The effects of medial gastrocnemius muscle fatigue on regional modulation of the ankle plantarflexors during standing external perturbations

The ankle plantarflexor muscles [medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL)] were shown to exhibit direction-specific regional modulation of muscle activity during external perturbations. This study investigated the effect of MG muscle fatigue on plantarflexor muscle activation patterns. It was hypothesized that fatigue of MG would be compensated for by changing the activity of the SOL and LG to maintain balance. Using external perturbations of 1% body mass, the unipedal standing balance of 20 volunteers was tested before and after low-frequency fatigue was induced in MG via electrical stimulation. High-density surface electromyography (HDsEMG) was used to determine the amplitude and barycenter location of the muscle activation in three perturbation directions: 0° (front), 60° left of front (60 L), 60° right of front (60 R). Kinetic analysis was used to evaluate the center of pressure (COP) displacements. Two-way repeated measures ANOVA showed that prefatigue, the HDsEMG amplitude, and barycenter of MG and LG were regionally modulated with perturbation direction (<i>P</i> < 0.01 for both parameters and muscles), whereas in SOL, only the barycenter location modulated across the perturbation direction (<i>P</i> < 0.01). Following MG fatigue, there was no observed difference in MG barycenter location across perturbation directions, and MG amplitude increased only in the front direction. Both LG and SOL maintained the regional muscle activation patterns across perturbation directions (<i>P</i> < 0.01 for both), and the HDsEMG amplitude increased only in LG (<i>P</i> < 0.001). Therefore, MG fatigue resulted in loss of regional activation in MG but did not affect regional activation in the unfatigued plantarflexors.<b>NEW & NOTEWORTHY</b> Regional modulation of electromyography (EMG) with perturbation direction within each plantarflexor (evidenced by the EMG barycenter shift) was lost in fatigued medial gastrocnemius (MG) but was maintained in unfatigued synergists. Regional modulation of EMG following directional perturbations was associated with center of pressure (COP) displacement, reinforcing that the shift in EMG barycenter was serving to oppose the perturbation. Responses to directional perturbations helped counteract low-frequency fatigue of MG by decreasing postural sway and increasing EMG amplitude in both MG and lateral gastrocnemius (LG).