Neuromuscular response to sustained low-level muscle activation: within- and between-synergist substitution in the triceps surae muscles

The intent of this study was to investigate physiological recruitment strategies employed by the triceps surae (TS) muscles during sustained low-level activation. Surface-recorded electromyographic (EMG) signal data were recorded from the medial gastrocnemius (MG), lateral gastrocnemius (LG), and medial soleus (MS) muscles while eight subjects sustained static plantarflexion contractions at 10% of their maximum voluntary contraction (MVC) for 1 h. Fine-wire EMG activity was simultaneously recorded from electrodes located at three sites within the MG muscle. Correlation coefficients were computed among root mean square (RMS) amplitude values recorded from MG, LG, and MS to investigate between-synergist substitution, as well as between the three wire channels to investigate local substitution within the MG motor unit pool. Over the 1-h test, EMG amplitude in LG and MS increased linearly, and there was a moderately strong (R ²=0.662, P<0.023) positive correlation in the detrended activity between LG and MS, suggesting that these muscles generally acted together. When the data were divided into 5-min blocks, regression analysis on the partial correlation data revealed that MS and LG were correlated over the duration of the contraction (P<0.001) suggesting co-activation synergism, whereas MG and LG and MS and MG demonstrated a tendency toward trade-off synergism. In five of eight individuals at least one wire channel pair within the MG was negatively correlated, suggesting that there was some form of substitution between motor units or motor unit pools. The other three individuals maintained correlated activity between all three pairs throughout the 1-h contraction.