Stride-to-Stride Fluctuations and Temporal Patterns of Muscle Activity Exhibit a Stronger Relationship in Running-Induced Fatigue.

During gait, the temporal patterns of stride-to-stride fluctuations and muscle activation exhibit similar responses. This study examined if increasing task demands such as fatigued high intensity running affects this relationship. Eleven experienced runners completed two 400-m runs (R1, R2) with a 3-min break to induce fatigue. Stride Time Intervals (STI) were measured using inertial sensors (IMUs), while Inter-Muscle Peak Intervals (IMPI) were derived from the electromyographic (EMG) activity of the Vastus Lateralis and Gastrocnemius Medialis. We calculated the Hurst exponent and coefficient of variation (CV) for all time series. The association (matching) between STI and IMPI metrics was analyzed using Pearson's correlation. To assess fatigue status, we monitored muscle oxygen saturation (SmO2) and EMG frequency characteristics. Hurst exponent was estimated from the Hurst-Kolmogorov process which is proposed as suitable for short time series. Baseline SmO2 immediately prior to the second run was significantly lower for R2 (p < 0.001). This was supported by EMG fatigue signatures: median frequency decreased and root mean square increased in both muscles. Significantly higher Hurst values were observed for all time series, whereas CV increased significantly only for the STI and Vastus Lateralis IMPI in R2 (p < 0.05). The STI-IMPI correlations (for CV and Hurst) increased significantly in R2 for both VL and GM (r = 0.66 to 0.96). The changes in EMG characteristics indicate less flexible neuromuscular control and possibly provide a physiological explanation for the increased Hurst exponent (persistency). Fatigue strengthened the coupling between stride-to-stride fluctuations and muscle activation. This suggests high-intensity fatigue causes the locomotor system to reorganize into a more tightly coupled, less flexible state.