Long-latency and voluntary responses to an arm displacement can be rapidly attenuated by perturbation offset

Feedback control of our limbs must account for the unexpected offset of mechanical perturbations. Here we examine the evoked activity of elbow flexor and extensor muscles to torque pulses lasting 22-152 ms and how torque offset impacts activity in the long-latency (45-100 ms) and voluntary epochs (120-180 ms). For each pulse width, we found a significant attenuation of muscle activity approximately 30 ms after the offset of torque compared with when the torque was sustained. The brief time between the offset of torque and the attenuation of muscle activity implicates group I afferents acting through a spinal pathway, because this route is the only one fast enough and short enough to be responsible. Moreover, elbow muscle activity in the subsequent 20-45 ms following torque-offset was approximately 35% smaller than when the torque was sustained. These results show that a fast spinal process can powerfully attenuate corrective responses of the arm to a torque perturbation.