Kinematic Coordination and Muscle Synergy Patterns of Grasping During Lower-Limb Locomotion

Human motor behaviors typically require coordination between lower-limb rhythmic tasks and upper-limb voluntary tasks. This study investigates the synergy strategies of human motion at the joint and muscle levels to explore the control mechanisms of cooperative tasks, such as grasping during walking or running. We designed novel motor experiments and analyzed kinematic characteristics, ipsilateral joint angles and EMG signals. Using principal component analysis on joint angle trajectories and non-negative matrix factorization on 16-muscle activities, we extracted kinematic primitives and muscle synergies to reveal underlying synergy mechanisms. The results demonstrate that the kinematic and muscle synergy mechanisms of cooperative tasks exhibit significant grasp-position dependence, and strong correlations exist between weight coefficients and specific kinematic characteristics. Additionally, cooperative tasks can be explained by integrating pre-existing muscle synergy patterns derived from lower-limb rhythmic and upper-limb voluntary tasks. The activation components of lower-limb locomotion are almost preserved, whereas those of grasping tasks are partially obliterated in some cases. These findings indicate that cooperative tasks induce functional coupling between lower-limb and upper-limb movements at both joint and muscle levels, exhibiting significantly stronger inter-limb coordination than individual voluntary tasks. This work introduces novel experimental paradigms to systematically study inter-limb coordination strategies, providing valuable insights into human motion control mechanisms.