Frequency peaks of tremor, muscle vibration and electromyographic activity at 10 Hz, 20 Hz and 40 Hz during human finger muscle contraction may reflect rhythmicities of central neural firing

Abstract

The output from the central nervous system to muscles may be rhythmic in nature. Previous recordings investigating peripheral manifestations of such rhythmic activity are conflicting. This study attempts to resolve these conflicts by employing a novel arrangement to measure and correlate rhythms in tremor, electromyographic (EMG) activity and muscle vibration sounds during steady index finger abduction. An elastic attachment of the index finger to a strain gauge allowed a strong but relatively unfixed abducting contraction of the first dorsal interosseous (1DI). An accelerometer attached to the end of the finger recorded tremor, surface electrodes over 1DI recorded EMG signals and a heart-sounds monitor placed over 1DI recorded vibration. This arrangement enabled maintenance of a constant overall muscle contraction strength while still allowing measurement of the occurrence of tremulous movements of the finger. Ten normal subjects were studied with the index finger first extended at rest and then contracting 1DI to abduct the index finger against three different steady forces up to 50% of maximal voluntary contraction (MVC). Power spectral analysis of tremor, EMG activity and muscle vibration signals each revealed three frequency peaks occurring together at around 10 Hz, 20 Hz and 40 Hz. Coherence analysis showed that the same three peaks were present in the three signals. Phase analysis indicated a fixed time lag of tremor behind EMG of around 6.5 ms. This is compared with previous measurements of electromechanical delay. Other experiments indicated that the three peaks were of central nervous origin. Introducing mechanical perturbations or extra loading to the finger and making recordings under partial anaesthesia of the hand and forearm demonstrated preservation of all the peaks, suggesting that they did not originate from mechanical resonances or peripheral feedback loop resonances. It is concluded that, at least for a small hand muscle, there exist not one but a number of separate peak frequencies of oscillation during active contraction, and that these oscillations reflect synchronization of motor units at frequencies determined within the central nervous system. It is proposed that the multiple oscillations may be a means of frequency coding of motor commands.

Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults

“…There is evidence that old adults exhibit increased coactivation during slow abduction-adduction movements of the index finger (1,24,35). Furthermore, the broader peak in the power spectrum for the old adults (50-125 Hz vs. 25-50 Hz) likely reduced phase differences between the frequency components of the signal (28) and resulted in less alternating activation between the agonist and antagonist muscles. Thus the altered temporal activation of the agonist and antagonist muscles in old adults can consequently impair the ability of an old individual to accelerate the limb segment and accurately match a target velocity (9).…”

Bilateral motor unit synchronization is functionally organized

“…MU synchronization at a certain frequency band implies a common, rhythmic input (McAuley et al 1997) that can be quantiWed via conventional coherence analyses between surface EMGs (cf. Miller and Sigvardt 1998).…”

“…In the latter case, MU synchronization reXects periodicities in the Wring of common presynaptic input to MUs that modulates the EMG amplitude and is likely caused by presynaptic synchronization (Kirkwood et al 1982). Although rhythmically synchronized input to MUs may arise from various levels of the nervous system (see McAuley and Marsden 2000, for a review), most researchers seem to agree that it arises at a supraspinal level (Farmer et al 1993;McAuley et al 1997;Grosse et al 2002;Semmler 2002). As such, MU synchronization might be functionally organized and may act as a binding mechanism integrating spatially distributed, neural activity.…”

One central oscillatory drive is compatible with experimental motor unit behaviour in essential and Parkinsonian tremor

“…Equation (13) indicates that the EMG or force (equivalent for acceleration) signals are filtered versions of the neural drive to muscles. The effect of the filters on the relative power of tremor harmonics (7/2/7/1) is obtained by substituting equation (13) in equation (1).…”

“…Equation (13) indicates that the EMG or force (equivalent for acceleration) signals are filtered versions of the neural drive to muscles. The effect of the filters on the relative power of tremor harmonics (7/2/7/1) is obtained by substituting equation (13) in equation (1). The Results will show that, because the average action potential and twitch waveforms vary across subjects, muscles, and conditions, their filtering effect partly hinders the discriminatory power of the neural drive to muscle for identifying the tremor pathology.…”