Recruitment of the plantar intrinsic foot muscles with increasing postural demand.

BACKGROUND The aim of this study was to determine the difference in activation patterns of the plantar intrinsic foot muscles during two quiet standing tasks with increasing postural difficulty. We hypothesised that activation of these muscles would increase with increasing postural demand and be correlated with postural sway. METHODS Intra-muscular electromyographic (EMG) activity was recorded from abductor hallucis, flexor digitorum brevis and quadratus plantae in 10 healthy participants while performing two balance tasks of graded difficulty (double leg stance and single leg stance). These two standing postures were used to appraise any relationship between postural sway and intrinsic foot muscle activity. FINDINGS Single leg stance compared to double leg stance resulted in greater mean centre of pressure speed (0.24 m s(-1) versus 0.06 m s(-1), respectively, P ≤ 0.05) and greater mean EMG amplitude for abductor hallucis (P ≥ 0.001, ES=0.83), flexor digitorum brevis (P ≤ 0.001, ES=0.79) and quadratus plantae (P ≤ 0.05, ES=0.4). EMG amplitude waveforms for all muscles were moderate to strongly correlated to centre of pressure (CoP) medio-lateral waveforms (all r ≥ 0.4), with muscle activity amplitude increasing with medial deviations of the CoP. Intra-muscular EMG waveforms were all strongly correlated with each other (all r ≥ 0.85). INTERPRETATIONS Activation of the plantar intrinsic foot muscles increases with increasing postural demand. These muscles are clearly important in postural control and are recruited in a highly co-ordinated manner to stabilise the foot and maintain balance in the medio-lateral direction, particularly during single leg stance.