Thirty-six participants performed bilateral and unilateral isometric elbow flexion trials at what they perceived to be 100, 75, 50, and 25% of maximal effort. Absolute bilateral deficits ranged from -16% at 25% effort to -10% at 100% effort. The deficit included a component independent of consciousness and a component inversely related to intensity attributable to perceptual differences between unilateral and bilateral tasks. Forty-two participants performed bilateral and unilateral isometric elbow flexion trials at 100, 80, 60, 40, and 20% of maximal effort. Perception of effort in submaximal bilateral trials was consistently and significantly higher (5.5-9.6%) than corresponding unilateral trials. These data suggest that the bilateral deficit exists at submaximal levels of effort and is based on perceptual and physiological components. (Contains 7 figures and 2 tables.)