The primary motions that occur during throwing are abduction, horizontal adduction, internal
rotation, and external rotation. Shoulder abduction is a motion that should stay constant throughout throwing. The arm stays abducted approximately 90 to 100 degrees until the ball is released. This has been identified as the ideal angle for the throwing arm, and variance in this angle alters the stress load and increases risk of injury. The shoulder has less hyperangulation forces with lower ball deliveries, which may explain fewer injuries with pitchers who deliver side arm, or “submariner” style. However, the side-arm delivery is not the optimal angle to release the ball because the ball’s flight is flat and easier to hit. The change in trunk position also can lower the arm and change the abduction angle as well.

The motion of horizontal adduction is an important position relative to hyperangulation. There is approximately 30 degrees of motion between a horizontal abduction position and a horizontal adducted position. The elbow is flexed approximately 90 degrees and quickly extends to 20 degrees (3,4,5).

Wilk et al. have recently described a concept called total motion (5). Total motion is the thrower’s maximal external rotation plus maximal internal rotation. Many have published findings that pitchers have greater amounts of external rotation than position players do (6,7). However, this is compensated for by a decrease in internal rotation. Some theorize that the posterior capsule scars and tightens with repetitive throwing, leading to restriction of internal rotation, particularly if the athlete becomes a thrower later in the teen years. Others suggest that the glenoid itself remodels during bone growth to a point where internal rotation has a bony block. The concept of total motion takes this into consideration, since a pitcher’s normal arc of motion is shifted compared with position players and his or her nondominant arm, but the total motion is nearly the same for both.

The baseball pitch can be categorized into specific phases, each with specific muscle firing patterns and biomechanical activities. From beginning to end, the average pitch takes under 2 seconds to fully complete. However, deficiencies in any phases can dramatically alter performance and increase the risk of injury.

Windup (Balance). The thrower assumes this readying position as he or she faces the target to prepare to throw. This phase generates little energy, and only minimally loads the shoulder. Pitchers take a step backward with the lead leg, keeping the push-off leg on the rubber. The athlete stays balanced, and the weight is placed over the push-off leg letting the trunk/core start to generate the arm into external rotation (early cocking). Windup is optimal when the weight is shifted from the push-off leg to the leading leg. The hands separate at the end of the windup, the body rotates 90 degrees, and the stride leg is elevated as the body faces the batter. The body should be balanced as the stride leg reaches maximal height and the late cocking phase. The forearm is slightly pronated, the shoulder slightly internally rotated, and the elbow flexed (Fig. 26.1A) (8). Muscle firing is low at this point.

Early Cocking (Stride). The stride motion begins at the end of the windup phase as the lead leg moves toward the target or plate. The key to high arm velocity is to keep the trunk coiled as long as possible and the delivery arm close to the body, to store the energy before the transfer. As the lead leg moves toward the target (the ball), the energy is uncoiled and the push-off leg begins to move the body forward. As the stride foot makes contact with the ground, the arm is semicocked (Fig. 26.1B). The stride length of a pitcher is slightly less than his or her body height (3,9).

Mechanics often break down at this point. If the stride foot is too closed, the pitcher will tend to throw across the body. If the stride foot is too open (flying out), then the stored energy dissipates. Good foot position is slightly closed to neutral when the lead foot hits the ground. Early activation of the deltoid occurs, while the supraspinatus, infraspinatus, and teres minor fire late.

Late Cocking. Hip rotation starts in late cocking to prepare for acceleration. The shoulder is at
maximal external rotation, which eccentrically loads and stretches the internal rotators. The shoulder has a large amount of energy available from the stored energy of the legs, hips, and trunk. At this point, tremendous stress is on the anterior shoulder. The forearm continues to stay pronated throughout this phase. The subscapularis and pectoralis major fire to form an anterior wall to prevent humeral subluxation.

The wrist begins to cock in preparation for the throw and final release. The elbow remains flexed between 60 and 75 degrees, while the extensor carpi radialis brevis and longus show high levels of activity. The medial elbow encounters high valgus load (Fig. 26.1C).

The deltoid firing decreases from early cocking, but the supraspinatus, infraspinatus, and teres minor reach maximal activity. The subscapularis, pectoralis major, and latissimus dorsi eccentrically fire to stabilize the humeral head as it moves posteriorly, then anteriorly in the later phases of throwing.

Arm Acceleration. The arm is ready to move to the target. The elbow begins to extend, closely followed by the beginning of humeral internal rotation. The trunk begins to flex as the medial elbow endures tremendous load, specifically the medial collateral ligament (MCL) and the pronator/flexor muscle group (3). The pronator teres, and flexor carpi radialis and ulnaris maximally fire at this point (Fig. 26.1D).

The triceps fires early in the phase, while the pectoralis major, latissimus dorsi, and serratus anterior fire late. The humerus moves through horizontal abduction to neutral, so the posterior capsule and rotator cuff absorb minimal shear stress. This may explain why some pitchers are still able to play with profound posterior cuff weakness or injury. Arm acceleration is completed when the ball is released.

Arm Deceleration. The most violent phase of throwing occurs after the ball is released. The arm and body continue to follow toward the target, and the leg kick helps to decelerate the body’s momentum. The lead leg begins to extend as the push-off leg comes to meet it. Arm deceleration has reached its end point when internal rotation is at 0 degrees (Fig. 26.1E). The posterior rotator cuff and shoulder muscles fire eccentrically to slow internal rotation and prevent distraction at the glenohumeral joint, and the long head of the biceps adds traction to slow down the arm. Deceleration requires the largest amount of muscle activity from the elbow flexors. Significant eccentric activity occurs in the latissimus dorsi, subscapularis, and infraspinatus muscles.

Follow-through. The follow-through phase dissipates all the energy used to accelerate and decelerate the arm. It adds nothing to the velocity or control of the pitch, but a proper follow-through minimizes the risk for injury. Shoulder abduction holds at about 100 degrees, and horizontal adduction increases to 35 degrees. The follow-through should have a longer path to allow time to disperse energy. Large body regions should be involved, such as trunk flexion. Follow-through ends with the throwing shoulder and hand at the opposite knee (Fig. 26.1F). Shoulder distraction and risk for injury increase if the arm ends up toward the target instead of across the body at the opposite limb.

As stated previously, pitching delivery in the above phases is different mechanically from the throwing form of position players. Infielders often throw off-balance and side-arm rushing to get the batter or runner out. Their deliveries are usually much shorter, with quicker cocking and release phases. Outfielders often use a crow-hop delivery, which consists of a hop, skip, and throw. Crow hops can also be used as a drill for throwers having difficulty involving their core or trunk.

Electromyographic studies have reported that the supraspinatus is most active at late cocking, as anterior translation and migration off the glenoid are being controlled (10,11). The upper trapezius, supraspinatus, and deltoid are maximally recruited during the early cocking phase (12). The infraspinatus and teres minor are most active during the late cocking and follow-through phases. The serratus anterior and subscapularis muscles are also active from acceleration to follow-through. The rotator cuff is not a prime mover in throwers, but rather a stabilizer, particularly after 90 degrees of elevation. The latissimus dorsi, pectoralis muscles, and subscapularis muscle are the prime movers during the internal rotation and acceleration phase. The lower and middle trapezius continues to stabilize the scapula through acceleration. Furthermore, the core or trunk generates significant power, since the latissimus dorsi originates from the thoracolumbar fascia and lumbopelvis. Thus, latissimus stretches should be part of the warm-up.

Significant force couples exist at the shoulder complex. The joint couples are the rotator cuff/deltoid, subscapularis/infraspinatus, and trapezius/serratus anterior (13). The serratus and lower trapezius muscles work together at about 120 to 140 degrees of elevation to limit the amount of scapular posterior tilt, which helps the acromion clear the glenohumeral joint. Happee and Vander Helm noted 40% thoracoscapular muscle activity with forward arm movement versus 18% noted at the rotator cuff (14). During deceleration, the teres minor, lower trapezius, subscapularis, and posterior deltoid are recruited most to resist the amount of force moving forward toward the target (12).

Knowledge of concentric and eccentric muscle firing and points of peak activity allows the clinician to prevent injury, locate the source of pain, and treat injury more effectively. Part of the history with any throwing injury is knowing exactly at which phase of throwing the pain occurs, such as upon ball release versus follow-through versus late cocking. This information is crucial to diagnosis and treatment.

Many have hypothesized that the breakdown in the kinetic chain is the reason for shoulder problems (14,15,16). The ground-reaction force is the beginning of the kinetic chain and defines the art of throwing, and disputes the contention that it is an arm (shoulder)-only activity. The transfer process in throwing starts from the ground, to the legs, then the trunk, the shoulder, next to the arm, and finally to the ball. Kibler and co-workers note that the hip and trunk provide the most energy (51% to 54%), while the shoulder provided only 13% of the energy and 21% of the force to the link in overhead activities (16,17).