Characterization of kinematic synergies during upright reaching tasks

Upright reaching performed in different directions places distinct postural and coordination demands, as each direction requires specific adjustments of trunk and lower-limb segments to maintain stability while reaching. These direction-dependent changes make these tasks particularly informative for examining how whole-body coordination adapts to varying postural requirements. In this context, kinematic synergies provide a low-dimensional description of multi-joint coordination. We characterized how these direction-dependent demands shape whole-body coordination by extracting kinematic synergies underlying the movements and examining their variation across tasks. Healthy adults carried out unilateral and bilateral forward reaching, along with a lateral reaching task, while whole-body kinematics were recorded. Joint angle trajectories from twelve degrees of freedom were selected and decomposed using Principal Component Analysis to identify dominant coordination components. Across all reaching directions, two kinematic synergies reconstructed joint angle trajectories. The first synergy captured upper-limb elevation, while the second reflected adjustments of trunk, hip, and ankle joints. While this structure was maintained across tasks, synergy weights showed direction-dependent modulations. The lateral reach, in particular, had smaller inter-individual similarity of the second synergy, suggesting flexibility in managing mediolateral balance. These findings indicate that upright reaching relies on a compact yet adaptable coordination structure integrating arm movement with posture-related joint contributions. The consistency of the extracted synergies across individuals, along with the increased variability in lateral reaching, highlights both stereotyped and flexible features of whole-body coordination. These insights reinforce synergy-based models of motor organization and offer applications for rehabilitation and robotic systems that leverage natural coordination patterns.