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EFFECTS OF UNILATERAL LOWER LIMB SUSPENSION AND SLEEP RESTRICTION ON BALANCE AND MOTOR UNIT RECRUITMENT

Abstract

BACKGROUND: Unloading and sleep restriction - two common stimuli experienced during space flight - independently impair balance and recruitment of anti-gravitational muscles such as the soleus. Unilateral Limb Suspension (ULLS) simulates the impact of spaceflight on musculoskeletal function, but effects on balance are unclear. The examination of balance in astronauts assumes paramount significance within the realm of space exploration, as it underpins crucial facets encompassing their safety, mission efficacy, and holistic well-being. OBJECTIVE: To investigate the interactive effects of sleep restriction and ULLS on balance regulation and lower extremity motor unit recruitment. METHODS: Healthy, active participants (18-50y M and F) will use forearm crutches and a modified shoe (Brooks Revel 5) with a 5cm rocker-style stack for continuous ambulation over a 13-day period while maintaining their regular daily activities. This protocol aims to unload the left leg under free-living conditions. After a 10d lead-in period, subjects will be randomly assigned to either adequate (21:00-0:600) or restricted (01:00-0:600) sleep for the final 3d of the study. Balance assessments including: 1) single-leg stance (15s) with eyes open and closed in both the shod and unshod conditions and 2) a simulated fall from a 30cm platform will be conducted before and after ULLS using force plates (VALD). Sway area and time to stabilize will be primary outcome measures. High density surface EMG (hdEMG) of the soleus and tibialis anterior will be decomposed into individual motor unit action potential trains (MUAPTs, Delsys NeuroMap) during balance tasks. MUAPT outcomes include amplitude, duration, firing frequency, and inter-pulse-interval. ANTICIPATED RESULTS: We hypothesize that ULLS and sleep restriction will independently and additively impair balance, with a more pronounced effect when both factors are combined. We expect an increase in center-of-pressure sway area in the left leg after ULLS, especially in the sleep-restricted group. Based on preliminary data, we anticipate increased firing frequency and reduced inter-pulse-interval in response to both sleep restriction and ULLS, while the amplitude and duration of MUAPTs remain unchanged. This predicts a noticeable rise in firing frequency after ULLS, amplified in the sleep-restricted group.

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