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 statistical 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 a s well as phases changed significantly with different positions of the hand s but not during the recordings when the position was left unchanged. Besid es confirmation that frequency and phase are stationary and reliable parame ters during short-term recordings under controlled laboratory conditions, t hese results are of particular interest for ambulatory long-term tremor mea surements. A higher variability of the estimated parameters reported in lon g-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 reliable results. (C) 2001 John Wiley & Son s, Inc.