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

• Bidirectional information flow was estimated between sensorimotor cortex and finger muscle by directed transfer function (DTF) during steady-state contraction.
• Ascending β-band DTF was greater in the thumb muscle than in the little finger muscle.
• Greater ascending β-band DTF was associated with higher EMG stability.

Objectiveβ-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.MethodsIn 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.Resultsβ-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.ConclusionsOur 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.SignificanceThe results show the differential contribution of the ascending part of the corticomuscular network depending on the functional organization.