Transcranial direct current stimulation of primary motor cortex modulates post-contraction potentiation

Surface electromyographic activity (sEMG) of the biceps brachii (BB) during weak elbow flexion has been reported to immediately increase after strong elbow flexion even while exerting consistent force; this phenomenon is called “post-contraction potentiation” (PCP). To determine whether the central nervous system is involved in PCP, we investigated the effect of transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) during PCP. Initially, the participants were instructed to perform successive muscle contraction tasks with different forces: 2% (Test 1); then 25%, 50%, or 100% (conditioning contraction [CC]); and again 2% (Test 2) of maximum voluntary contraction (MVC). In subsequent experiments, the CC intensity was set at 50% MVC, and tDCS (anodal, cathodal, and sham) was applied to the M1 before the task. In the last experiment, TMS was applied to M1 to evaluate the excitability of the corticospinal tract during Tests 1 and 2. The CC intensity at 50% or 100% MVC generated PCP, but didn’t at 25% MVC. Anodal tDCS significantly decreased the magnitude of PCP, while cathodal tDCS showed an increase in magnitude compared to sham tDCS. The BB motor-evoked potential amplitude during Test 2 was lower compared to that during Test 1. These findings suggest that changes in the excitability of the corticospinal tract and resultant changes in the activation pattern of motor unit activity play a role in generating PCP.