Spatial and temporal modulation of intra-limb coordination in kinematic synergies during hindlimb split-belt treadmill walking in rats

Effective walking requires precise control of the spatiotemporal patterns of leg motions, involving both inter-limb and intra-limb coordination to maintain balance and generate propulsive force. Studies using a split-belt treadmill, which imposes asymmetrical walking conditions, have suggested that temporal modulation predominates over spatial modulation when evaluating locomotor parameters such as stance duration and stride length. However, since locomotion involves coordinated multi-joint motions, analyses based on these parameters, such as the duration and/or length, may not fully capture how modulations are implemented at the level of inter-segmental coordination within a limb. Therefore, it remains unclear whether locomotor adaptation at the multi-joint level is driven solely by temporal modulations of motor commands as inferred from the changes in a range of factors that can affect coordinated movement. Here, we examined intra-limb coordination during hindlimb bipedal locomotion in rats walking under tied and split-belt conditions. Using kinematic synergy analysis, multi-joint motions were decomposed into temporal and spatial components to determine whether locomotor adaptation is primarily governed by temporal modulations or also involves spatial reconfiguration. Our findings indicate that, in addition to temporal modulations, spatial modulations of intra-limb coordination are significantly reconfigured under asymmetrical walking conditions. These findings suggest that locomotor adaptation involves not only timing adjustments but also spatial reconfiguration of joint coordination, providing insight into the control strategies underlying stable locomotion.