Variability of frequency and phase between antagonistic muscle pairs in pathological human tremors.

We investigated the electromyographic activity (EMG) of flexor and extensor muscles with different hand positions in patients with essential (ET) and parkinsonian (PD) tremor. Using a previously developed bootstrap method and standard cross-spectral analysis, we performed sta- tistical tests to assess the effect of hand position on: (1) the frequency of the EMG; and (2) the phase between the EMGs recorded from antagonistic muscle pairs. Frequency as well as phases changed significantly with dif- ferent positions of the hands but not during the recordings when the position was left unchanged. Besides confirmation that frequency and phase are stationary and reliable parameters during short-term recordings under con- trolled laboratory conditions, these results are of particular interest for am- bulatory long-term tremor measurements. A higher variability of the esti- mated parameters reported in long-term recordings may perhaps reflect a patient's mobility only. Our study shows that long-term recording systems should have the means to monitor the patient's movements to provide reli- able results.