It has been shown that maximal, bilateral efforts result in both a force and EMG deficit when compared to maximal, unilateral activation of the same musculature. It is unclear whether this deficit is the result of interactions of central or peripheral origin. The first aim study investigated the bilateral performance index (BPI (%) = [100 x bilateral force/(right unilateral + left unilateral forces)] - 100) for maximal, isometric, extensor torques about the knee joint in three groups of subjects: untrained (never lifted weights), cyclists (leg musculature trained reciprocally), and weightlifters (legs trained bilaterally). The BPI for the weightlifters (+7.0 ± 5.0%) was significantly (p < 0.05) greater than the BPI of the cyclists (-4.0 ± 6.3%) or the untrained subjects (-9.7 ± 5.2%). These results indicate that the inhibitory mechanisms previously proposed to act during bilateral efforts are inadequate, and that excitatory factors must be present to achieve a BPI > 0. The second aim study showed that the BPI can be altered as a result of three weeks of bilateral isometric strength training. The BPI's for the control and unilateral training groups were not significantly different pre- to posttraining. However, the BPI of the bilateral training group increased significantly (p < 0.05) from -3.7 ± 6.9% prior to training, to +4.2 ± 4.4% after training. These findings indicate that bilateral strength training can alter the relationship between unilateral and bilateral force output. The third aim study demonstrated that subjects with a positive BPI (+6.8 ± 4.3%) responded differently to an afferent perturbation (electrical stimulation) than subjects with a negative BPI (-10.0 ± 5.2%). The negative BPI group showed a 5.7 ± 3.4% facilitation in force during contralateral electrical stimulation. This was significantly (p < 0.05) less than the 16.5 ± 7.5% facilitation shown by the positive BPI group. These results indicate that afferent feedback can alter the force output in the contralateral limb, and may thereby play a role in unilateral-bilateral force differences.