Thirteen days of unloading and three days sleeprestriction modify soleus motor unit firing frequency and balance during single leg stance

Both unloading and short sleep independently impair muscle recruitment during isometric contractions, but their combined effects on balance regulation remain unclear. This study assessed postural control and motor unit activity of the soleus (SOL) during single leg stance (SLS) before and after short-term (13days) unilateral lower limb suspension (ULLS) with and without sleep restriction (SR). We hypothesized that ULLS and SR Additively impair balance and motor unit recruitment. Twelve healthy subjects (4M, age: 38.4±7.6y; 7F, age: 24.3±4.9y, 1TGM, age: 26y) underwent SLS balance assessment pre and post 13d ULLS by wearing a modified shoe with a 5cm rocker-style stack on the right foot and ambulating with crutches to unload the left leg. During a 10d lead-in period, subjects slept between 21:00 and 06:00 each evening. For the final three days, they were randomly assigned to either sleep adequate condition (SA, 21:00-06:00) or SR (01:00-06:00). Balance was assessed during a 15-second SLS with hands on hips and eyes open with unshod. Portable force decks (Vald performance) were used to measure center of pressure (CoP, square millimeters mm 2 ) and mean velocity (MV, millimeters per second mm·s -1 ). Eleven participants completed four-pin surface electromyography (sEMG) sensors (Delsys Galileo) were applied to the SOL and signals were decomposed using propriety software (Delsys Neuromap) into motor unit action potential trains (MUAPTs). Only MUAPTs with >80% accuracy were analyzed. Regression analyses were used to determine the relationship between amplitude in millivolts (mV) and firing rate in pulses per second (pps) using the formula y=A·e Bx , with A- and B- terms compared via linear mixed models. We observed a significant sleep x limb x time interaction for CoP displacement and MV (both p =0.040) where ULLS and SR tended to increase measures, and this effect was greater in the ULLS+SR condition. However, no significant sleep x limb x time effect was noted for SOL A- ( p =0.443) or B-term ( p =0.386). Thirteen days of unloading significantly impaired postural control (increased CoP displacement and mean velocity) which was worsened by SR. A- and B-terms demonstrated no significant differences in the amplitude-firing rate relationship of the SOL. Thus, changes in soleus motor unit recruitment cannot explain the altered balance regulation following ULLS and SR. This work is supported by a NASA Kentucky Space Grant Consortium Research Initiation Award (RIA-22-043) (LMB). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.