Leg muscle activity during tandem stance and the control of body balance in the frontal plane.

OBJECTIVES: We investigated the pattern of activity of the tibialis anterior (TA), soleus (SOL) and peroneus longus (PER) muscles of both legs during tandem stance, in order to highlight their respective role in maintaining balance. METHODS: Twelve young healthy subjects were asked to stand with feet in line for successive 15s-epochs, on a dynamometric platform with (EO) and without (EC) vision. EMG was recorded from the six muscles simultaneously. Collected signals were displacement of the centre of feet pressure (CoP) and EMG. Variables calculated for each recorded epoch were mean level, variability and distribution between legs of EMG, and cross-correlation between EMG and CoP traces and between EMG of homonymous muscles. RESULTS: CoP motion was larger along the medio-lateral (M-L) than antero-posterior (A-P) axis, and larger with EC than EO particularly in the M-L axis. Muscle activity was larger in the rear than in the front leg, as expected, except for PER. Activity increased with the increase in M-L CoP oscillations, except for SOL, which was tonically active, both legs, regardless of the amplitude of the oscillations. Manipulating vision had no effect on the variability of the EMG for equal mean levels of activity, for any muscle. Cross-correlation between EMG of rear leg muscles and M-L CoP sway gave higher coefficients for TA and PER than SOL, and appropriate time-delays between TA or PER and CoP motion, indicating a role of these muscles in the control of M-L sway. Except for the tonically active SOL, the homonymous muscles of the two legs were active out-of-phase, indicating a mutual push-pull action of the pairs. This was confirmed by the reciprocal activation of TA and PER of the same leg. CONCLUSIONS: Overall, in spite of a large inter-trial and inter-subject variability, the neural command to the leg muscles during tandem stance implies a task-sharing rule, whereby SOL keeps the body upright while the reciprocal PER and TA activities produce the alternate impulses necessary for body stabilization in the frontal plane. SIGNIFICANCE: Knowledge of the normal mode of control of balance in frontal plane can foster new investigation in both posture and gait control, in addition to offering tools for understanding balance problems of elderly persons and patients at risk of fall.