Preserved coordination structure and distance-related changes in task-variable stabilization: A PCA–UCM analysis of inter-joint synergies during a 10-km run

Background Endurance running performance depends on how multi-joint synergies distribute control across propulsion, absorption, and stability, yet process-level evidence on the along-distance evolution of inter-joint coordination during long runs remains scarce. Methods Fifteen trained runners completed a constant-speed 10-km treadmill run. PCA was used to extract inter-joint synergy features across the 1–10 km segments. UCM was then applied to relate synergy variability to fluctuations in COM fore-aft (AP) velocity. Results The first three principal components consistently explained the majority of variance: PC1 (ankle-driven distal propulsion), PC2 (hip-dominant proximal stabilization/propulsion), and PC3 (ankle–knee cross-plane coupling). With increasing distance, the contribution of distal rotational DOFs (e.g., left-ankle internal/external rotation) decreased, whereas hip loadings remained stable. UCM showed V UCM ≈ 0.180–0.190 (stable), V ORT rising from 0.013 at 1 km to 0.029 at 10 km, and a large but slightly declining synergy index ΔV (0.75–0.88) as load accumulated; a V ORT peak / ΔV trough emerged around 6 km. Conclusions Across the 10-km run, coordination structure remained largely preserved, whereas VORT increased and ΔV decreased, indicating a modest distance-related increase in task-variable-affecting variability and a reduction in UCM-based synergy strength.