We recorded the late electromyographic (EMG) responses to predictable and u
npredictable stretches of the wrist flexor and extensor muscles during ball
istic movement or isometric contractions. We simultaneously recorded the ac
companying cerebral responses. Our findings suggest that the late EMG respo
nses are influenced by suprasegmental (cerebral) phasic mechanisms that see
m to have a dual functional role, being involved in the control of limb sti
ffness and in a servomechanism to return the displaced limb to its intended
position. (C) 2000 John Wiley & Sons, Inc.