The pattern of EMG activity in agonist/antagonist muscle pairs has been extensively studied, but has not proved to be particularly helpful, since the activity is quite variable. Alternating and co-contracting patterns of EMG activity have been documented in most pathologic types of tremor, but the pattern may vary over a period of a few minutes, from co-contracting to alternating in the same agonist/antagonist muscle pair, and in some patients, rhythmic activity is con fined to the agonist (antigravity) muscles alone.  Similarly, a synchronous rather than an alternating burst pattern has been considered a hallmark of dystonia (assumed to represent co-contraction), but its diagnostic accuracy is debatable.