Figure 1.

Model structure and setup for reaching. (A) Model structure. There are three main modules in the model: brain, spinal cord, and biomechanical arm modules. The brain module consists of the basal ganglia (BG) and cortex, and sends out motor commands into the spinal cord module. The spinal cord module combines the motor commands with afferent feedback from the mechanical arm to generate motoneuron outputs controlling arm muscles for movements. (B) Setup for the center-out reaching tasks. The arm model simulates 2D reaching movements as a human subject performs with a robotic manipulandum. Reaching tasks are defined as moving the arm from the initial position at the center of the circle to one of target positions (sample target positions are shown by blue dots). For all simulations, the target reaching distance was fixed at 0.2 m and the reaching time was set to 1 sec. Angles θ₁ and θ₂ represent the shoulder and elbow joint angles, respectively, with respect to the vertical axis (gray lines).