Limb-specific modulation of muscle synergies and segmental coordination during curved running

Curved running imposes asymmetrical mechanical demands on the lower-limbs and thus provides a model to test the adaptability of the locomotor system. This study examined how curved running alters neuromuscular modularity and lower-limb intersegmental coordination relative to straight-line running. Surface EMG recordings from 14 bilateral lower-limb muscles were analysed using non-negative matrix factorisation to extract muscle synergies separately from the inner and outer limb, while segmental kinematics were used to quantify intersegmental coordination. Four synergies were identified in each limb. Although the overall modular organisation remained largely preserved, curved running induced systematic changes in both temporal and spatial activation patterns. Synergy activation timing occurred earlier in the gait cycle, particularly for modules associated with push-off and late swing, while spatial recruitment patterns diverged between the inner and outer limbs. Adaptations were limb-specific: the inner limb displayed greater reweighting and reduced complexity, while the outer limb showed earlier temporal shifts. Higuchi's fractal dimension indicated reduced complexity in touchdown and late swing synergies but increased complexity in push-off. Kinematic analyses showed that curved running modified intersegmental coordination, with divergence of the covariation plane between inner and outer limbs, reflecting their distinct functional roles in redirecting versus propelling the body. Together, these findings demonstrate that curved running preserves the fundamental organisation of locomotor modules while eliciting flexible, limb-specific adaptations in neuromuscular and kinematic coordination. The results support the view that human locomotion combines robust rhythmic structure with context-dependent modulation to accommodate asymmetric mechanical demands.