Reduced gravity weakens muscle activation coupling and destabilizes bimanual force control

Gravity provides a fundamental organizing constraint on coordinated bimanual motor output. However, the frequency-specific neural mechanisms that support force control under reduced gravity remain unclear. We examined bimanual isometric force coordination across graded gravity levels (0, 0.25, 0.50, 0.75, and 1 g) during parabolic flight. Twelve adults produced bilateral forces at three relative phase patterns: 0°, 90°, and 180°. Electromyography (EMG) was recorded from bilateral triceps, and intermuscular coherence was quantified using wavelet-based surrogate thresholding across four frequency bands (α: 5-13, β: 13-30, low-γ: 30-60, and high-γ: 60-100 Hz). Performance was indexed by mean force and coefficient of variation (CV) as a measure of variability. Reduced gravity lowered the mean force and increased force variability. These performance changes were accompanied by reduced intermuscular coherence in the 5-60 Hz frequency range. Hilbert-based continuous relative phase (CRP) analysis revealed that bimanual phase-locking strength was preserved across gravity levels. Still, the most challenging task (90°) showed greater absolute phase error at 0 g than at higher gravity levels, consistent with increased coordination difficulty under reduced gravity. Additionally, at 90°, greater reductions in β coherence with decreasing gravity were associated with better steadiness. Thus, individuals who down-regulated β the most tended to maintain the most stable force at 0 g. These findings identify frequency-specific neural signatures through which gravity shapes motor stability and highlight beta and low-gamma band synchrony as candidate targets for countermeasures to support skilled coordination in altered gravity.