Impact Of Shoulder Exoskeleton Assistance On Human Upper-Limb Motor Primitives

Human upper-limb movements are known to exhibit robust kinematic features, including bell-shaped velocity profiles and the two-thirds power law. These features are interpreted as signatures of motor primitives underlying human movement planning. While wearable exoskeletons are increasingly used to reduce physical load, their influence on these motor primitives remains unexplored. This study investigates whether upper-limb kinematic motor primitives are maintained during movements performed with a shoulder-support exoskeleton. Five healthy participants performed vertical and complex three-dimensional upper-limb movements under three conditions: no exoskeleton, low-support exoskeleton, and high-support exoskeleton. Movement elements were extracted based on velocity zero-crossings of hand kinematic. The experimental velocity profiles of these elements were compared to a bell-shaped profile, and the scaling relationship between mean velocity and movement displacement was evaluated. Results showed strong agreement with bell-shaped velocity profiles, with high correlations to the theoretical profile (0.86-0.87), and scaling exponents close to the theoretical two-thirds value (confidence intervals range of [0.57, 0.73]). Statistical analysis revealed no significant effects of exoskeleton assistance level or movement type. These preliminary findings suggest that shoulder-support exoskeleton assistance does not disrupt the fundamental kinematic structure of upper-limb motor primitives, maintaining predictable kinematic motor primitives despite load redistribution. This supports the use of model-based assistance, intent inference, and primitive-based control strategies in wearable robotics, assuming that fundamental movement structure remains intact under exoskeleton support.