Fig 1.
Stimulation electrodes were placed on the posterior tibial nerve to elicit an M-wave and or H-reflex. The response was recorded by surface electrodes placed on the soleus muscle. The resultant muscle contraction would induce a plantar flexion moment in a closed kinematic chain, causing a perturbation in standing posture. Unilateral or bilateral stimulation would provide sideway or forward perturbations. Red dots represent the places marked for measuring BOS and feet locations.
Fig 2.
(A) Sample M-max with almost no observable H-reflex. The inset is an amplification of the same signal to show EMG activity, (B) Sample of COP trajectory deflection.
Fig 3.
M-max amplitude was monitored for consistency of stimulation.
Normalized values of M-max (each value divided by its mean), is shown here for each side of stimulation. The box plot shows the mean and standard deviation for all subjects in each condition. No statistical differences were observed among these three conditions.
Fig 4.
(A) Density of Equilibrium points in the AP and ML directions. Leftward perturbation (stimulation of the left side) caused a higher density of EPs in the perturbation phase. This was statistically significantly higher than the bilateral stimulation condition. Statistically significant differences were found between left and right side stimulation for both EPx and EPy and between left and bilateral stimulation. (B) Duration of perturbation. The error bars are Standard Error of the Means.
Fig 5.
Vector analysis of perturbation.
(A) Sample of a typical subject. From right to left: Thirty seconds of unperturbed COP, COP perturbation without vectors (For clarity), which took about 300 seconds, COP perturbation with vectors overlayed (same as the middle graph). (B) Vector analysis results for all subjects is shown in this graph. The shaded area represents the SD of each perturbation direction with vectors representing the direction and amplitude of perturbation. (C) Same as in figure B but instead of vectors, actual points are plotted. The five trials for each stimulation direction (left, right, both) were averaged for each subject to represent the response profile of that subject. These average points are plotted for all 15 subjects. There are, therefore, 45 points for each direction. A line connecting the center (0,0) to each point would constitute its vector.