Changes in FDB and soleus muscle activity after a train of stimuli during upright stance

background: evidence of self-sustained muscle activation following a brief electrical stimulation has been reported in the literature for certain muscles. objectives: this report shows that the foot muscle (flexor digitorum brevis - fdb) shows a self-sustained increase in muscle activity during upright stance in some subjects following a train of stimuli to the tibial nerve. methods: healthy subjects were requested to stand upright and surface emg electrodes were placed on the fdb, soleus and tibialis anterior muscles. after background muscle activity (bga) acquisition, a 50 hz train of stimuli was applied to the tibial nerve at the popliteal fossa. the root mean square values (rms) of the bga and the post-stimulus muscle activation were computed. results: there was a 13.8% average increase in the fdb muscle emg amplitude with respect to bga after the stimulation was turned off. the corresponding post-stimulus soleus emg activity decreased by an average of 9.2%. we hypothesize that the sustained contraction observed in the fdb following stimulus may be evidence of persistent inward currents (pic) generated in fdb spinal motoneurons. the post-stimulus decrease in soleus activity may have occurred due to the action of inhibitory interneurons caused by the pics, which were triggered by the stimulus train. conclusions: these sustained post-stimulation changes in postural muscle activity, found in different levels in different subjects, may be part of a set of possible responses that contribute to overall postural control.