The purposes of this study were to compare internal and external rotation torque measurements of the dominant arm during concentric and eccentric muscle actions, to determine the relationship between peak and average torques, and to compare shoulder rotator capabilities of tennis and nontennis playing women. Twenty healthy nontennis players and 20 healthy intercollegiate tennis players performed concentric-eccentric cycles at 60 degrees /sec angular velocity while seated with the glenohumeral joint at 45 degrees abduction and in the scapular plane (30 degrees horizontal flexion). No significant differences were observed between the two groups on peak or average torques within 115 degrees range of motion, on average torques within +/- 30 degrees of neutral, or on peak/average torque ratios (p > 0.05). Regardless of muscle action, the internal rotators produced significantly greater peak and average torques during eccentric actions than the external rotators (p < 0.01). Eccentric muscle actions produced significantly greater torques than concentric actions (p < 0.01). Peak torques were highly related to average torques within 115 degrees range of motion (r = 0.85-0.93) and less related within +/- 30 degrees of neutral (r = 0.76-0.91). This study provides comparative data for a test position and protocol that may be applicable early in rehabilitation. J Orthop Sports Phys Ther 1991;13(1):40-46.