Sensorimotor control of contact force.

Interacting with objects in the environment introduces several new challenges for motor control: the potential for instability, external constraints on possible motions and novel dynamics. Grasping and manipulating objects provide the most elaborate examples of such motor tasks. We review each of these topics and suggest that when sensory feedback is reliable, it is used to adapt the motion to the requirements imposed by the object. When sensory feedback is unreliable, subjects adapt the stiffness of muscles and joints to the task's requirements. One of the simplifications introduced in the control of such movements is a reduction in the effective number of degrees of freedom (sensorimotor axes and muscle synergies) and recent findings and methodological considerations relevant to this topic are discussed.