Objective: To assess motor SI during real life motor tasks.

Background: SI is a neurophysiological mechanism in the human nervous system that focuses neuronal activity in a center-surround way [1]. Motor SI facilitates individuation of movements, by inhibiting unwanted muscle activation [2]. So far, motor SI has been described only during phasic muscle activation of a hand muscle. In this study, we investigated SI during real life tonic and phasic motor tasks, by assessing the modulation of corticospinal excitability in the ADM muscle when it is the active (center) or the non-active (surround) muscle.

Method: In a group of healthy volunteers we measured activation of the ADM muscle (RMS EMG amplitude) and its corticospinal excitability (MEP peak to peak amplitude) during three tasks: 1. “Writing”: writing with a pen on a paper 2. “Pen”: tonically holding a pen against gravity (pinch) and 3. “Bottle”: tonically holding a water bottle against gravity (grasp).  In the first two tasks, the ADM is in the surround, and in the third task it is one of the agonists.  Then for each subject, the relationship between EMG activation and corticospinal excitability of ADM was modeled, during tonic and phasic activation, as a center muscle (active tonic and active phasic tasks). These individual-subject models allowed predictions of the corticospinal excitability, for levels of EMG activity that matched the initial three tasks. Lower corticospinal excitability during a real-life task compared to the active tasks, with matched EMG activity, demonstrates presence of SI.

Results: Data from 17 subjects showed that MEP amplitude in ADM during the “Pen” task was significantly smaller compared to the active tonic task, with matched EMG activity (p<0.001). This was not the case for the “Bottle” task (p=0.590). During “Writing”, the corticospinal excitability in ADM was weakly but significantly smaller than the tonic active task (p=0.048) and strongly and significantly smaller compared to the phasic active task (p=0.010).

Conclusion: SI is present in ADM when the little finger is not actively involved in a motor task (pinching during the “Writing” and “Pen” tasks) but absent when the little finger is actively involved (grasping during the “Bottle” task). This study introduces a new assessment method for determination of mSI during real life motor tasks. In addition, for the first time, SI is shown to be present during tonic muscle activation.

References: 1. Hartline, H.K., H.G. Wagner, and F. Ratliff, Inhibition in the eye of Limulus. J Gen Physiol, 1956. 39(5): p. 651-73. 2. Sohn, Y.H. and M. Hallett, Surround inhibition in human motor system. Exp Brain Res, 2004. 158(4): p. 397-404.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/surround-inhibition-si-in-the-human-motor-system-during-real-life-motor-tasks/