In order to understand the possible mechanisms contributing to enhanced concentric performance in stretch-shortening cycle exercises in vivo the present study examined knee extension torque, EMG activity and fascicle length of the vastus lateralis muscle in maximal and submaximal human movements. Maximal concentric knee extensions (120 s-1) were done after pre-stretch and pre-isometric conditions by nine volunteers. During shortening at the knee angle of 115 (180 = extended) the knee extension torque was found to be greater in pre-stretch condition (272 vs. 248 Nm, p < 0.05) although the torque level prior to shortening was smaller than in pre-isometric condition (268 vs. 314 Nm, p< 0.05). At the moment of torque enhancement the EMG activity levels or fascicle lengths did not differ between the conditions. It is proposed that besides specific experimental conditions the present enhancement may be related to longer fascicle length prior to shortening (by 4.1 cm, p< 0.05) in pre-stretch condition and to modified length-tension properties. Fascicle length behaviour was found to play an important role also in unilateral, submaximal sledge-jump conditions where pre-loading was altered but the concentric net impulse and joint angular movements were the same. In repeated drop jumps with greater pre-load the changes in fascicle length were smaller than in the counter movement jump that was characterized by a lower force and activity level in the eccentric phase. Results from the present maximal and submaximal loading conditions suggest that the benefits of stretch-shortening cycle muscle function may come through different interactive mechanisms that may be task specific.