Unilateral Lower Limb Suspension Impairs Balance and Firing Rate of Soleus Small Motor Unit Action Potential Trains...

Postural control, which is largely determined by coordinated activation of dorsi- and plantar-flexors, is impaired following spaceflight. Unilateral lower limb suspension (ULLS) mimics spaceflight-induced reductions in motor unit recruitment, but its effects on balance are unknown.

To assess postural control and motor unit action potential train (MUAPT) firing rate of the tibialis anterior (TA) and soleus (SOL) during single leg stance (SLS) before and after 13d ULLS.

Thirteen healthy subjects (6 men, 7 women; age: 18-44y) completed 15s SLS pre- and post-ULLS. Center of pressure (CoP) ellipse area, total excursion (TE), and average velocity (AV) and surface electromyography (sEMG) signals from four-pin grid array sensors (Delsys Galileo) applied to TA and SOL were measured. The relationship between MUAPTs amplitude (mV) and mean firing rate (pps) for each subject were modeled with the formula y=A·e Bx where A represents the y-intercept and B represents the exponential decay coefficient. Data were compared via linear mixed models.

CoP ellipse area (pre: 761.4±431.1; post: 2443.9±426.5 mm², p=0.010), TE (pre: 620.5±114.5; post: 1028.3 ± 113.4 mm, p=0.006), and AV (pre: 41.4 ± 7.6; post: 68.5 ± 7.5 mm·s⁻¹, p=0.006) were greater in the unloaded, but not loaded, limb post- compared to pre-ULLS. A significant limb × time interaction was observed for the SOL MUAPT amplitude-firing rate relationship where the y-intercept was less post-ULLS in the unloaded, but not loaded, limb compared to pre-ULLS (p=0.005). Firing rate of small (<0.025 mV), but not large, MUAPT of the SOL, but not TA, was significantly less post-ULLS compared to pre-ULLS in the unloaded limb.

ULLS impairs balance and firing rate of low-amplitude MUAPT in SOL, but not TA.