Throwing and batting each require repetitive motions that can result in injuries unique to baseball. Fortuantely, advances in operative and nonoperative treatments have allowed players to return to competition after sustaining what previously would have been considered a career-ending injury. Once a player has been deemed ready to return to throwing or hitting, a comprehensive, multiphased approach to rehabilitation is necessary to reintroduce the athlete back to baseball activities and avoid re-injury. This article reviews the biomechanics of both throwing and hitting, and outlines the phases of rehabilitation necessary to allow the athlete to return to competition.
Throwing
Biomechanical Overview
The overhead throwing motion is complex and involves full body coordination from the initial force generation through the follow-through phase of throwing. The “kinetic chain”—the concept that movements in the body are connected through segments culminating with the highest energy in the final segment—is paramount to achieving the force and energy needed for throwing.1-8 The kinetic chain begins in the lower body and trunk and transmits the energy distally to the shoulder, elbow, and hand, ending with kinetic energy transfer to the ball.3-5,7 The progression of motion through the kinetic chain during throwing includes stride, pelvis rotation, upper torso rotation, elbow extension, shoulder internal rotation, and wrist flexion. Disruptions in this chain due to muscle imbalance or weakness can lead to injury downstream, particularly in the upper extremity.3,7,9
The importance of the kinetic chain can be highlighted in the 6 phases of throwing motion. These include wind-up, early arm cocking, late arm cocking, arm acceleration, arm deceleration, and follow-through (Figure 1).1,2,9,10
The wind-up phase starts with initiation of motion and ends with maximal knee lift of the lead leg; its objective is to place the body in an optimal stance to throw.3-5,7 There are minimal forces, torques, and muscle activity in the upper extremity during this phase, but up to 50% of throw speed is created through stride and trunk rotation.6 During the early cocking phase, the thrower keeps his stance foot planted and drives his lead leg towards the target, while bringing both arms into abduction. This is coupled with internal rotation of the stance hip, external rotation of the lead hip, and external rotation of the throwing shoulder. This creates linear velocity by maximizing the length of the elastic components of the body. Elbow, wrist, and finger extensors are also contracting during this phase to control elbow flexion and wrist hyperextension.3
The late cocking phase begins when the lead foot contacts the ground and ends with maximum shoulder external rotation.3-5 Lead foot contact is followed by quadriceps contraction to decelerate and stabilize the lead leg. This is followed by rotation of the pelvis and upper torso. The result is energy transfer to the throwing arm with a shear force across the anterior shoulder of 400 N.4 The shoulder stays in 90° of abduction, 15° of horizontal adduction, and externally rotates to between 150° and 180°. This produces a maximum horizontal adduction moment of 100 N.m and internal rotation torque of 70 N.m.4 Simultaneously, the elbow generates maximum flexion and a 65 N.m varus torque.7 Forces about the elbow are generated to resist the large angular velocity experienced (up to 3000°/second). This places an extreme amount of valgus stress along the medial elbow, particularly on the ulnar collateral ligament. The shoulder girdle and rotator cuff muscles simultaneously act to stabilize the scapula and glenohumeral joint.
The arm acceleration phase is from maximal shoulder external rotation until ball release.3-5 In this phase, the thrower flexes his trunk from an extended position, returning to neutral by the time of ball release while the lead leg straightens. The shoulder stays abducted at 90° throughout while the rotator cuff internal rotators and scapular stabilizers contract to explosively internally rotate the shoulder, creating a maximal internal rotation velocity greater than 7000°/second by ball release.1,4,7 The elbow also begins to extend, reaching maximum velocity during mid-acceleration phase from a combination of triceps contraction and torque generated from rotation at the shoulder and upper trunk.3 Finally, the wrist flexors contract to move the wrist to a neutral position from hyperextension as the ball is released.
During arm deceleration, the shoulder achieves maximum internal rotation until reaching a neutral position and horizontally adducts across the body. This is controlled by contraction of the shoulder girdle musculature; the teres minor has the highest activity.3,4 The greatest forces produced during the throwing motion act at the shoulder and elbow during deceleration and can contribute to injury.2 These include compressive forces of greater than 1000 N, posterior shear forces of 400 N, and inferior shear forces of 300 N.4,7