Hand posture comparison in Synergy Space

In studies of human movement control, an important question is how the central nervous system (CNS) controls movements in the presence of a large number of degrees of freedom (DoFs) at all levels of the control architecture. It is known that CNS groups the multiple DoFs into representative functional units also called synergies for simplification of the task. Hence, comparing two different hand postures in the synergy space, rather than joint angle space may provide insightful information about efforts needed (in terms of synergy components) to transform from one hand posture to another hand posture. Working with synergy space may also provide information about how CNS deals with system with multiple DoFs. We developed an index called posture similarity index (PSI) which measures the similarity of two postures by projecting hand posture from the joint angles into the synergy space. A large value of PSI represents high similarity between postures whereas a lower value represents less similarity between postures. This index uses principle of synergies and nicely captures effort required for kinematic transformation. Using this index as a feature, possible set of representative postures can be identified. The other hand postures can be derived from a possible set of representative postures with relatively less efforts for kinematic transformation.