Abstract
Muscles involved in rapid, targeted movements about a single joint exhibit a triphasic electromyographic pattern. The first burst of activity in the agonist muscle (AG1) accelerates the limb towards the target while the antagonist (ANT) burst acts as a brake and the second agonist (AG2) burst dampens terminal oscillations. Early work involving such protocols as movement perturbations (limb blocking) suggested that for short movements, the entire pattern was prepared and initiated in advance (Keele, 1968; Wadman et al., 1979). In contrast, recent TMS evidence has suggested that each burst may be prepared and executed separately (Irlbacher et al., 2006; MacKinnon & Rothwell, 2000). The purpose of this study was to investigate the independent initiation of each component of the triphasic pattern with movements of different amplitudes. Participants performed rapid elbow extension movements to various targets (10, 20, 40, & 60 degrees). On some trials a startling acoustic stimulus (SAS), which is thought to initiate prepared movements at short latency, was delivered at the onset of the AG1 burst with the intent to examine preparation of the ANT. The SAS significantly shortened the AG1-ANT interval in the 10 and 20 degree movements but not in the 40 and 60 degree movements. The results of this study suggest that the ANT burst was prepared and initiated independently from the AG1 burst and that preparation of the ANT occurred online for movements of larger amplitudes.Acknowledgments: This study was supported by NSERC