Synergies in Grasping.

The reach-to-grasp movement is a prototype of human movement coordination. Since the pioneering work of Jeannerod (Attention and performance, ix. Erlbaum, Hillsdale, NJ, pp 153-169, 1981), this movement is generally considered to be a coordinated combination of hand transport and grip formation. One of the main theoretical reasons for choosing transport and grip as building blocks is that they are anatomically independent: one can determine for each muscle, joint or brain area whether it belongs to transport or grip. We have proposed a different view on grasping, in which the coordination problem is formulated as one related to the movements of the digits (Smeets and Brenner in Motor Control 3:237-271, 1999). According to this view, both the transport of the hand and the formation of the grip emerge from the combination of independent digits' movements towards the objects' surface. This independency of the digits resembles the independence of synergies (as discussed in the chapter of d'Avella). Different synergies are activated independently, but a single muscle can be part of several synergies. In this chapter we will present three types of experiments that were designed to test to what extent the individual digits' movements can be considered the building blocks of the reach-to-grasp movement.