Late electromyographic activity following stretch in human forearm muscles: physiological role

We have previously found that the late reflex electromyographic (EMG) responses to muscle stretch are simultaneously present in both the stretched muscle and its antagonist, suggesting that it may be involved in the control of limb stiffness rather than in a servo-mechanism that returns the limb to an initial or intended position. Nevertheless, in these earlier experiments the size of the late EMG activity was always greater in the stretched muscle than its antagonist, suggesting also a possible servo-role for these responses. To clarify this, we have now recorded the late EMG activity simultaneously from both the right extensor and flexor carpi radialis muscles in ten subjects under four experimental conditions. These conditions differed in which muscle was activated voluntarily prior to the stretch stimulus and in which muscle was stretched by the pertubing force. In all cases subjects were asked to maintain the limb in a fixed position and oppose any displacement that occured. The amount of integrated EMG activity for both the M2 and M3 components of the late response was greater in the agonist muscle (i.e., the muscle actually stretched by the perturbing force) than the antagonist. The ratio of agonist to antagonist activity, however, was significantly larger for the M2 than for the M3 component. These results suggest that both the M2 and M3 components of the components of the late EMG response have a dual functional role, being involved both in a servo-mechanism and in the control in limb stiffness. Each component, however, seems to have a distinctive role. M2 seems more involved in the servo-role, whereas M3 seems more involved in the control of limb stiffness.