Intrinsic foot muscle activity and spatial characteristics assessed by high-density surface electromyography in...

Intrinsic foot muscles work together to control foot biomechanics and react to body sway complexity. This study used high-density surface electromyography (HDsEMG) with a wide detection area to capture the properties of muscle activity and explored the control strategies of the muscles during static standing under different base of support postures and loading conditions. Twenty-six participants performed six static standing tasks: wide and narrow stances, each with 50% and 90% loading, rear leg during tandem stance (TR), and one-leg stance (OLS). In all conditions, the load was applied to the right leg, and only the right foot was measured. Using HDsEMG placed transversely across the plantar surface, the muscle activity of the abductor hallucis (ABH), flexor digitorum brevis (FDB), and abductor digiti minimi (ADM) was assessed from the corresponding columns. The centroid of the muscle activity and its trajectory length were calculated from the spatial distribution of HDsEMG during each task. Friedman's test was conducted to compare the average of muscle activity and trajectory length of the centroid across different postural conditions. Muscle activity and centroid trajectory length increased with a narrower base of support and a greater load. Under TR conditions, muscle activity was lower, whereas centroid trajectory length was longer than that under OLS. Differences in the patterns of muscle activity and HDsEMG centroid displacement across postures suggest that intrinsic foot muscles may adopt posture-specific neural control strategies.