Corticomuscular coherence (CMC) is an oscillatory synchronization of 15–35 Hz (β-band) between electroencephalogram (EEG) of the sensorimotor cortex and electromyogram of contracting muscles. Although we reported that the magnitude of CMC varies among individuals, the physiological mechanisms underlying this variation are still unclear. Here, we aimed to investigate the associations between CMC and intracortical inhibition (ICI) in the primary motor cortex (M1)/recurrent inhibition (RI) in the spinal cord, which probably affect oscillatory neural activities. Firstly, we quantified ICI from changes in motor-evoked potentials induced by paired-pulse transcranial magnetic stimulation in M1 during tonic isometric voluntary contraction of the first dorsal interosseous. ICI showed a significant, negative correlation with the strength of EEG β-oscillation, but not with the magnitude of CMC across individuals. Next, we quantified RI from changes in H-reflexes induced by paired-pulse electrical nerve stimulation to the posterior tibial nerve during isometric contraction of the soleus muscle. We observed a significant, positive correlation between RI and peak CMC across individuals. These results suggest that the local inhibitory interneuron networks in cortical and spinal levels are associated with the oscillatory activity in corticospinal loop.

Oscillatory activity in the sensorimotor cortex is coherent with 15-35 Hz band (β-band) muscle activity during tonic isometric voluntary contractions. In human subjects with higher corticomuscular coherence, prominent grouped discharge associated with a significant silent period was observed in electromyographic (EMG) signals. We examined the potential effects of β-band corticomuscular coupling on new ballistic movement as assessed by reaction time (RT). First, we quantified the coherence between electroencephalographic (EEG) signals over the sensorimotor cortex and rectified EMG signals from the tibialis anterior muscle during tonic isometric voluntary dorsiflexion at 30% of maximal effort in 15 healthy subjects. Subjects were divided into 2 groups [i.e., those with significant EEG-EMG coherence (COH+, n = 8) and those with no significant coherence (COH-, n = 7)]. Next, subjects performed ballistic contractions from a preliminary state of sustained contractions in reaction to auditory signals. RT was defined as the interval between the signal and the response onset measured by force. There were no intersubject differences in RT between COH+ and COH-. However, when the trials performed by COH+ subjects were divided into 2 groups depending on whether clear grouped discharge in the β-band was observed in the EMG (GD+ or GD-) just prior to the reaction, RT was significantly longer in the GD+ than in the GD- trials. We found that the magnitude of EEG-EMG coherence just before the reaction was significantly greater in the GD+ than in the GD- trials. These results suggest that generation of a new movement is delayed when corticomuscular coupling is elevated.