Biomechanical responses to incremental backpack loading during stair ascent in male adolescents

Backpack carriage is common among adolescents, yet its biomechanical responses during stair ascent remain unclear. This study investigated the effects of incremental backpack loads (0, 10, 15, and 20% body weight) on stair-ascent biomechanics in 16 male adolescents. Three-dimensional motion analysis, kinetics, surface electromyography, and dynamic stability measures were assessed using SPM1D. Increasing loads significantly altered AP-MOS, COM-COP inclination angle, vertical GRF, lower-limb joint mechanics, and muscle activation, while traditional spatiotemporal parameters remained unchanged. Load-related adaptations occurred during single-support, double-support, and swing phases, involving altered activation of key lower-limb muscles. These findings suggest that adolescents maintain external stair-ascent performance through internal biomechanical adjustments under increased backpack loads.