The 'M2' electromyographic response to random perturbations of arm movements is missing in long-trained monkeys.

Adult monkeys who have been under training over a long period of time show a loss of the M2 component in their biceps electromyographic response to brief, random perturbations of an alternating arm movement, the M1 component being apparently enhanced. These changes of the stretch-induced responses indicate a long-term plasticity of the sensorimotor system of monkeys. This result also provides hints for the origin and possible functional significance of the short- and long-latency components of the electromyographic response in subjects with less experience.