Rotational adaptation: Humeral head retroversion in throwing athletes.
Biomechanics; 4/1/2003; Chant, Chris B.
Byline: Chris B. Chant, MSc
The rotational stress placed on the shoulder while throwing a baseball is tremendous. During a normal overhead throw, the internal rotators and adductors of the shoulder generate large amounts of energy, which act on the humerus to forcefully rotate the shoulder internally from a position of abduction and extreme external rotation. In pitchers, during the cocking phase of throwing, the shoulder may be externally rotated as much as 180[degrees].1,2 During the acceleration phase, which begins when the shoulder is at maximal external rotation and ends at ball release, internal rotation can reach a peak angular velocity close to 7000[degrees]/sec and has been shown to be as high as 9000[degrees]/sec in professional major league pitchers.1,3 This translates into 80[degrees] of rotation occurring in as little as 30 ms.3 Elite pitchers can impart a velocity close to 160 km/h on a ball at the time of release, after which eccentric muscular forces completely stop arm motion within approximately 350 ms.1,3 Compressive forces on the shoulder averaging 1090 N have been reported during deceleration of the arm.4
Because the mechanics of pitching are repetitive and easily reproduced, most throwing studies have focused on the biomechanics and structural adaptations of the shoulder in elite pitchers. However, players at every position on the baseball field are required to throw a ball. Thus, arm and throwing strength are important to any successful player. Although pitchers spend more time overall throwing the ball, all players spend a considerable amount of time throwing before games and during practices.
As a result of greater repetitive stress being placed on one shoulder than the other, athletes who participate in overhead sports such as baseball and tennis display different flexibility patterns between their dominant and nondominant upper extremities.5-10 Much of the literature on this topic focuses on the decreased internal rotation found in the stroke arm of highly skilled tennis players compared to the nonstroke arm. Within this population, the loss of internal rotation seems to be progressive and correlates with increasing age and number of years of tournament play.7
Large differences in external and internal rotation between dominant and nondominant arms have also been demonstrated in baseball pitchers at both the professional and collegiate levels.5,6,8,10 In addition to the side-to-side differences in shoulder range of motion seen in pitchers, Brown and colleagues also reported increased external rotation and decreased internal rotation in the dominant arms of position players at the major league level compared to their nondominant, nonthrowing arms.8
Most investigators have deduced that these adaptations in glenohumeral flexibility occur as a result of changes in the soft tissue in and around the shoulder.6,8,10 The general belief has been that the increased external rotation results from stretching of the anterior capsule and glenohumeral ligaments, and that the decreased internal rotation results from tightening of the posterior capsule and surrounding musculature. However, recent studies of team handball players11 and baseball players5,12 suggest that an osseous adaptation, in the form of increased retroversion of the humeral head, may contribute to the different flexibility patterns of the throwing arm. In theory, a larger angle of retroversion would allow greater external rotation before the anterior capsule and glenohumeral ligaments limit additional movement. By the same token, a larger angle of retroversion would result in less internal rotation since the humeral head would be limited by the posterior capsule earlier in the throwing cycle. Such an osseous adaptation, along with changes in soft tissue, may indeed contribute to the differences in side-to-side ROM seen in throwing athletes.
The overhead baseball pitch can be divided into five distinct phases: windup, arm cocking, arm acceleration, arm deceleration, and follow-through. The relative torques, forces, and muscle activity surrounding the shoulder during the wind-up and follow-through phases are relatively low.2,4 Most of the mechanical stress on the shoulder occurs during the arm cocking, arm acceleration, and arm deceleration phases of the baseball pitch.
The arm-cocking phase begins at the end of the windup and continues until the shoulder is at maximal external rotation. As the thrower drives the lead (stride) leg forward to generate momentum toward the desired target, the shoulder begins to horizontally abduct and externally rotate while the elbow remains in a flexed position. By the time the stride foot makes contact with the ground, the arm has been abducted to approximately 90[degrees] and is in an externally rotated position. The hips, trunk, and shoulders then rotate toward the target, and the trunk extends. As the upper body is turning toward the target, the throwing arm continues to externally rotate. Just prior to maximal shoulder external rotation, large shoulder internal rotation torques are generated. The internal rotators of the shoulder are eccentrically loaded during this period, as they are producing a substantial internal rotation torque on the proximal humerus while the upper arm, forearm, hand, and ball are still externally rotating.4
At the end of the arm-cocking phase, the combination of glenohumeral rotation, scapulothoracic rotation, and trunk extension result in the throwing arm achieving a maximal shoulder external rotation of 150[degrees] to 180[degrees].1,2 The extreme external rotation of the shoulder, coupled with 90[degrees] to 100[degrees] of abduction and significant horizontal extension at maximal shoulder external rotation, is thought to result in anterior capsule laxity and associated anterior instability.13 Although capsular laxity and shoulder instability may seem unfavorable to most athletes, it has been suggested that the extreme external rotation achieved during the pitching motion may be necessary for the performance of elite throwers.4 Specifically, the greater the maximal external rotation of the shoulder, the larger the angle over which the throwing athlete can accelerate the upper arm and ball prior to release.4
The arm-acceleration phase starts when the humerus begins to internally rotate about the shoulder and ends at the time of ball release. This is an extremely powerful and explosive part of the throw lasting less than 0.1 seconds.2 At the initiation of this phase, the trunk begins to flex from its previously extended position. The shoulder internal rotators continue to contract, now concentrically, to help produce an extremely high maximal internal rotation velocity, often close to 7000[degrees]/sec. Dillman3 reported a mean maximal angular velocity of 6940[degrees]/sec plus/minus 1080[degrees]/sec for internal rotation of the shoulder in 29 elite throwers. This maximal internal rotation velocity occurs just before the end of the arm acceleration phase (i.e., at ball release) while the shoulder is still at approximately 90[degrees] of abduction, as it has been throughout the entire phase.1,4
Arm deceleration begins at ball release. The primary purpose of this phase is to comfortably decelerate the throwing limb using eccentric muscular forces, which will completely stop arm motion within approximately 350 ms.1 During the first 50 ms after ball release, a vigorous active deceleration force is generated by the biceps and muscles of the posterior shoulder girdle while the shoulder continues to move into a position of horizontal flexion and internal rotation.1 Compressive forces placed on the shoulder while decelerating the arm have been reported to average as much as 1090 N in elite throwers.4
The final phase of the overhead throw, the follow-through, can be described as a passive phase, during which the body is merely catching up with the arm.
Musculoskeletal changes in athletes
In sports that require greater use of the dominant arm than the nondominant arm, there is overwhelming evidence demonstrating a number of different dominance-specific musculoskeletal changes in athletes. Greater muscle hypertrophy of the dominant arm compared to the nondominant arm is quite common in overhead sports such as tennis and baseball.8-10 Increased bone density and bone size in the one upper extremity compared to the other has also been documented for many athletes and has been related to both static and dynamic forces.14,15
A variety of different mechanical forces have been shown to affect the growth and development of the dominant humerus in athletes participating in overhead sports.15 Krahl14 presented evidence of an increase not only in bone density and diameter, but also in longitudinal bone growth in the stroke arms of 20 high-ranking professional tennis players. They attributed these adaptations to mechanical stimulation and hyperemia of the constantly strained extremity and thus regarded them as biopositive adaptations.
Osseous changes due to imbalances in muscle size and strength are evident in many neuromuscular disorders. For instance, deformation of the femoral head in individuals with cerebral palsy has recently been attributed to specific disease-related muscular forces and pressures.16 Beck and colleagues16 reported on three case studies in which the muscle and tendon of the gluteus minimus caused lateral notching of the femoral head by hindering its migration out of the acetabulum.
Osseous deformities involving the glenohumeral joint have been associated with Erb's palsy.17 Specifically, posterior displacement of the humeral head is quite common in these patients, who have a persistent brachial plexus palsy. The overall deformity of the joint has been found to progress with age during the growing years.18 Thus, evidence supports the theory of muscular forces affecting the growth and development of bone generally and of the humerus specifically.
Effects of muscular forces on humeral torsion
Krahl19 demonstrated the effects of muscle on the torsion of the humerus as early as 1947. In that paper, Krahl discussed the differences in humeral torsion observed between individuals with different-sized muscles and provided evidence that this torsion is subject to change during growth and until maturity. He suggested that there is a hereditary primary genetic torsion as well as a secondary torsion brought about by muscular forces acting above and below the proximal humeral physis. He felt that contraction of the internal and external rotators of the humerus caused a turning of the humeral shaft with respect to the proximal humeral epiphysis, directing the growth of the bone in a spiral fashion. Krahl also suggested that the torsion angle increases (retroversion decreases) by approximately 10[degrees] during childhood and adolescence, finally stabilizing at approximately age 20, coinciding with skeletal maturity.19 Thus, it is plausible that the increased humeral retroversion found in the dominant arm of many competitive baseball players occurred at the proximal physis as a result of repetitive throwing practice during adolescence.
The adolescent thrower
Due to the unique aspects of the developing skeleton, the effects of repetitive throwing on adolescents are not well understood. The weakest part of growing bone is the physis. Thus, although the adolescent athlete is vulnerable to the same injuries as the adult, the presence of the physis changes the patterns of injury. A mechanical stress that would cause injury to the soft tissue in an adult may fracture the physis in the adolescent.20 Fractures through the proximal physis in young baseball players are not uncommon and have been attributed to repetitive throwing of long duration and high frequency.21,22
Dotter22 first described Little Leaguer's shoulder. He believed that pitching caused a fracture through the proximal physis of the humerus in a 12 year-old baseball player. Since this time there have been numerous case reports describing Little Leaguer's shoulder, which is characterized by pain when pitching and is associated with widening of the epiphyseal plate shown on radiographic analysis.21,23
Barnett23 described this condition as being an inflammatory response and identical to adolescent capital femoral epiphyseolysis. It has been shown that an inflammatory process within the physis can have different and unpredictable effects on the linear growth of bone.24 Because as much as 80% of humeral length has been shown to occur at the proximal physis,25 and because the throwing athlete subjects this area to such high loads, it is not unreasonable to suspect some sort of osseous adaptation in the area of the proximal humeral physis in response to the same forces that can lead to injury.
Three major factors point toward humeral bone growth being shaped by the repetitive forces placed on the arm during the throwing motion. First, bone growth has been shown to increase in an athlete's dominant arm due to consistent stress from sporting events.14,15 Second, the torsion angle of the humerus changes throughout the growth period, but stops once skeletal maturity is reached.19 Finally, rotational stress applied to the proximal humeral physis during throwing has been linked to changes in growth plate cartilage, such as Little Leaguer's shoulder and widening of the physis.21-23 Therefore, it is reasonable to postulate that increased humeral head retroversion could be produced in athletes who frequently practice pitching prior to skeletal maturity.
Recognizing the possibility of osseous changes will aid in the prevention of throwing injuries. A lack of flexibility in the shoulder is often cited as being a cause of chronic shoulder pain. Stretching techniques are often prescribed to decrease tightness in the posterior capsule and surrounding musculature and to gain back some of the lost internal rotation that results from the repetitive microtrauma to the dominant shoulder of the throwing athlete. However, Pieper11 reported that the throwing arm of international handball players with chronic shoulder pain did not exhibit the increased humeral head retroversion that was present in those not suffering from chronic pain. He suggested that those athletes who do not undergo an osseous adaptation to the repetitive stress of overhead throwing have more strain on their anterior capsules with less external rotation and develop chronic shoulder pain due to anterior instability. Hence, the shoulder pain of athletes who do not exhibit an increase in humeral retroversion may not be relieved by stretching of the posterior capsule, and the possibility of underlying anterior instability should not be ignored.
Evidence of an osseous contribution to the loss of internal rotation should be taken into consideration during clinical evaluations of these athletes, and stretching of the shoulder complex should be closely monitored and individualized for each athlete. This knowledge could prove helpful in training prescriptions as well as in the treatment and prevention of injury.
Chris Chant, MSc, is a first-year medical student at The Flinders University of South Australia in Adelaide. Previously, he was a clinical kinesiologist at Physiotherapy One/Physiotherapy 2000 in Mississauga, ON.
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