The diagnosis and the follow-up of pathological tremor (rest, essential or cerebellar) are difficult, especially in early stage [1Elble, et al. Factors influencing the amplitude and frequency of essential tremor Mov Disord 1994 ;  9 : 589-596 [cross-ref]

Cliquez ici pour aller à la section Références]. This study defines tremor quantification indexes from a spectral analysis of EMG signals.

Bilateral EMG of the wrist flexors/pronators (FlPr) and extensors/supinators (ExSu) were recorded (F_e=1000Hz) in one patient (50 y) with a left side tremor clinically observed. Eight conditions were tested during 10s: at rest without any distraction (1), with cognitive distraction (2), with contralateral motor task (3), posture with the wrist pronated (4), semi-pronated (5), supinated (6), flexed elbows with proned wrist close to the thorax (7), walking (8). The EMG signal envelop is obtained by a band-pass [20;450Hz] and a low-pass (30Hz) filters. A power spectral density study (DSP) from 1.5 to 12Hz leaded to the calculation of the main frequency, the maximum DSP and the rhythmicity index (IR=area under the main frequency peak/total area) [2Velickovic, Gracies Keys to identifying and treating tremor Geriatrics 2002 ;  57 : 32-36

Cliquez ici pour aller à la section Références].

The main frequency is similar in all conditions (FlPr G 5.4±0.1Hz, ExSu G 5.6±0.3Hz; FlPr D 7.2±0.3Hz, ExSu D 7.4±0.4Hz, mean±standard deviation). The maximum DSP is higher on left side independently to the task (e.g. Rest, ExSu G 42.33mV²/Hz, D 0.45mV²/Hz). The IR is also higher on left side with a reduced difference with cognitive or motor distraction (muscles averaged, tasks 4–8, G 47±13%, D 27±13%; tasks 2–3, G 49±16%, D 39±20%). The maximum DSP and the IR are higher in extensors than in flexors in each task (postures 4-7, FlPr G 38±18%, ExSu G 43±17%).

This method of tremor quantification, based on the spectral density analysis of EMG signals, shows invariant tremor main frequency, whereas the maximum DSP and the IR depend on the muscle. Only the IR seems to be impacted by the task (without vs. with distraction) and also could be used in the follow-up of tremor patient.