Muscle spindle contributions to the stretch-evoked electromyogram: nerve block studies

Application of external torque to cause joint rotation evokes electromyogram (EMG) activity in the stretched muscles. These responses were studied in the tibialis anterior (TA) and flexor carpi ulnaris (FCU) muscles of normal human subjects in conjunction with a nerve block procedure by a local anesthetic agent. In both muscles, differences were observed between the myotatic and postmyotatic portions of the evoked EMG response. During recovery from a complete nerve block, the late components of the EMG (postmyotatic and stabilizing responses) recovered to the preblock magnitude faster than the early components (myotatic and late myotatic responses). Following partial nerve block, the late EMG components were diminished to a lesser extent and recovered faster than the early EMG components. This study suggests that peripheral afferent inputs are obligatory for the myotatic and late myotatic EMG responses. The effects of peripheral afferent inputs on postmyotatic responses are to modulate these later responses. However, the absence of peripheral afferent inputs will not prevent or even delay postmyotatic responses.