Self-paced treadmill speed differentially affects gait variability and stability in walking and running.

Self-paced treadmills allow individuals to continuously regulate belt speed and may more closely replicate overground locomotion than fixed-speed conditions. However, their effects on gait variability and stability, particularly during running, remain incompletely understood. This study compared self-paced (SFP) and constant-speed (CON) treadmill locomotion during walking and running at matched average speeds. Twenty-eight healthy adults completed SFP and CON walking and running trials. Spatiotemporal characteristics were assessed using mean values and coefficients of variation (CV) for stride length, step frequency, step width, and stance ratio (duty factor in running). Global stability was evaluated using detrended fluctuation analysis (DFA), and local dynamic stability using the maximum Lyapunov exponent. During walking, SFP slightly increased stride length (p = 0.004) and reduced stance ratio (p = 0.025), while step width and step frequency were unchanged. CV increased for stride length (p < 0.001), step frequency (p < 0.001), and stance ratio (p < 0.001), whereas step width variability remained unchanged. DFA scaling exponent was increased during SFP walking (p < 0.001). Local dynamic stability decreased in the thigh (p < 0.001) and foot (p < 0.001). In running, no significant differences were observed between SFP and CON for mean spatiotemporal variables, their variability, or global stability. Local dynamic stability improved in the trunk (p = 0.036), shank (p < 0.001), and foot (p = 0.002). These findings demonstrate gait-mode-specific responses to self-paced treadmill speed regulation. Walking appears more sensitive to externally imposed speed constraints, whereas running preserves its spatiotemporal characteristics and dynamical organization.