Voluntary ankle contraction reproduces centre of pressure features commonly attributed to automatic postural control

Postural control is often assessed through centre of pressure (COP) analysis to infer underlying control mechanisms. For example, during dual-tasking, measures such as sample entropy have been interpreted as reflecting changes in automaticity, whereas shifts in frequency content have been associated with stiffening, often attributed to ankle co-contraction. However, the specificity of these metrics remains unclear.

The objective of the present study was to compare the effects of dual-tasking and voluntary ankle contraction on COP and electromyographic (EMG) activity to determine whether commonly used metrics reflect specific control mechanisms.

Fifteen healthy young adults completed 60-second standing trials under four conditions: baseline, voluntary contraction of the muscles around the ankle, and two dual-task conditions of different difficulty. EMG was recorded from the medial gastrocnemius, tibialis anterior, rectus femoris, and biceps femoris. COP measures (sway area, mean velocity, sample entropy, and wavelet-derived frequency content), EMG activity, co-contraction index, and intermuscular coherence were compared across conditions.

Dual-tasking and voluntary ankle contraction both altered sample entropy and frequency content, although their effects differed across axes and frequency bands. Dual-tasking reduced sway area, whereas voluntary ankle contraction did not. Dual-tasking produced little change in EMG activity, whereas VAC elicited a more pronounced neuromuscular response, particularly in TA activity and the co-contraction index.

COP metrics should be carefully interpreted in isolation, as control strategies associated with dual-tasking and voluntary ankle contraction may produce overlapping outcomes. Studies lacking neuromuscular measures or experimental context should therefore be interpreted cautiously.