Baseball rehabilitation and prehabilitation often focuses on the dominant throwing arm for obvious reason the incidence of injury is higher. In position players the non dominant arm plays an important role in hitting mechanics and should be evaluate to prevent injury. For a right handed hitter who also throws right handed, the left arm is used to decelerate the the swing during hitting.
Eccentric strength, which is the ability for a muscle to contract while the muscle is lengthening, is required to decelerate the swing. An easy example of eccentric strength is the controlled descent of a bicep curl, how slow/smooth the weight can be lowered is based on the eccentric strength of the bicep (the bicep is both lengthening and contracting). During a forceful swing, the follow through and slowing of the bat is controlled mostly by the non dominant arm. There are different styles of follow through that impact the amount of eccentric strength required of the non dominant arm. Single arm follow through puts the greatest demand on the arm.
Two arm follow through allows both hands to decelerate the swing, although the demand is still greater on the non dominant arm. Some positions players throw right handed and hit left handed which puts increased demand on their dominant arm to decelerate the swing since their dominant arm is leading in their batting stance. In this case different training/ strengthening can be implemented to prevent injury related to swinging. A proper wholesome strengthening program for a position player who both throws and hits would require evaluation of the eccentric strength of various muscle groups that would be active during deceleration of the swing. The muscles active during swing deceleration would be the internal rotators of the shoulder, pectoral complex, shoulder extensors and shoulder adductors. All of these muscles are eccentrically controlling the deceleration of the swing. Without strengthening of these muscles injuries from excessive joint torque such as cartilage damage, labral injuries and dislocation are likely higher because of the lack of muscular support. Evaluation of weakness related to the shoulder complex of the non dominant arm is an important factor to keep young athletes competing on the field and having fun.