How modular footwear affects gait stability on inclines: a phase-dependent gait analysis

Walking on inclined surfaces alters gait mechanics and increases metabolic demand. Although modular footwear with angled outsoles has been proposed to reduce locomotor effort during uphill and downhill walking, its effects on phase-dependent gait stability and multidirectional centre-of-mass (COM) variability remain poorly understood. A secondary analysis was performed using a publicly available dataset of 10 healthy male adults (mean age: 27.6 ± 5.9 years) walking at 1 m/s across 15 combinations of treadmill inclination (−6°, 0°, +6°) and sagittal-plane modular outsole angle (−12° to +12°). Phase-dependent gait variability was quantified using the Gait Tube Stability (GTS) framework, which evaluates multidirectional COM-velocity variability through three-dimensional variability ellipsoids across the gait cycle. Positive and negative outsole inclination conditions altered phase-dependent COM-velocity variability, particularly during mid-stance and toe-off phases. Vertical variability demonstrated the greatest sensitivity to outsole geometry, whereas anterior–posterior variability remained comparatively preserved across conditions. Ellipsoid volumes differed significantly from the reference condition and demonstrated moderate-to-strong correlations with total multidirectional variability (r ≥ 0.63, p < 0.05). These findings suggest that modular outsole geometry influences gait stability in a phase-dependent and direction-specific manner. The results further demonstrate the utility of the GTS framework for detecting subtle biomechanical adaptations associated with footwear interventions and may inform the future design of rehabilitation and performance-oriented footwear systems.