Task-Specific Rescue Loading Redistributes Lower Limb Joint Work During Simulated Sprinting Across Fatigue States in Firefighters

Firefighters routinely perform short-distance sprints while wearing or carrying occupational gear. Nevertheless, how fatigue interacts with task-specific loading to shape the distribution of mechanical work across lower-limb joints remains poorly understood. This study aimed to determine how fatigue state and task-specific rescue loading affect sprint performance, impact loading, lower-limb joint mechanics, joint work contribution, and muscle activation in male firefighters. Fifty-three male firefighters completed four simulated rescue sprinting tasks under five fatigue conditions: non-fatigue, whole-body fatigue, and mild, moderate, and severe local knee extensor fatigue. Tasks included unloaded sprinting and 7, 20, and 30 kg task-specific rescue-load conditions. Kinematic, kinetic, and electromyographic data were collected during the stance phase of the dominant limb. Mild local fatigue was associated with a sprinting speed comparable to the non-fatigue condition and higher than that under whole-body or moderate-to-severe local fatigue. Progressively heavier task-specific rescue loads decreased sprinting speed, prolonged stance time, increased knee work contribution, and decreased ankle work contribution, indicating a redistribution of stance-phase mechanical work from the ankle toward the knee. This compensatory pattern was further modulated by treadmill-induced global locomotor fatigue. In conclusion, this ankle-to-knee redistribution may help firefighters complete high-load rescue sprinting, but it may also increase knee mechanical demand, particularly under global locomotor fatigue.