Do the rotator cuff tendons of young athletic subjects hypertrophy in response to increased loading demands?

Background: The rotator cuff is composed of muscle and tendon units. Although muscle has been shown to adapt to mechanical loads, the response of human tendon is not well defined. We hypothesized that increased loading demands on the rotator cuff of young trainees would cause an adaptive muscle response but not an adaptive hypertrophic tendon response. Methods: The hypertrophic response of the rotator cuff tendon, shoulder strength, aerobic fitness, and the lean body weight of 70 young male recruits were studied before and after a 1-year course of elite infantry training. Shoulder strength was assessed by the maximum number of pull-ups done and the rotator cuff thickness by ultrasound measurement of the supraspinatus thickness. Aerobic physical fitness was assessed by maximum oxygen consumption (Vo₂ max). Lean body weight was measured by skin-fold thickness. Results: The mean number of pull-ups done increased from 17.5 to 21.7 (P = .01), but the supraspinatus thickness at the beginning of training (6.1 mm) was unchanged at the end of the training. Vo₂ max increased from 57 to 64 mL/kg/min (P = .0001). Lean body weight increased from 58.3 to 64.7 kg (P = .0001). Conclusions: As a result of increased loading, the strength of the rotator cuff muscles of young trainees increased, but by the parameter of hypertrophy, no evidence was found of a parallel adaptive response of the rotator cuff tendon.