. During deceleration, the elbow compressive force jumps over 800 N [4,31].

When the tension forces or chronic changes of the capitellum or radial head

secondary to the compressive loads are left unbalanced or unaccounted for, it is

not surprising that athletes can develop failure of the medial collateral ligament.

Fleisig and colleagues [8,22,32] noted that a single throw in elite baseball players

approaches the ultimate tensile strength of the medial collateral ligament.

The skeletally immature thrower

Although the fundamental concepts of throwing biomechanics apply to the

developing elbow, a number of factors make it difficult to directly translate the

forces and specific mechanics of the adult thrower to the skeletally immature

thrower. The creation of forces is dependent on a mature versus immature

throwing mechanic, the age of the athlete, the weight of the athlete, the height and

stride length of the athlete, the length of each kinetic segment (forearm, arm, and

so on), the strength in each muscle group along the kinetic chain, and a

coordination of the activation and firing of those muscle groups.

Much has been studied and written about the general developmental

milestones in the pediatric population (eg, crawling, walking, talking). Less is

known about the development of a throwing motion in children. Wild [33,34]

first described the developmental sequences for bhard overhandQ throwing. Thirty-two children were filmed while throwing forcefully at multiple times from

age 2 to age 12. She studied how the children, at different ages, recruit different

parts of their body to achieve a forceful throw. Wild and others initially were

describing what is now known as the btotal-body phenomenonQ to the devel- opmental throwing sequence [33–37]. Further research into the development of

an overhead throwing motion has led to a bcomponent approachQ [38–40]. Roberton and Halverson [41] published the developmental sequences for

components of the overarm throw for force, which include the developmental

sequences for three body parts involved in the overarm throw: the trunk (pelvis-

spine), the humerus, and the forearm.

The developmental throwing milestones are a consistent sequence of events

that maturing children progress through when developing a normal throwing

mechanic (Fig. 3) [33,34]. The first few stages begin with the young athlete

Fig. 3. (A–C) The first three developmental phases of throwing include a push, cocking or trunk

rotation, and a step in the direction of the target but all with the body facing the target. (D,E) The final

developmental phases of throwing are heralded by a body turn to perpendicular to the target, followed

by a gradual progression to a mature throwing mechanic.

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544536

Fig. 3 (continued).

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544 537

facing the target, torso parallel. First the young child simply pushes the ball (age

2–3). Next, she learns to cock her arm with some torso rotation to gain

momentum (age 3–5). In an attempt to make the ball go farther in Stage III, she

learns to take a step toward the target (age 5–6). This usually begins with an

awkward ipsilateral step before adding coordination and stepping with the

contralateral leg. The next major development occurs when the young athlete

begins the sequence with her torso facing perpendicular to the target (age 7 and

above). This allows for a more mature development of windup, stride, cocking,

acceleration, and follow-through. Very few injuries happen to children in the

early stages of a developing throwing mechanic. In each stage in which the child

begins parallel and not perpendicular to the target, it is difficult if not impossible

to create enough torque and force development along the kinetic chain to

negatively affect the ligaments or growth plates. As the sequence of throwing

approaches a more mature throwing mechanic, peak velocity improves and the

timing of ball release associated with peak arm velocity improves [41]. Before a

mature throwing mechanic is achieved, children are unlikely to throw hard

enough or frequently enough to exceed the limits of their structure.

Various authors [42–44] have reported that the pace of developing a mature

throwing mechanic may be delayed in females compared with age-matched males

[Fig. 4]. Indeed, Jones in 1961 [45] may have been the first to show that

Fig. 4. The percentage of mature throwing patterns by grade and gender. From left to right the

percentage of both males and females with those patterns increases from kindergarten to eighth

grade, with intermediate decreases in some years; however, females remain delayed throughout

the age progression. (Data from Butterfield SA, Loovis EM. Influence of age, sex, balance, and

sport participation on development of throwing by children in grades K–8. Percep Mot Skills 1993;76:

459–64.)

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544538

bthrowing like a girlQ represented simply an immature throwing mechanic and not a specific gender-related anatomic or physiologic finding. Historically, when

evaluating ball velocity, gender has been a reasonable predictor; however, if the

athletes are grouped by stages of development, gender explains no more then 2%

additional variance [46]. The concept that gender alone causes an abnormal

throwing mechanic should be discarded. What is witnessed is more likely an

immature throwing mechanic.

Tullos and King [47] recognized that by early adolescence young Little

Leaguers had developed a similar sequential pattern of throwing to that of adults.

Campbell and colleagues [48,49] have documented a mature throwing mechanic

by the age of 8 to 9 in some Little Leaguers. Fleisig and coworkers [7] performed

a kinematic and kinetic study to compare pitching biomechanics among various

levels of competition, in order to see if proper mechanics were taught and learned

early in a pitcher’s career. Two hundred and thirty-one male baseball pitchers

were included in the study (23 youth, 33 high-school, 115 college, and 60 pro-

fessional level athletes). Kinematic (position and velocity), kinetic, and temporal

parameter were calculated and compared among the four levels. All velocity and

kinetic parameters increased with competition level. Only 1 out of the 17 position

and temporal parameters showed a significant difference. The increases in joint

forces and torques in the higher level athlete were due to increased strength and

muscle mass. Therefore, it seems that the biomechanics between the different

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544 539

levels of competition were not significant, except for the velocity parameters,

which increase as the athlete’s strength increases and the body matures [7].

Although the sequence appears normal, it is not possible for immature

throwers to develop the same levels of forces across their elbows as seen in adult

throwers. Humerus and forearm length are less than the mature adult, implying a

shorter moment arm to develop torque. Muscle mass, as shown by Fleisig et al [7]

is smaller in the young athlete. Muscle strength and quickness continue to

advance as the young athlete matures, but muscles are not as powerful or agile as

in the adult athlete. These facets, as well as the relative low pitch count in the

youngest of throwers, may explain why injuries are less commonly seen in the

8- to 11-year-old throwers. Relatively speaking, however, the protective concepts

of shorter moment arm, reduced muscle mass, and reduced motor power remain

true for the 12- to 14-year-old pitchers, who are the most common age group to

develop overuse injuries to their elbows. Why does this occur?

Biomechanics and elbow injuries in the skeletally immature throwing athlete

Elbow pain in young baseball pitchers is associated with many factors,

including age, weight, height, number of pitches thrown during a season,

satisfaction with performance, lifting weights, and playing outside of the league

[50]. Age and the timing of the child’s growth spurt appear to be important

factors, because the physis appears to be at increased risk of injury during active

phases of growth. The athlete’s increased height implies longer moment arms in

the kinetic chain to transfer for forces. The athlete’s higher weight implies more

potential energy to be transferred along the kinetic chain during forward stride.

The total pitch count gradually increases as the child thrower matures. By age 11

to 14, young athletes have begun to separate themselves out as position players,

leaving those determined to be pitchers to have an increased total number of

throws. Increased practice with parents and coaches, and the increased demands

and total innings that come with success, further increase the total pitch count.

The energy and forces across the maturing elbow grow with the total number of

pitches, and are magnified by a gradual increase in the speed of the ball thrown

from relative low speeds to 45 to 55 miles per hour by age 11 or 12. Andrews and

Fleisig [51] performed a survey of US baseball to determine recommendations for

pitch counts, and came up with 52 F 15 pitches per game at age 8 to 10, 68 F 18 pitches per game for 11 to 12 year olds, and 76 F pitches for 13 to 14 year olds. Lyman et al [52] noted a 35% increased risk of elbow pain if young athletes were

throwing 75 to 99 pitches per game and over 600 pitches per season. The impact

of pitch type has received special focus in the literature on young throwing athlete

[53–56].

Historically, coaches and physicians have discouraged young pitchers from

throwing curve balls, for fear of increased forces across the medial elbow and

subsequent increased risk of injury. In a recent study [52], 476 pitchers ranging in

age from 9 to 14 were prospectively studied. Nearly 7% of all pitching ap-

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544540

pearances resulted in elbow pain, and 28% of pitchers admitted to having elbow

pain at least once during the season. In studying pitch types, including the slider,

the curveball, and the change-up, it was found that only the slider had a

significant relationship to elbow pain, and that it had an overall 86% increased

risk of elbow pain among the pitchers who threw it. The use of the change-up was

associated with 12% risk reduction for the development of elbow pain. Albright

et al [57] noted that the elbow injury rate was higher in pediatric throwers with

poor technique (52%) as opposed to those who used proper technique (9%). In

their study, side arm motion was implicated; however, the effect of throwing a

curve ball on the skeletally immature elbow remained unproven.

Seventeen to forty-five percent of preadolescent and adolescent baseball

pitchers aged 9 to 14 years have elbow discomfort while pitching [16,58–61].

The presence of a mild flexion contracture occurred in 4% to 12%, and valgus

deformity in 3% to 37%. Radiographic changes may be seen in 28% to 100% of

young pitchers. Radiographic changes include hypertrophy, separation, and frag-

mentation of the medial epicondylar physis; osteochondrosis, flattening or frag-

mentation of the capitellum; radial head hypertrophy or fragmentation; olecranon

physeal widening or spurring; or asymmetric valgus alignment of the elbow. The

etiology of these findings is directly related to overuse and the biomechanics of

the throwing motion [62]. The cocking and acceleration phases create a medial

tension stress over the medial collateral ligament that chronically pulls on the

medial epicondyle. The same valgus load creates chronic compressive forces over

the capitellum and radial head, leading to hypertrophic compensation or vascular

compromise. The compressive forces across the capitellum and radial head

continue during the deceleration phase, as the biceps and brachialis fire to slow

extension and decelerate the forward motion of the arm. The olecranon is loaded

during active extension of the elbow, and may impinge in the olecranon fossa

during cocking and during follow-through. Campbell and colleagues [48,49]

performed some interesting studies comparing pitchers in four age groups: 9 to

12 years old, 13 to 16 years old, collegiate, and professional. They discovered

that younger athletes had slower ball velocity, less angular velocity of the upper

torso, less elbow extension, less shoulder internal rotation, less shoulder internal

rotation torque, and less shoulder anterior force, even when compensated for

body weight. The elbow in the youth pitcher’s elbow did have increased varus

torque forces in the acceleration phase compared with the adult thrower.

So with all of these forces, why doesn’t a pitcher destroy his elbow with every

pitch? The answer lies in the coordinated, balanced, progression of forces trans-

ferred in a carefully sequenced pattern up the kinetic chain. Timing and coor-

dination are important for pitch accuracy, especially the timing of ball release

with peak velocity [63,64]. Some flaws may reduce the forces across the elbow

and reduce the risk of injury. Backward lean in the balance position and early ball

release were associated with a decreased risk of elbow pain [52]; however, if the

athlete tries to throw with the same speed with poor push-off or core rotation, he

must make up the momentum somewhere, and the shoulder or elbow usually

suffer. The distal segment velocity is directly related to the efficiency of the

M.R. Hutchinson, S. Wynn / Clin Sports Med 23 (2004) 531–544 541

proximal/distal segment articulation and the proximal segment angular velocity

[65,66]. The timing of muscle firing and coordination of movement along the

kinetic chain is a developed skill. If the kinetic sequence and transfer of forces are

coordinated, balanced, and smooth, the athlete can throw at the same speed with

less detrimental forces across the joints.

Biomechanical concepts for injury prevention

The best time to correct improper biomechanics, and thereby prevent elbow

injury and pathology, is at the beginning of a pitcher’s career, which usually starts

during youth or Little League baseball. The two primary targets of intervention

are overuse and technique. Current Little League rules advise limiting young

throwers to six innings per week with 3 days rest between outings [67]. Total

pitch counts have been implicated, and should be monitored carefully. Little

Leaguers and their parents have dreams of playing professional baseball. Care

must be taken to include total practice pitches at full speed and with different

teams when evaluating total throwing exposure. In reviewing major league

pitching staffs, it became apparent that most pitchers began pitching at age 15 and

began throwing breaking pitches at age 17. No Little League All-Star pitcher has

become a professional level pitcher [68].

The second target of prevention is optimizing technique. Children should not

be allowed to throw as hard as they can until a mature throwing mechanic is

documented. Poor coordination or weakness at any segment proximally in the

kinetic chain will directly influence the forces seen at more distal segments. If

balance is poor and the sequence is uncoordinated, excessive forces will be

placed at the weak links in the chain. For young developing throwers, the weak

link is likely to be the tension medial side of their elbow or the lateral

compression side of their elbow. Attention to these issues of biomechanics in the

young throwing elbow can help to reduce the overall risk of injury in these

developing athletes.

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