“You can’t speed it up if you can’t slow it down.” This adage means that you can only move with the speed your body can decelerate. For that reason, deceleration strength is a crucial component for the health and performance of baseball players, and it involves a balance of strength on either side of the throwing shoulder.
The Decelerating Muscles
The primary decelerators are the rotator cuff muscles. The muscle on the frontside rotates the arm forward to accelerate it toward the target—the rotation toward the throwing direction is internal rotation. The rotator cuff muscles on the backside bring the arm into the layback position and create a pre-stretch of the internal rotators before the throw—the rotation of the shoulder toward the layback position is external rotation.
Deceleration occurs when muscles lengthen in the opposite direction of the primary movement. When the arm lays back, the internal rotators (frontside) slow the rate of layback. Then after ball release, the rotator cuff muscles on the backside decelerate powerful rotation of the arm in the direction of the throw.
Balanced rotator cuff strength helps withstand the three prominent biomechanical forces in throwing.
The Rotator Cuff Balances Forces During Throwing
A balanced rotator cuff strength helps withstand the three prominent biomechanical forces in throwing:
- Internal Rotation Shoulder Torque is the rotational force component that occurs during the deceleration effort of the upper arm in layback.
- Elbow Valgus Torque is the rotational force that opens up the inner elbow, creating stress on the ulnar collateral ligament. The UCL is the ligament famously repaired by Tommy John surgery. During pitch delivery, peak internal rotation torque and valgus torque coincide close to the maximum layback of the arm. Strength loss due to fatigue of the acceleration muscles decreases the layback of the shoulder. The body then increases the layback at the forearm to make up for the loss in shoulder motion, which increases torque at the inner elbow.
- Shoulder Compressive Force is the most significant deceleration force that stabilizes the shoulder at the instant of ball release. Suppose strength is unbalanced with stronger internal rotators compared to the external rotators. In that case, compressive forces become too high to withstand and cause excessive strain on the rotator cuff muscles on the back of the shoulder.
