The effect of initiation prediction and non-prediction on muscle relaxation control

Abstract

[Purpose] This study aimed to examine the difference in the excitability of the primary motor cortex between initiation-predictive and non-predictive tasks, where the onset of muscle relaxation is predicted and not predicted, respectively. [Participants and Methods] Seventeen participants were asked to perform rapid muscle relaxation either through an initiation-predictive or non-predictive task. The baseline was set at 20 percent of the maximum voluntary contraction. Motor-evoked potentials and H-reflexes elicited by transcranial magnetic stimulation and median nerve electrical stimulation, respectively, were measured. The mean stimulation time from the onset of relaxation was calculated, and the motor-evoked potentials and Hoffmann's reflexes elicited during the first (immediately before relaxation) and second half (long before relaxation) were compared. [Results] The amplitude of the motor-evoked potential significantly increased in both initiation-predictive and non-predictive tasks when compared to the baseline, indicating increased excitability of the primary motor cortex. The motor-evoked potential from the initiation-non-predictive task, but not the initiation-predictive task, was associated with increased excitability of the primary motor cortex immediately before relaxation. [Conclusion] Variations in the predictability of motor movements are associated with changes in muscle relaxation control in the central nervous system.

Keywords: Motor-evoked potential; Muscle relaxation.

Fig. 1.

Experimental setting. Overall setup during the experiment. (A) Initiation-predictive task: While maintaining 20% maximum voluntary contraction (MVC) and tone, there was a visual signal (falling line) on the computer monitor, which was presented twice before the onset of relaxation (predictive tone 1 and 2). The participants perform muscle relaxation as soon as possible after the trigger sound. Transcranial magnetic stimulation (TMS) or H-reflex stimulation is performed 50 ms and 75 ms before the trigger sound, respectively. (B) Initiation-nonpredictive task: Muscle relaxation is performed as soon as possible after the trigger sound while maintaining 20% MVC. TMS or H-reflex stimulation is performed at 50 ms and 75 ms after the trigger sound, respectively.

Fig. 2.

Outline of the difference between Long before Relaxation and Immediately before Relaxation, and the time measurement of motor-evoked potential and H-reflex. The black dots indicate each amplitude of motor-evoked potential (MEP)s or H-reflexes.