Long-latency stretch reflexes of the human elbow extensors during voluntary relaxation: differences between agonistic muscles.

Reflex electromyographic (EMG) responses of elbow extensor muscles to unexpected elbow flexion were recorded in the absence of initial tonic activity from subjects instructed not to resist the stretch. The monosynaptic component M1 was present only in the anconeus muscle and only for high accelerations. The acceleration value at which the long-latency components M2 and M3 appeared was lower for anconeus than for triceps brachii. Increases in peak acceleration of stretch resulted in decreases in M2 and M3 latencies and increases in M2 and M3 magnitudes in both muscles. However, M2 and M3 latencies for anconeus were shorter than those of triceps brachii, except at high acceleration values. Furthermore, the magnitude of M2 and M3 components of anconeus activity increased faster for low accelerations than for high accelerations, whereas those of triceps brachii increased in proportion to the acceleration. These differences between anconeus and triceps brachii were similar to those described earlier for voluntary movements. It is suggested that the motoneurons of all elbow extensor muscles may be recruited as a single motoneuron pool following Henneman's size principle, irrespective of whether the activity is voluntary or reflex in origin.