Role of motor unit activity in flexor digitorum brevis to maintain balance during forward leaning

Our purpose was to compare the influence of motor unit activity in Flexor Digitorum Brevis (FDB) and Soleus (SOL) on force fluctuations during three forward-leaning tasks. Ground reaction forces and high-density EMG signals were collected from 19 males when leaning forward at 25%, 50%, and 75% of maximal forward leaning force. EMG amplitude increased with percent of leaning and was greater for SOL than FDB, but there were no differences in force fluctuations across tasks. Differences in motor unit activity indicated that the relative contribution of the two muscles to the control of balance varied across tasks as confirmed by the association between the fluctuations in neural drive [standard deviation of the filtered cumulative spike train (SD of fCST)] and force [coefficient of variation (CoV) for force]. Specifically, the correlation values were greater for FDB at the lower target forces. Correlation analyses revealed that synaptic noise (CoV for interspike interval) was weakly correlated with the CoV for force, whereas the variability in shared synaptic input (SD of fCST) was strongly correlated with the CoV for force. This finding suggests that the relative influence of the two muscles on the fluctuations in force during forward leaning varied with task requirements.