Ascending beta oscillation from finger muscle to sensorimotor cortex contributes to enhanced steady-state isometric contraction in humans.

Objective: β-Band corticomuscular coherence is suggested as an electrophysiological mechanism that contributes to sensorimotor functioning in the maintenance of steady-state contractions. Converging evidence suggests that not only the descending corticospinal pathway but the ascending sensory feedback pathway is involved in the generation of β-band corticomuscular coherence. The present study aimed to investigate which pathway, descending vs. ascending, contributes more to the stability of muscle contraction, especially for human intrinsic hand muscles.

Methods: In this study, we assessed directed transfer function (DTF) between magnetoencephalography signals over the sensorimotor cortex (SMC) and rectified electromyographic (EMG) signals recorded during steady-state isometric contraction of the right thumb muscle (flexor pollicis brevis, FPB) or right little finger muscle (flexor digiti minimi brevis, FDMB) in 15 right-handed healthy subjects.

Results: β-Band DTF was statistically significant in both descending (SMC→EMG) and ascending (EMG→SMC) directions, and mean phase delays for each direction were in agreement with the conduction time for the descending corticospinal and ascending sensory feedback pathways. The strengths of the β-band DTF (EMG→SMC direction) were greater in the FPB muscle than in the FDMB muscle, while the strengths of the β-band DTF (SMC→EMG direction) were not different between the two muscles. Moreover, the β-band DTF (EMG→SMC direction) was greater in the "Stable" period than in the "Less Stable" period within the FDMB muscle. Greater DTF (EMG→SMC direction) was positively associated with the stability of muscle contraction.

Conclusions: Our findings suggest that ascending β-band oscillatory activity may promote a steady-state isometric contraction by efficiently transmitting sensory feedback from finger muscles to the sensorimotor cortex.

Significance: The results show the differential contribution of the ascending part of the corticomuscular network depending on the functional organization.