Running speed and cognitive load differentially modulate gait variability and fractal dynamics.

Gait variability reflects both the magnitude and temporal structure of stride fluctuations. While running speed influences gait dynamics, how cognitive load modulates these effects across spatial and temporal domains remains poorly understood. Seventeen trained runners completed treadmill trials (8-16 km·h-1) under single-task (RUN) and dual-task (RUN + CT; concurrent cognitive task) conditions. Variability magnitude (CV) and fractal scaling (DFAα) were quantified for step length and step time. Linear mixed-effects models and individual-difference analyses assessed the effects of Speed, Condition, and their interaction. DFAα increased with speed for step length and decreased for step time (both p < 0.001), while cognitive load consistently reduced DFAα across domains. In contrast, CV of step length decreased with speed whereas cognitive load effects on variability magnitude differed from those observed for temporal structure, revealing a dissociation between variability magnitude and temporal organization. At the individual level, runners with stronger speed-dependent temporal organization during RUN exhibited larger dual-task-related reductions in DFAα (r = -0.49, p = 0.047). Strong coupling between spatial and temporal DFA measures was also observed for both speed-dependent slopes (r = 0.71, p = 0.0015) and dual-task changes (r = 0.86, p < 0.001). Together, these findings show that mechanical demands shape baseline gait dynamics, whereas cognitive load produces a robust reduction in temporal organization across running speeds. Individual reliance on speed-dependent temporal structure predicts vulnerability to attentional interference, highlighting temporal organization as a sensitive marker of cognitive-motor interaction.