Electromyographic response to displacement of different forelimb joints in the squirrel monkey

The electromyographic (EMG) reflex response evoked in muscles stretched by imposed angular joint displacement has previously been studied in a variety of muscles and species. In most muscles studied, the EMG response consists of an initial burst of activity at latencies comparable to tendon tap responses followed by one or more bursts of EMG activity occurring at latencies less than somatosensory reaction time. The longer latency bursts or peaks of activity have often been assumed to be of similar origin in functionally diverse muscles. The present experiments were performed to examine the EMG response to imposed joint displacement in several different muscles of the squirrel monkey forelimb. The EMG was studied in muscles stretched by elbow, wrist, and metacarpophalangeal joint extension. Early (M1) and long (M2) latency peaks of activity could be observed in the EMG responses of short head of biceps (SHB), flexor carpi ulnaris (FCU), and flexor digitorum profundus (FDP), at latencies which are similar to those reported in other primate species allowing for differences in the size of the animals. The intervals between these peaks did not correspond to the period of oscillations in tension which occurred after the onset of the joint extension. The M2 peak consistently occurred later in SHB than in FCU and FDP. The M1 peak was smaller in distal than in proximal muscles and was frequently absent in FDP at resting levels of EMG activity. The ratio of M1 to M1 + M2 activity for different muscles revealed that proximal muscles had prominent early with less prominent long latency activity, whereas distal muscles had minimal early latency activity with prominent long latency activity. The onset of EMG activity approached tendon tap latencies in FCU at high base line EMG levels but in most cases occurred at approximately twice tendon tap latencies in FDP at high base line EMG levels. The results demonstrate the presence of significant differences between the EMG response to joint displacement in the proximal-distal series of forelimb flexors. Differences in the reflex response to joint displacement are discussed in view of twitch times and motor unit composition of the muscles studied.