Movement Dynamics of Treadmill Running Within the Different Exercise Intensity Domains in Male Runners

The aetiology of fatigue differs between exercise intensity domains, but limited research has explored non-linear gait dynamics across them. To address this, this study aimed to investigate movement variability and stability in moderate, heavy, and severe intensity domains, and to examine how these dynamics evolve over time. Twenty-one male runners performed treadmill trials at speeds corresponding to these domains, with lower limb joint angles and centre of mass displacement recorded using 3D motion analysis and analysed at the start, middle, and end of each trial. Non-linear dynamics of joint angles and centre of mass displacement including local dynamic stability, dimensionality, and variability were assessed using Lyapunov exponent (LyE), correlation dimension (CoD), and MeanSD, respectively. Repeated-measures ANOVAs revealed a significant effect of intensity across most parameters, including CoD and LyE at the ankle, knee, and hip (all p < 0.05), as well as MeanSD at the hip (p < 0.05). Furthermore, a significant effect of intensity was observed in 6 out of the 9 parameters measured at the centre of mass. Effects of time were sparse, with interaction effects largely confined to centre of mass variability. Post hoc analyses indicated that higher intensity domains were associated with reduced local dynamic stability, lower dimensionality, and increased variability at some sites. Changes over time were not exclusive to the severe intensity domain, suggesting that metabolic stress does not play an exclusive role in regulating movement dynamics during running. Collectively, these findings suggest that between-domain differences in gait dynamics were more consistent with speed-related task constraints than with cumulative fatigue or metabolic stress alone.