In this study, we aimed to investigate changes in spinal excitability, SEP, and SPS during voluntary muscle contraction. We found that the F-wave appearance rate increased during the isometric gripping task and demonstrated that this natural variation increases during the pinching task. It is possible that the increase in the F-wave appearance rate during the pinching task is caused by suppression of the corticospinal tract, which converges on the spinal anterior motor nerve, and inhibitory systems such as the higher control system.
The potential source of N20 is considered to be the 3b area, which is interpreted simply as the stage at which sensory stimulation has reached the primary sensory cortex via the thalamus [7]. Furthermore, P25 is considered a component derived from a higher level than the 3b area [8]. This suggests that the submaximal isometric pinching force used in our current study suppresses the somatosensory input of a higher level than the 3b area. Previous studies on SEP gating during voluntary movement have reported that gating does not occur in the components corresponding to N20 [9, 10]. For these reasons, although the electrophysiological input that is projected to the primary somatosensory area is the same for a given amount of physical stimulation (regardless of the presence or absence of the motor task), this electrophysiological input is suppressed during the subsequent more complex process of information processing.
In this study, the accuracy rate for cutaneous stimulation at the sensory threshold was used as an index of SPS, and this parameter was reduced by submaximal isometric muscle contraction. This is considered the result of presynaptic inhibition of peripheral sensation during motor output. During muscle contraction, some sensory inputs derived from skin sensory receptors are already suppressed within peripheral nerves, i.e., before reaching the neural circuits of the brain and spinal cord [11]. The numerous neural circuits in the cerebral cortex and spinal cord play various important roles during motor behavior. The results of this study suggest that there is a mechanism that facilitates the necessary circuits by inhibiting unnecessary circuits that are related to motion during motor output.
Furthermore, it is possible that the threshold of cutaneous surface sensation was increased by the motor output. Most of the decrease in the accuracy rate observed in this study was due to the increased inability to recognize the sensory stimulation. This indicates the need for a test in which participants could recognize sensory information correctly. There was a certain degree of freedom regarding the stimulation site and filament stimulation intensity, as the sensory stimulation was applied manually in this study. This implies that the sensory threshold during motor output does not increase uniformly at all sites but varies according to each site or receptor.
This study demonstrates the neurophysiological aspect of changes in subjective sensation during muscle contraction and that subjective sensations in humans are correlated with the attenuation of sensory potential.