Unilateral lower limb suspension affects motor unit action potential train characteristics in a size-dependent manner during high force isometric knee extension.

Unilateral lower limb suspension (ULLS) mimics spaceflight-induced decreases in voluntary muscle strength and muscle recruitment. The purpose of this study is to examine the effects of short-term ULLS on quadriceps motor unit action potential train (MUAPT) characteristics during high-intensity isometric contractions. Thirteen healthy adults (7 women and 6 men; age: 18-44 yr) completed bilateral knee extension assessments, including maximal voluntary isometric contraction (MVIC), interpolated twitch, and trapezoidal ramp contractions, before and after 13-day ULLS. Surface electromyography (sEMG) signals from four-pin grid array sensors were decomposed into MUAPTs. Exponential models between MUAPT amplitude and mean firing rate (MFR) and interpulse interval (IPI) standard deviation and linear models between MUAPT amplitude and activation threshold were created for each subjects' limbs pre- and post-ULLS. Limb × time interactions were analyzed using linear mixed models. MVIC (166.6 ± 62.7 vs. 188.9 ± 54.8 Nm, P < 0.001) and voluntary activation (72.5 ± 24.1% vs. 82.0 ± 15.0%, P = 0.018) were significantly reduced in the unloaded limb post-ULLS. Average MUAPT amplitude was not significantly different pre- vs. post-ULLS in either limb (P = 0.322). The exponential function for the MUAPT amplitude MFR relationship and IPI SD relationship was significantly greater post-ULLS in the unloaded limb. Similarly, the slope of the activation threshold was significantly greater in the unloaded limb post-ULLS. Short-term ULLS reduces knee extensor strength and impairs MUAPT characteristics in a size-dependent manner.NEW & NOTEWORTHY Motor unit action potential trains respond differently to disuse based on size of the motor unit during 90% maximal isometric contraction. Small motor units display reduced mean firing after disuse. Large motor units display reduced activation threshold and greater interpulse interval variability.