Foot phase-response during voluntary oscillations

When studying coupled oscillations of hand and foot, we approximated the extremities to pendula forced by sinusoidal inputs and described their input-output phase relations by measuring the phase difference between the onsets of the EMG bursts and the onset of the related movements. However, while hand response was fitted by the simple pendulum model, foot response did not, since below 1.5Hz movement onset led the force onset. This obvious paradox indicated that the initial part of the foot movement was sustained by forces (elastic recoil of antagonists, joint structures, etc) other than contraction of its prime movers. In this perspective, it might be argued that elastic recoil promotes foot motion up to recovery of the equilibrium position, and that contraction starts when equilibrium is crossed. Should this be true, EMG-movement phase relation had to be measured with respect to crossing of equilibrium, not to movement onset. To verify this, the equilibrium position of the foot, resting on a rotating platforms, was measured in 5 subjects (ankle angle = 91±6° SD); then the foot was voluntarily oscillated at various frequencies (0.2 ÷ 3Hz) around its equilibrium. The phase relations were measured between the onset of the tibialis anterior EMG and i) the peak plantar flexion; ii) the equilibrium crossing during dorsiflexion. The frequency responses determined by the two techniques showed a parallel sigmoidal decay. The EMG-equilibrium curve started near zero (-10±10° at 0.2 Hz), reached -95±22° at 3 Hz and could be well fitted by a pendulum model (R2 = 0.93±0.05). The EMG-peak plantar flexion curve started at 59±16° (0.2Hz) and reached –9±13° at 3 Hz. Parallelism of the two curves suggests that referring the EMG onset to the peak plantar flexion introduced a systematic phase shift which may be removed by translating to zero the curve starting point. Approximation of the foot movement by a simple pendulum model seems therefore legitimate.