Fine-wire electromyography of the transverse head of adductor hallucis during locomotion

Background: Previous literature on the transverse head of adductor hallucis (AddH-T) has largely focused on muscle morphology. This data provides insight into muscle architecture, yet fails to inform it's functional implication during walking. The role of the AddH-T, which runs parallel to the distal transverse metatarsal arch, has never been studied using fine-wire EMG during locomotion.

Research question: The purpose of this study is to explain a novel method of recording fine-wire EMG of the adductor hallucis muscle of the foot, and secondly, to report phasic AddH-T muscle activity during level walking on hard and soft surfaces.

Methods: Ultrasound-guided fine-wire EMG was recorded from the AddH-T of each foot, in ten asymptomatic young adults. Participants completed ten walking trials per experimental conditions (hard and soft surface). Ensemble averages were calculated from the time normalized linear-envelopes of each participant, and represented from 0 to 100 percent of the gait cycle.

Results: Using the described ultrasound-guided fine-wire protocol, successful EMG signals were generated in 19 of 20 feet. When walking over hard or soft flooring, the AddH-T muscle has two bursts in EMG, occurring between 0-20 % and 50-65 % of the gait cycle. The magnitude of peak activity was often reduced at initial contact when walking over foam. 45 % of participants experienced a third burst in EMG activity at midstance, corresponding to 30-40 % of the gait cycle.

Significance: This study has successfully explained a novel method of recording finewire electromyography (EMG) of the adductor hallucis (transverse head) muscle of the foot. Results suggest that the AddH-T stabilizes the forefoot at initial contact and toeoff, while further anchoring the hallux during propulsion. These results provide preliminary insight into the functional role of the AddH-T during human locomotion.