Lower limb force production and bilateral force asymmetries are based on sense of effort

Previous research suggests that individuals use a sense of effort, more than proprioceptive feedback, to gauge force production in their upper limbs. We have adopted an isometric force matching task to determine if force asymmetry between lower limbs during bilateral force production results from a neural mechanism related to sense of effort. We hypothesized that subjects attempting to produce equal lower limb forces would generate equal percentages of their bilateral maximum voluntary strength rather than equal absolute limb forces. Ten subjects performed isometric lower limb extensions on an exercise machine. Subjects attempted to match forces in their lower limbs at three different submaximal levels (20, 40, and 60% of their weaker limb peak force during bilateral maximum voluntary contraction). Subjects received visual feedback of only the target and stronger limb force. Results showed that subjects consistently produced less force in their weaker limb during all force matching levels when normalized to their unilateral maximum voluntary contraction force (ANOVAs 20% P = 0.0473, 40% P = 0.0012, 60% P = 0.0007). As predicted by our hypothesis, normalizing force magnitudes by bilateral maximum voluntary contraction forces revealed no significant differences between limbs at all force levels (ANOVA P = 0.8490). Regardless of whether humans produce maximal or submaximal forces, limb force asymmetry appears to be related to neural factors rather than differences in mechanical capabilities between the limbs. Our findings have implications for bilateral asymmetries during movement in healthy and neurologically impaired populations.