Foot sole and ankle muscle inputs contribute jointly to human erect posture regulation

Abstract

In order to assess the relative contribution and the interactions of the plantar cutaneous and muscle proprioceptive feedback in controlling human erect posture, single or combined vibratory stimuli were applied to the forefoot areas and to the tendons of the tibialis anterior muscles of nine standing subjects using various vibration frequency patterns (ranging from 20 to 80 Hz). The variations in the centre of foot pressure, ankle angle and the EMG activities of the soleus and tibialis anterior muscles of each subject were recorded and analysed. Separate stimulation of the plantar forefoot zones or the tibialis anterior muscles always resulted in whole-body tilts oppositely directed backwards and forwards, respectively, the amplitude of which was proportional to the vibration frequency. EMG activity of ankle muscles also varied according to the direction of the postural responses. However, the same vibration frequency did not elicit equivalent postural responses: in the low frequency range, tactile stimulation induced stronger postural effects than proprioceptive stimulation, and the converse was the case for the higher frequency range. Under sensory conflict conditions, i.e. foot sole-flexor ankle muscle co-stimulation, the direction of the body tilts also varied according to the difference and the absolute levels of the vibration frequencies. In all cases, the resulting postural shifts always corresponded to the theoretical sum of the isolated effects observed upon vibrating each of these two sensory channels. We proposed that tactile and proprioceptive information from the foot soles and flexor ankle muscles might be co-processed following a vector addition mode to subserve the maintenance of erect stance in a complementary way.

Figures

Figure 1. Typical forward and backward postural responses induced by stimulating separately the tibialis anterior muscles (top) or the forefoot zones of both soles (bottom)

Traces show the mean displacements (thick lines) and the standard deviations (thin lines) of the CoP in the antero-posterior (Y) and lateral (X) planes after 2.5 s of vibration at 80 Hz.

Figure 2. Mechanical and EMG changes recorded for one subject in response to tactile (A) or proprioceptive (b) stimulation at 80 Hz

La, latency of the whole-body tilt; E, early EMG responses preceding any body displacement; L, late EMG responses developed after the onset of the whole-body tilt. Note that data recordings correspond to the first 500 ms previbration and 1 s vibration to detail the initial events. TA, tibialis anterior; Sol, soleus.

Figure 3. Mean amplitudes of the forward and backward Y CoP displacements induced by isolated proprioceptive (top) or tactile (bottom) stimulation according to the vibration frequency applied

Figure 4. Mean postural responses induced by combined proprioceptive and tactile stimulation at the same frequency (20, 40, 60 or 80 Hz)

□, mean experimental amplitudes of the Y CoP displacements obtained under all the Δ0 co-stimulation conditions. ▪, expected mean values resulting from the theoretical sums of the isolated effects induced by vibrating separately the two sensory channels. Note that no significant differences were found between the experimental and theoretical values.

Figure 5. Mean postural responses induced by combined proprioceptive and tactile stimulation according to the difference in vibration frequency used

Each bar corresponds to the mean amplitude of the Y CoP displacements recorded for all the subjects under the various patterns with frequency differences of the same magnitude (Δ20, Δ40 or Δ60 Hz) in favour of the tactile (T) or the proprioceptive (P) stimulation.

Figure 6. Comparisons between the mean experimental (○) and theoretical (•) amplitudes of the postural effects in response to different combined stimulation conditions

For each set of posturographic data, the co-vibration pattern is indicated by two frequency values, which correspond to the tactile and proprioceptive stimulations. Note that whatever the co-stimulation condition, no difference was found between the mean experimental and theoretical values.

Figure 7. Oriented postural responses induced by transcutaneous electrical stimulation of the forefoot and/or heel zones of the soles

The plantar regions were stimulated by pairs of surface electrodes (•): delivery of rectangular pulses (0.5 ms duration, 100 Hz) at non-painful intensity (1.2 × perception threshold). Individual final positions of the CoP after 2.5 s of stimulation for 5 subjects are shown (○); their means are represented (□). Vectors show that body tilts are contralaterally oriented with respect to the stimulation sites.