Uncovering movement synergies during reaching and grasping of common objects for the upper limb

Understanding how the upper limb is coordinated during everyday activities is essential for developing strategies to restore motor function in individuals with impaired upper limbs. One prominent theory is that upper limb movements are controlled through synergies, which are coordinated patterns of joint movements. While many studies have supported the existence of synergies, there exist gaps where only a narrow range of tasks were examined, data capture relied on restrictive tracking tools, or analysis focused solely on the hand, limiting naturalistic movements and generalizability, leaving key gaps in understanding how hand and arm movements jointly contribute to upper-limb synergies. To address these gaps, we collected full upper-limb kinematics from 20 participants performing 80 common reach and grasp activities using markerless motion tracking. We identified a set of four upper limb postural synergies that were able to accurately reconstruct grasp postures on an unseen participant's data, suggesting that a small set of global movement patterns underlie most common actions. These results demonstrate that upper-limb kinematics can be effectively described within a compact, low-dimensional synergy space that is generalized across different participants and activities, with potential applications in assistive technologies and rehabilitation.