Contribution of intracortical inhibition in voluntary muscle relaxation

Abstract

Terminating a voluntary muscle contraction is an important aspect of motor control, and yet, its neurophysiology is unclear. The objective of this study was to determine the role of short-interval intracortical inhibition (SICI) by comparing SICIs during relaxation from a power grip versus during a sustained power grip at the matching muscle activity level. Right-handed healthy young adults gripped and relaxed from power grip following auditory cues. The relaxation period was determined as the time for the flexor digitorum superficialis (FDS) muscle to reach its pre-contraction baseline level after the cue to relax. SICI during relaxation was obtained at different times into the relaxation period in two separate studies (70, 80, 90 % into relaxation in Study 1; 25, 50, 75 % into relaxation in Study 2). In addition, SICI during sustained contraction was assessed while subjects maintained a power grip at the matching FDS EMG levels (obtained during relaxation, for both Studies). Results showed that the mean SICI was greater during relaxation than during sustained contraction at the matching muscle activity level in both Studies (p < 0.05), indicating increased activation of intracortical inhibitory circuits for muscle relaxation. SICI gradually increased from 25 to 50 and 75 % into relaxation (Study 2, p < 0.05), but did not change from 70 to 80 and 90 % into relaxation (Study 1). MEP decreased with progression of relaxation (p < 0.05) in both Studies, reflecting gradual decreases in corticomotor excitability. This work supports the hypothesis that relaxation from a voluntary muscle contraction involves inhibitory activity in the primary motor cortex.

Fig. 1

Experimental set up for both Studies 1 and 2

Fig. 2

RMS EMG during a single grip-and-relax trial for determination of the muscle relaxation period

Fig. 3

(a) RMS EMG for the entire grip-and-relax trial in Experiment 1 (for both Studies 1 and 2). Stimulation was delivered during relaxation after grip. (b) Raw EMG during relaxation, showing MEP (note different time scales; for both Studies 1 and 2). (c) RMS EMG showing a sustained contraction trial in Experiment 2 (for both Studies 1 and 2). (d) Raw EMG showing the stimulation timing and MEP at the comparable background FDS RMS EMG during Experiment 2 (for both Studies 1 and 2).

Fig. 4

(a) Mean ± SE SICI obtained during 70%, 80% and 90% into the relaxation period during the grip-and-relax trials (Experiment 1) and sustained contraction trials at the matching background EMG levels (Experiment 2) in Study 1. SICI was 41% greater during relaxation than sustained contraction (p < 0.05). (b) Mean ± SE nonconditioned and conditioned MEPs obtained during 70%, 80% and 90% into the muscle relaxation period (Experiment 1) and during sustained contraction at the matching background EMG levels (Experiment 2) in Study 1.

Fig. 5

(a) Mean ± SE SICI obtained during maximal power grip, at 25%, 50% and 75% into the relaxation period during the grip-and-relax trials (Experiment 1) as well as at sustained contractions at the matching background EMG levels (Experiment 2), and at rest in Study 2. SICI was 32% greater for relaxation than sustained contraction (p < 0.05). (b) Mean ± SE nonconditioned and conditioned MEPs obtained during maximal power grip, during 25%, 50% and 75% into the muscle relaxation period (Experiment 1), during sustained contraction at the matching background EMG levels (Experiment 2), and at rest in Study 2.

Fig. 6

Mean ± SE background FDS and EDC RMS EMG at 25%, 50%, and 75% into the relaxation period. The error bar denotes a standard deviation