Background: Describing the response of spatiotemporal gait characteristics, and related variables such as variability and stiffness, to different stressors is important to better understand spring-mass model. Research question: This study aimed to examine the effect of fatigue induced by a running protocol on spatiotemporal gait parameters, step variability and vertical (Kvert) and leg stiffness (Kleg) during running on a treadmill. Methods: Twenty-two trained male endurance runners performed a 60-min time trial run. An analysis of spatiotemporal parameters (contact time [CT], flight time [FT], step frequency [SF] and step length [SL]), step variability (in terms of coefficient of variation [CV]) and stiffness was conducted in two different conditions: non-fatigued (before the protocol) and fatigued (after the protocol). Results: The pairwise comparisons (i.e., non-fatigued vs. fatigued condition) indicated that temporal parameters (i.e., CT and FT) experienced significant changes (p=0.001 and <0.001, respectively). Step variability increased in presence of fatigue, with higher CV in CT (p=0.039), FT (p=0.005), SF (p=0.046) and SL (p=0.027) after the running protocol. The Kleg experienced a reduction in the fatigued condition (p<0.001) whereas the Kvert remained unchanged (p=0.602). Significance: The results indicate that fatigue induced by a 60-min time trial run causes some adaptations in spatiotemporal gait characteristics and stiffness in trained endurance runners. Specifically, in the presence of fatigue, the athletes showed greater CT and shorter FT, higher step variability and lower leg stiffness. Highlights: • This study highlights the effect of fatigue on step characteristics and stiffness. • Fatigue causes an increase of ~4 % in the contact time during running. • In a fatigued condition, runners show a ~15 % longer flight time. • In the presence of fatigue, step variability increases during running. • Fatigue induced by 60-min time trial run alters stiffness by reducing leg stiffness.
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