Muscle activity during forelimb reaching movements in rats

The absolute timing, and correlation in time of activity in a number of shoulder, elbow and wrist muscles in the rat was analysed in relation to onset and termination of the extension phase of a skilled forelimb reaching movement. Movement onsets were analysed separately in the component upward, forward and medial directions. On average, movements in the upward and medial directions occurred together, prior to forward movement. Latissimus dorsi activity was the earliest muscle event, occurring approximately 150 ms prior to movement, whilst onset of teres major activity, possibly related to paw elevation, had the highest temporal correlation with movement onset. Triceps activity was strongly time locked to the end of the reach, and may have provided final extensor thrust to complete the reaching movement. Although it is possible to speculate on possible roles of particular EMG bursts, multiple (often anatomically antagonistic) muscles were active at all stages of the reach, and no unique muscle relationships to initiation of individual movement components could be identified. It is concluded that reaching movements are produced by temporal variation in distributed activity among all available muscles.