Effects of 48-H natural hypobaric hypoxia on intermuscular coherence during a repetitive upper-limb task.

Natural hypobaric hypoxia (HH) is a relevant environmental stressor in occupational and sport settings, yet its short-term effects on frequency-specific coupling between antagonist muscles during dynamic upper-limb tasks remain unclear. This study examined whether acute and 48-h natural HH modifies biceps-triceps intermuscular coherence (IMC) during repetitive reaching. Ten healthy lowlanders (4 males/6 females) completed the same 180-s metronome-paced reaching task under normobaric normoxia (NN), acute HH, and after 48 h of continuous exposure at 3,600 m. Surface EMG was recorded from the biceps brachii (BB) and triceps brachii (TB), and magnitude-squared coherence was quantified across delta, theta, alpha, beta, and gamma bands. Peripheral oxygen saturation (SpO₂) was monitored to confirm the hypoxic stimulus. Results SpO₂ decreased by approximately 9% during acute HH and 10% after 48 h compared with NN. IMC showed frequency- and exposure-duration-dependent increases under HH. Mean theta coherence increased during both acute and 48-h HH compared with NN, with the largest increase after 48 h. Theta peak coherence and consistency area index were also higher after 48 h than in NN. In higher frequency bands, mean beta coherence increased by approximately 131% and mean gamma coherence by approximately 111% after 48 h compared with NN. Forty-eight hours of natural HH was associated with increased BB-TB coherence during repetitive reaching, particularly in theta, beta, and gamma bands. These findings suggest that sustained altitude exposure may promote a more strongly coupled agonist-antagonist activation pattern during repetitive upper-limb movement. IMC may therefore help detect HH-related changes in neuromuscular coordination, with potential relevance for monitoring repetitive motor performance in occupational settings at altitude.