Tennis is an asymmetric sport with specific muscle-activation patterns, especially eccentrically in the supraspinatus, infraspinatus, and teres minor during the complex and rapid serving motion. In particular, the external rotators are highly loaded eccentrically during the deceleration phase by resisting shoulder internal rotation first, then sharing the deceleration of horizontal adduction at the glenohumeral joint with the rest of the body. A high percentage of athletes will have infraspinatus atrophy without apparent clinical significance. The external rotation strength deficits, whilst apparently not affecting tennis performance, has been reported as a risk factor for shoulder pain (Clarsen et al, 2014).
Clarsen, B., Bahr, R., Andersson, S. H., Munk, R., & Myklebust, G. (2014). Reduced glenohumeral rotation, external rotation weakness and scapular dyskinesis are risk factors for shoulder injuries among elite male handball players: a prospective cohort study. Br J Sports Med, 48(17), 1327. bjsm.bmj.com/content/48/17/1327.abstract