Influence of preceding muscle activity on movement-related cortical potential during superimposed ballistic...

The purpose of current study was to clarify the influence of preceding muscle activity on the force production and movement-related cortical potential (MRCP) during superimposed ballistic contractions. The participants performed the ballistic force production at 40 % of maximum voluntary contraction (MVC) using the isometric abduction force of the metacarpophalangeal joint of the index finger. They were asked to match the peak of force curve with a horizontal target line displayed on the computer monitor. We compared the MRCP amplitude during force exertion detected from Fz, C4, C3, Cz and Pz electrodes during ballistic force production with (active condition) and without (resting condition) preceding muscle activity. The results showed that the MRCP amplitudes of Fz, C4, C3 and Cz electrodes were significantly smaller for the active condition than the resting condition. This was the case even though the peak force values during both conditions were identical. This result suggests that the facilitation of spinal motoneuron excitability by preceding muscle activity could reduce the required central motor command to produce the identical force level. In addition, we examined the MRCP amplitude during ballistic force production of the active condition without a visually displayed target. In this condition, the participants had to perform the force production based on aiming point of target force level (40 %MVC). As a result, the mean of peak force without a visual target was 54 %MVC, which overshot the aiming force level. However, the MRCP amplitudes of five electrodes during the 54 %MVC force production in the active condition were equivalent to the case of the 40 %MVC force production in the resting condition. These results suggest that the MRCP amplitude is consistent with participants' sense of effort involved in the force production, rather than the actual produced force level.