Maximum bilateral contractions are modified by neurally mediated interlimb effects.

The force exerted by a single limb during a maximal bilateral contraction has been found to be less than the force associated with a maximal unilateral contraction. The purpose of this study was to determine whether this bilateral deficit is due to neural mechanisms. For one experiment, three groups of subjects (untrained, cyclists, and weight lifters) performed maximal one- or two-limb isometric tasks for which the two-limb combinations were either both legs or the left arm and the right leg. The untrained subjects exhibited a bilateral deficit, the cyclists did not, and the weight lifters produced a bilateral facilitation. Although the changes in electromyogram did not completely parallel the changes in force, variability in the filtered electromyogram associated with the maximal contraction was too great for reliable interpretation. The arm-leg task demonstrated that the bilateral deficit affects only homologous contralateral muscles. For the second experiment, two groups of subjects (bilateral deficit and bilateral facilitation) performed maximal left leg contractions while the right leg either rested or was activated with electrical stimulation. All subjects produced an increase in the maximal voluntary left leg force during right leg electromyostimulation. The magnitude of the increase was greatest for the bilateral facilitation subjects. These results suggest that interlimb interactions during maximal bilateral contractions are mediated by neural mechanisms.