By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

Abstract

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

Keywords: Step duration; step length; triceps surae.

Figure 1.

Gait initiation. Green, red, and blue indicate slow, normal, and fast walking velocity, respectively. (A) shows the time‐course of gait initiation variables in a representative subject (each trace is the average of 12 trials). Mechanical variables (AP and Ver GRF and CoM velocity) and the EMG envelopes of SOL, GM, GL, and TA are shown from top to bottom. Vertical dotted lines indicate the instant of foot off (FO), dashed lines the instant of foot contact (FC). In (B, left) the horizontal bars indicate the grand mean ± SD of duration of single stance. In (B, right) the bars depicts the grand mean ± SD of step length. In (C, left) the plot depicts the grand mean ± SD of CoM AP momentum at FO (circles) and FC (triangles). In (C, right) the plot depicts the grand mean ± SD of CoM Ver momentum at FO (circles), minimum value (square) and FC (triangles). In (D, left) the horizontal bars are the grand mean ± SD of the end of activity of the EMG activity of SOL with respect to FO (t = 0). The dashed lines indicate the grand mean of the time of FC. In (D, right) the time instant of the end of soleus activity with respect to FO is plotted against the single‐stance duration (all trials collapsed). There is a strict correspondence between the duration of muscle activity and duration of single stance. The linear equation is: y = 0.49x + 0.14 (E) shows the average values of the level of EMG activity of SOL (left) and the average CoM vertical acceleration (right) measured in the four equal time‐windows, in which the single stance was divided. The mean level of EMG increases with the progression of the single stance and increases in parallel with the acceleration.

Figure 2.

Balance recovery. (A) shows the time‐course of balance‐recovery variables in a representative subject (each trace is the average of 12 trials). Same layout as in Figure 1. In (B, left) the horizontal bars indicate the grand mean ± SD of duration of single stance. In (B, right) the bars depicts the grand mean ± SD of step length. In (C, left) the plot depicts the grand mean ± SD of CoM AP momentum at FO (circles) and FC (triangles). In (C, right) the plot depicts the grand mean ± SD of CoM Ver momentum at FO (circles), and FC (triangles). In (D, left) the horizontal bars are the grand mean ± SD of the end of activity of the EMG activity of SOL with respect to FO (t = 0). The dashed lines indicate the grand mean of the time of FC. In (D, right) step length is plotted against the single‐stance duration (all trials collapsed). There is a strict correspondence between step length and the duration of single stance. The linear equation is: y = 2.87x + 0.14. (E) shows the average values of the level of EMG activity of SOL (left) and the average CoM vertical acceleration (right) measured in the four equal time‐windows, in which the single‐stance phase was divided. In BR, SOL EMG activity was very low during single stance for short steps, which caused CoM to accelerate quickly downwards. In normal steps, SOL EMG activity was high throughout single stance, which decelerated the CoM fall with respect to short steps where SOL EMG level was low. In long steps, SOL EMG was highest in late stance, which decelerates further CoM fall during this period.

Figure 3.

Disequilibrium torque. This figure shows the results obtained during GI (left) and BR (right). In (A) the mean (average of 12 trials) time‐courses of CoM‐CoP gap and of disequilibrium torque of a representative subject are shown. Same layout as Figure 1 In (B, upper panel) the bars show the grand mean ± SD of the CoM‐CoP gap at FC. In (B, middle panel) the bars show the grand mean ± SD of the disequilibrium torque at FC. (B, lower panel) shows the position of the CoM in the sagittal plane (CoM‐CoP gap) plotted against the CoM velocity (all trials collapsed). The linear equations obtained for GI and BR are y = 2.97 + 0.1 and y = 2.95x − 0.13. The high coefficient of determination and the equal slopes emphasizes the role of the CoM‐CoP gap in equally setting the velocity of the walking body regardless of the imposed task.