Linear and Nonlinear Kinematic Synergies in the Grasping Hand

The hand is tasked with creating a multitude of postures in everyday life in order to grasp, use, and manipulate objects. With more than 25 degrees of freedom (DoF), the hand is well equipped to dexterously achieve these tasks. It is hypothesized that the central nervous system (CNS) calls upon a few key movement patterns when controlling the hand; this has been supported by neural,muscle, and kinematic studies of reach and grasp movements. For example, a cup is often grasped with a similar hand motion irrespective of position or size. These few movement patterns are known as synergies.

Synergies, or movement primitives, are viewed as fundamental building blocks of movement. Using synergies, the CNS can simplify control of high DoF limbs: instead of controlling individual DoF, the CNS controls a few synergies that each encodes multiple DoF. Synergies, especially in the human hand present a unique testing environment for two reasons:the human hand has the most DoFs in the body and we and others have observed that hand movements in activities of daily living (ADL) involving hand grasping can be reconstructed with 90% accuracy using six synergies. These six synergies were computed using principal component analysis (PCA) and accommodated for more than 90% of the variance in the joint kinematics. This means that high dimensional control (25 DoF) control can potentially be reduced to low dimensional control (6 functional DoF).