Possible differences between the extent of muscle utilization during leg extensions performed bilaterally (BL, both legs acting together) and unilaterally were investigated in young males in college. Significantly less integrated electromyographic activity was recorded from the quadriceps muscles of the dominant leg during BL compared with UL maximal voluntary contractions (MVC). BL strength was significantly lower than the summed (UL; sum of 2 legs acting singly) UL value under isometric conditions [BL = 91 +/- 2.5% (mean +/- SE) of UL; P less than 0.01, n = 9]. By use of a modified isokinetic dynamometer, it was also shown that the effect of increasing the velocity of concentric contraction was significantly greater (P less than 0.001) on BL than UL force development; thus at 424 degrees/s mean BL MVC was only 51 +/- 3.5% of UL. Greater resistance to fatigue was shown in the BL condition in repeated concentric contractions (% decline over 100 MVC at 105 degrees/s was 28 +/- 2.7% for BL vs. 38 +/- 2.9% for UL, P less than 0.001). Results indicated that the extent of motor unit activation appeared to be reduced in BL relative to UL MVC. Compatible evidence from the strength-velocity and fatigability comparisons suggested that this reduction was due to a lesser utilization of the fast-twitch fatigable type of motor unit.