Continuous Relative Phase Angle and Variability: A Crossover Analysis of Duration and Surface Effects While Long-Distance Running Over Treadmill and Over-Ground Running.

Running coordination, quantified using continuous relative phase (CRP) and its variability, plays a key role in adapting to dynamic environments; however, how these measures behave during long-distance running on different surfaces remains unclear. This study compared lower-limb coordination and variability during prolonged running across treadmill and over-ground, focusing on how surface and duration affect movement patterns in sagittal-plane. Eleven healthy adults (nine males) completed 31-min runs at their preferred speed on both surfaces, on separate days, while data were collected using seven Opal Movement Monitoring inertial measurement units. CRP and its variability were examined across two-time intervals (initial and final 5 min) and two running surfaces, both over the full gait cycle and within the stance and swing phases. Overall, running duration and surface did not significantly affect coordination across the full gait cycle. However, ankle-knee coordination increased in the final 5 min during stance. Surface-by-duration interactions were observed in knee-hip and ankle-knee couplings during over-ground running. During the swing phase, ankle-hip coordination increased in the final 5 min on both surfaces, with additional interactions appearing in ankle-hip coupling during treadmill running. Coordination variability showed no significant differences across the gait cycle or within stance and swing phases. These findings suggest that lower-limb coordination patterns, rather than variability, are more sensitive to changes in running duration and surface. The results underscore the importance of considering external running conditions when evaluating coordination and optimizing gait performance in biomechanical assessments.