Biomechanical Responses to Level, Uphill, and Downhill Running Segments During an Outdoor Marathon.

This study investigated running biomechanics across varying slopes during a mass-participation road marathon. Although laboratory studies suggest specific gait modifications occur in response to varying slopes, data from outdoor competitive environments remain scarce. We monitored 39 distance runners using shoe-mounted inertial measurement units to capture spatiotemporal, footstrike, and wearable-derived loading data across level, uphill (+3%), and downhill (-3%) segments of the 2025 Hong Kong Marathon. Results demonstrated slope-dependent modifications, with uphill running characterized by slower velocity (-0.29 m/s, Cohen's d = -1.98), shorter stride length (-0.15 m, d = -1.55), and lower cadence (-5.00 steps/min, d = -1.21) compared with level ground (p < 0.001). Kinematically, participants adopted a more anterior footstrike pattern uphill (d = 1.48) and a more posterior footstrike pattern downhill (d = -0.62) relative to level running (p < 0.05). Analysis of wearable-derived loading metrics showed no differences across slope conditions once running velocity was included as a covariate. Running velocity emerged as the primary determinant of braking, impact, and shock accelerations, indicating that differences in self-selected running velocity may contribute substantially to loading responses across varying slopes during overground running. These findings highlight a discrepancy between controlled laboratory simulations and competitive overground running, emphasizing the role of wearable technology in capturing ecologically valid gait alterations to different terrains.