Side-to-side correlation of muscle activity in physiological and pathological human tremors

Objective: Many tremors occur always or often bilaterally. The question arises whether this could be explained by a common source or commonly transmitting pathways or by bilaterally represented, independent structures with the same oscillatory properties. A similar tremor frequency does not provide sufficient information to clarify this question. Methods: We analyze coherencies between surface electromyographies (EMG) to investigate if bilateral physiologic (PT), essential (ET), Parkinsonian (PD) and orthostatic (OT) tremors originate from a common source for both sides of the body. We show that commonly used techniques to test whether coherencies are significant could lead to false positive results for tremor EMGs. A new estimation procedure is proposed to test EMG tremor time series on their linear independence. We apply this test to bilateral tremors. Results: All measured EMG-pairs in OT (n=7) were highly coherent between both sides with reproducible coherency values of up to 0.99. All other investigated tremors, i.e. PT and enhanced physiological tremors (EPT, n=117), ET (n=76) and PD resting and postural tremors (n=70) do not show a significant side-to-side correlation. Conclusions: This finding shows that the pathophysiologies of OT and other pathological tremors are definitely different. Either they have different origins or different kinds of transmitting pathways. The proposed method might also be used to investigate other electrophysiological data and is a helpful, easy to use investigation for a daily clinical routine.