Jonathan D. Gelber, MD, MS; Lonnie Soloff, DPT, PT, ATC; Mark S. Schickendantz, MD

Disclosures

J Am Acad Orthop Surg. 2018;26(6):204-213. 

In This Article

Biomechanics of Pitching

The general stages of throwing are the windup, early cocking, late cocking, acceleration, deceleration, and follow-through (Figure 1). The large muscle groups in the lower extremity, core, and upper extremity act as part of a kinetic chain that coordinates transmission of force from the legs and trunk to the upper extremity for ball release.[13] The transition between the late cocking phase and the acceleration phase is the critical point at which many injuries occur.

Figure 1.

Illustration showing the phases of throwing.

In late cocking, the shoulder is abducted, and the anterior capsule is under major strain as it attempts to restrain anterior translation of the humerus. Repetitive and excessive humeral external rotation by throwers may increase shoulder laxity.[14] Repetitive hard throwing is also thought to cause tensile failure and attenuation of the anterior capsule. Furthermore, stretching of the coracohumeral ligament may occur during forced external rotation in the overhead athlete.[8] With the arm in the 90°-90° position, the undersurface of the posterosuperior rotator cuff makes contact with the posterosuperior glenoid labrum and may become pinched between the labrum and the greater tuberosity, resulting in internal impingement.[11,12] In addition, shear forces increase at the biceps anchor and the posterosuperior labral attachment.[7]

The transition from the late-cocking to the acceleration phase involves coordination of eccentric and concentric muscle activities, which optimizes both performance and functional stability throughout the rapid range of motion. During the late cocking phase, the shoulder is abducted and externally rotated, which is accomplished by contraction of the posterior structures and both stretching of anterior static structures and eccentric firing of the anterior dynamic stabilizing structures. The infraspinatus and teres minor contract while the subscapularis, pectoralis major, and latissimus dorsi form an "anterior wall" that provides stability to the anterior aspect of the shoulder joint.[15] At this point, the supraspinatus is the least active of the rotator cuff muscles.

To transition from the late-cocking phase to the acceleration buildup, the eccentric and concentric balance must shift. The posterior deltoid keeps the arm abducted during shoulder internal rotation. The anterior cuff must then fire to internally rotate the shoulder and generate sufficient torque to impart substantial energy to the ball. In addition, the large anterior muscle groups, such as the pectoralis major and latissimus dorsi, act to internally rotate the shoulder with concentric contraction. The posterior rotator cuff (ie, the teres minor moreso than the infraspinatus) acts as a force couple to these anterior structures by eccentrically firing, which provides a posterior stabilizing force.[16]

During acceleration, professional throwers rely less on the rotator cuff and more on the pectoralis major, latissimus dorsi, triceps, and serratus anterior muscles.[17] In contrast, deceleration relies more on the rotator cuff, especially during follow-through, when the rotator cuff acts eccentrically and is subject to tensile stress. During follow-through, the posterior capsule and posterior cuff are subject to substantial eccentric stresses, which can be up to 108% (±16%) of body weight.[6] Repetitive stress on the posterior cuff may lead to muscular fatigue, which can increase transfer of stress to the posterior capsule.[9] Chronic microtrauma and tearing of the posterior capsule may precipitate a fibroblastic healing response, increased collagen deposition, and loss of tissue compliance, resulting in a stiff posterior cuff and capsule.[10]

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