Abstract

Hypobaric hypoxia (HH) affects the nervous system’s ability to stabilize motor tasks, primarily through changes in neuromuscular activation. Previous studies have reported inconsistent findings regarding electromyographic responses under HH conditions, possibly due to a focus on individual muscle behaviors rather than intermuscular coordination. This study aimed to determine the effects of HH on surface electromyography-based cocontraction indices (CCIs) during a repetitive upper extremity task and to evaluate the impact of acute (<3 h) and prolonged (48 h) exposure. A cross-sectional study was conducted at 3,600 m above sea level, involving 12 healthy adults (5 males, 7 females). Surface electromyographic activity of the biceps and triceps brachii muscles was recorded during a repetitive reaching-retrieving task. Two CCIs were calculated. A significant effect of HH exposure time was observed, with higher CCIs after 48 h compared with acute exposure (F1,44 = 4.172; P = 0.047, g2p = 0.515). No significant interactions between task phases or movement durations were found. Exposure to HH for 48 h significantly increases CCIs, suggesting compensatory neuromotor responses to HH. These findings highlight the utility of CCIs as markers of neuromuscular alterations during HH and provide insights into the strategies used by the nervous system under extreme conditions. Future studies should explore these responses over longer periods and across diverse motor tasks. © © 2025 The Authors.