Figure 1.

Biomechanical model. The human hand was approximated as a cone with realistic parameters (table). It was attached to a stationary forearm via a two-degree-of-freedom joint. Four realistic muscle models [Virtual Muscle (Cheng et al., 2000)] actuated the cone to produce force or motion in the extension–flexion direction (in plane of drawing) and radial–ulnar direction (perpendicular to plane of drawing). Proprioceptive sensors attached to each muscle provided proprioceptive feedback to the SLR. The SLR integrated proprioceptive feedback and descending commands to provide motoneuronal excitation to the muscles. These projections were associated with 184 control inputs that were differentiated as SET and GO inputs (see also Fig. 2). The SET inputs continuously regulated the background activity in the spinal cord and the GO inputs produced the transition to a new state.