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1. Studies were made of the electromyogram (EMG) patterns associated with the performance of visually guided, step-tracking arm movements by normal humans. Subjects were instructed to make movement either 'accurately', 'as fast as possible' or 'fast and accurately'. Movements of 16, 32, 48 and 64 deg of arc were made with each instruction. Movements had durations of approximately 250-600 msec. 2. A 'triphasic' pattern of EMG activity was associated with all movements in this study. All bursts in this pattern were more clearly defined in faster movements whether the increased speed of movement was a result of increased movement amplitude or of the instruction-related 'intent' of the subject. 3. The magnitudes of the two agonist EMG bursts showed identical linear dependencies on movement amplitude. The slope of this relation was instruction-dependent, being greatest for 'fast' and least for 'accurate' movements. 4. The duration and time of onset of the initial agonist burst relative to the start of the movement were not dependent on movement amplitude or on instruction. In contrast, the time of onset of the second agonist burst depended on both movement amplitude and instruction, occurring earlier when movements were made faster. 5. The magnitude of the antagonist activity was instruction- but not amplitude-dependent. Duration and onset of this burst varied with both instruction and movement amplitude.

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