Motor variability in repetitive upper limb tasks under natural Hypobaric Hypoxia

Natural hypobaric hypoxia is a defining feature of high-altitude environments and may alter human motor strategies during repetitive work tasks. This study investigated whether acute exposure and 48 h of continuous natural hypobaric hypoxia modify movement variability during a standardized, submaximal upper-limb reaching task in healthy lowlanders. Ten adults performed a 180 s paced reaching-retrieving task (1 Hz) under three conditions: normobaric normoxia at sea level, acute natural hypobaric hypoxia within 3 h of arrival at 3,600 m, and after 48 h of continuous exposure at 3,600 m. Wrist triaxial acceleration was recorded and processed offline to quantify linear variability metrics (mean acceleration, acceleration standard deviation, root mean square) and nonlinear metrics (sample entropy, maximum Lyapunov exponent) across movement axes, directions, and phases. Multivariate analyses showed significant effects of condition (p < 0.001), axis (p < 0.001), and phase (p = 0.007), as well as significant condition-by-axis (p = 0.016) and condition-by-phase (p = 0.003) interactions. Compared with sea level, hypobaric hypoxia increased acceleration magnitude in task-relevant axes and reduced variability in non-primary axes and early movement phases, whereas nonlinear metrics remained unchanged. These findings suggest that natural hypobaric hypoxia selectively redistributes linear movement variability during repetitive upper-limb performance.