Analysis of physiological tremor mechanisms in surface electromyogram

Rhythmical grouping of action potential in the low frequency band have been observed during steady, constant force contractions. The origins of this rhythmical pattern have always been associated to tremor oscillations. However, literature concerning physiological tremor has shown that the origin of tremor remains inconclusive with various works providing valid arguments and counter-arguments for the various mechanisms proposed. Physiological tremor oscillation has consistently been associated with the following four mechanisms: a) synchronisation of motor units; b) mechanical feedback resonance; c) feedback resonance from neural delay and d) central oscillations. It is largely believed that the overall tremor oscillation is either the interaction effect of various oscillatory mechanisms acting in parallel with each other or one or other mechanisms acting predominantly in various situations. The objective of this research was to determine the role of neural feedback and muscle mechanics in the generation of physiological tremor oscillation. This research developed a neuromuscular model to simulate the individual and resulting oscillatory mechanisms of tremor. The oscillations found in EMG signal were also analysed empirically from EMG data from 54 subjects using Hilbert Transform and rectification to obtain the envelope of the signal. The time between rhythmical waveform patterns in the EMG signal, TC was found to be 78ms. This value corresponded to the supraspinal neural delay, 50-80ms reported in literature and the theoretical value of the supraspinal feedback simulated from the model. The theoretical model also reported a predominant mechanical resonance frequency in the resulting oscillation between muscle and spinal feedback and the resulting oscillation between muscle, spinal and supraspinal feedback. These observations coincide with the physiological tremor oscillations reported in literature: the mechanical-reflex and the 8-12Hz. The mechanical-reflex oscillation is defined to be the product of spinal feedback and muscle mechanics and is found to be highly influenced by the mechanical properties of muscle. The 8-12Hz component is reported to originate from the interaction of supraspinal and central mechanisms. TC has shown to coincide with the supraspinal neural delay time and the theoretical supraspinal value suggesting that TC correlates to the 8-12Hz component.<br>