Identifying Fatigue-Related Gait Patterns Using Multiple Inertial Measurement Units and Statistical Parametric Mapping: A Continuous Analysis of an Outdoor Full Marathon in Male Recreational Runners.

Background: Running is an effective exercise for personal fitness, yet many recreational runners suffer from running-related injuries. Prolonged running induces neuromuscular fatigue, interfering with an individual's preferred running gait and increasing the injury risk. This study aimed to examine gait patterns associated with fatigue in runners during a full marathon by analyzing lower limb segment and pelvis kinematics captured via multiple inertial measurement units (IMU).

Methods: Three IMUs were attached to measure the rearfoot, shank, and pelvis kinematics of 23 male recreational runners during an outdoor marathon. Data were extracted for nine time points: the baseline, and at the 5th, 10th, 15th, up to the 40th kilometer. Each segment's free acceleration and angular velocity during the stance phase at these nine timelines were analyzed using statistical non-parametric mapping.

Results: Male recreational runners exhibited a lower running speed (1.13 km/h, p < 0.001), lengthened stance time (0.009 s, p ≤ 0.001), and prolonged stride time (0.014 s, p < 0.05) after 35 km of running, alongside a smaller anterior and superior acceleration of rearfoot and shank during the propulsion phase (p < 0.05). With increasing running mileage, the rearfoot demonstrated a gradual increase in lateral acceleration and external rotation velocity during the propulsion phase (p < 0.01). The shank exhibited a progressive decline in anterior tilt velocity during the loading response phase (p < 0.05). Additionally, the pelvis displayed significantly greater anterior acceleration during propulsion at the 40 km mark (p < 0.01).

Conclusions: Male recreational runners exhibit a marked decline in performance only after 35 km. The progressive increase in rearfoot lateral acceleration and external rotation velocity during the propulsion phase, may be associated with a compensatory distal strategy to maintain balance stability. The gradual reduction in anterior tilt velocity of the shank during the loading response likely reflects a stiffness-enhancing mechanism in the lower limb to preserve locomotor efficiency under fatigue. The increased anterior acceleration of the pelvis at the 40 km mark suggests a proximal shift in propulsion mechanics due to fatigue. These findings underscore the necessity of long-distance protocols, continuous kinematic monitoring, and full stance-phase analysis to study running fatigue.