Figure 1.

Comparison of models for motor learning. A, In previous motor learning schemes, update of the motor command (joint torque or muscle activation) corresponds to a monotonic antisymmetric (in most cases, linear) function of the joint angle error or muscle length error. B, In our new model there is a proportional increase in the feedforward command in response to muscle lengthening, a proportional increase in response to muscle shortening with a slightly lower gain, and a decrease when the error is near zero. This is shown for both muscles of an agonist–antagonist pair. C, When a disturbance occurs, this model produces a scaled response by modifying the reciprocal activation by the amount indicated in the shaded region to generate an opposing force. D, When a large disturbance occurs, this model will produce an increase in coactivation (green area) to stiffen the joint. When no disturbance is present, the motor command is reduced (orange area) to decrease metabolic cost.