MECHANISM OF INJURY

Improper mechanics are believed to increase the stress placed on the throwing shoulder and elbow, resulting in increased chance of injury. This is especially true for the adolescent thrower because of the status of the epiphyseal growth plates.

Near the end of arm cocking, the shoulder is in extreme external rotation, the elbow is flexed approximately 90 degrees, and the forearm is in a valgus position. To resist the valgus stress, a large varus torque is produced at the elbow. Tension in the ulnar collateral ligament (UCL) provides nearly half of this varus torque. This high tension is near the ultimate tensile strength of the UCL, making it susceptible to injury. Because the growth plates, especially at the medial epicondyle, have not fully fused in younger athletes, the valgus stress generated by throwing tends to affect the growth plate rather than the UCL. The flexor-pronator muscle mass of the forearm also contributes varus torque; thus when the flexor-pronator mass is weak or fatigued, the load on the UCL may be even greater, further amplifying the risk of injury.

Repetitive tension can cause spur formation, leading to compression of the ulnar nerve. Mechanical compromise of the ulnar nerve from tension, friction or compression can lead to ulnar neuritis. Furthermore, ulnar neuropathy has been documented due to repetitively throwing breaking balls.

Two factors have been correlated with increased force on the elbow: increased shoulder rotation at foot contact and increased shoulder horizontal adduction. The presence of multiple improper mechanisms can increase loading of the elbow. The cumulative effect of increased loading from repetitive pitching may result in increased injury risk.

It is interesting to note that biomechanical studies at ASMI have shown that 10 out of 11 paramters during the throwing motion show no difference between levels of skill from youth through professional. These findings support the philosophy that players should be taught proper mechanics as early as possible. Use of a smaller, lighter baseball might allow youth pitchers to generate arm velocities more similar to older pitchers and might help the younger pitcher learn proper grips.