Various studies have indicated that human muscles in-vivo manifest a substantially similar, if not the identical force-velocity relationship established for isolated, maximally stimulated animal muscles. In the present study, fifteen healthy males and females, 18 to 38 years old and representing varied activity patterns from sedentary to athletic, performed maximal dynamic knee extensions on an isokinetic loading dynamometer. Maximal torque forces attained at a specific point in the range (30 degrees before full extension) and at seven loading velocities from 0 (isometric) to 288 degrees/sec were recorded. The maximum 30 degrees torques exhibited by the various subjects ranged from 29 to 245 Newton-meters. However, over the four lower test velocities (0, 48, 96 & 144 degrees/sec), all subjects exhibited less than a 15% deviation from their respective maximum 30 degrees torque values, which occurred most often at the 96 degree/sec test velocity. Maximal instantaneous power output at the 30 degree position ranged from 98 to 680 Watts. In all 15 subjects this was attained at and remained generally constant over the three highest test velocities (192 to 288 degrees/sec). A neural mechanism that restricts a muscle's maximal tension in-vivo is postulated as being responsible for the marked difference between the force-velocity relationship found for human muscles in-vivo and that exhibited by isolated animal muscles.