Decoupling of stretch reflex and background muscle activity during anticipatory postural adjustments in humans
We studied the evolution of stretch reXexes in relation to background electromyographic (EMG) activity in the soleus muscle preceding the onset of voluntary arm raise movements. Our objective was to investigate if changes in reXex EMG and muscle activity occur simultaneously and are similarly scaled in amplitude. Ten human subjects stood with each foot on pedals able to exert short dorsiXexor pulses during stance. Subjects were asked to product consistent voluntary arm raise movements to a target upon a visual cue. In ¼ of trials, no pulse perturbations were given, but in the remaining ¾’s of all trials pulses were given randomly during a 600-ms period, from 400 ms before until 200 ms after the onset of the movements. Perturbation trials were sorted into 20-ms bins post hoc, and the amplitude of the reXex EMG component was calculated and compared to the EMG activity obtained when no pulses were given. Results showed that despite exhibiting similar proWles over time, the background EMG consistently inhibited before the reXex EMG did. However, times of reactivation (rebound) were variable across subjects, with background EMG activating before reXex for some subjects and vice versa for others. The minimum values of inhibition, time of inhibition and time of rebound for background and reXex EMG measures did not show signiWcant linear correlations when all subjects’ data were considered. These results suggest that reXex and background EMG components of anticipatory postural adjustments evolve diVerently in time and amplitude. This has implications for the independent control of reXexes and voluntary muscle activity.