Mitigating the bilateral deficit: reducing neural deficits through residual force enhancement and activation...

The bilateral deficit (BLD) is characterized by a reduction in maximal voluntary torque during a bilateral contraction relative to the sum of left and right unilateral contractions. The BLD has been attributed to interhemispheric inhibition as a result of unilateral torque differences between limbs. If the BLD is the result of interhemispheric inhibition, lowering activation for a torque matching task, as shown in residual force enhancement (RFE), may help overcome the decrease in neural drive during bilateral contractions. Therefore, the purpose of the present study was to determine whether RFE could reduce the BLD.

Participants (n = 12) performed both isometric and RFE MVCs of the elbow flexors under three conditions: (1) unilateral-left; (2) unilateral-right; and (3) bilateral. To directly address the purpose of the study, a sub-group of participants that displayed both RFE and a BLD ("Responders", n = 6) were selected from the participant pool.

"Responders" displayed RFE (7.1 ± 5.3%) and an isometric BLD (BI: - 9.9 ± 3.2%). In the RFE state, the BLD was no longer significant (- 5.8 ± 7.9%), accompanied by the elimination of differences in biceps brachii EMG between arms (left: - 11.7 ± 10.3%; right: - 11.5 ± 13.2%), as seen during isometric contractions (left: - 12.0 ± 23.2%; right: - 21.1 ± 16.6%).

Residual force enhancement appears to mitigate the BLD, alleviating the effects of a decrease in neural drive by allowing more force for a given level of muscle activation when compared to a purely isometric contraction.