Figure 5. The response of muscle spindle primary afferents to stretch and release during voluntary contraction.

The unaveraged instantaneous firing rate of primary afferents, torque at the wrist joint, rectified EMG and joint angle are shown during stretch, hold and release. The subjects maintained a steady contraction. Unit A exhibits a high dynamic response to stretch, and unit B shows reduced dynamic response to stretch. Both afferents show maintained firing during muscle shortening.