Now showing 1 - 3 of 3
  • Publication
    The Influence of Force Level and Motor Unit Coherence on Nonlinear Surface EMG Features Examined Using Model Simulation
    Nonlinear features extracted from surface EMG signals have been previously used to infer information on coherent or synchronous activity in the underlying motor unit discharges. However, it has not yet been assessed how these features are affected by the density of the surface EMG signal, and whether changes in the level of muscle activation can influence the effective detection of correlated motor unit firing. To examine this, a motoneuron pool model receiving a beta-band modulated cortical input was used to generate correlated motor unit firing trains. These firing trains were convolved with motor unit action potentials generated from an anatomically accurate electrophysiological model of the first dorsal interosseous muscle. The sample entropy (SampEn) and percentage determinism (%DET) of recurrence quantification analysis were calculated from the composite surface EMG signals, for signals comprised of both correlated and uncorrelated motor unit firing trains. The results show that although both SampEn and %DET are influenced by motor unit coherence, they are differentially affected by muscle activation and motor unit distribution. The results also suggest that sample entropy may provide a more accurate assessment of the underlying motor unit coherence than percentage determinism, as it is less sensitive to factors unrelated to motor unit synchrony.
      373Scopus© Citations 2
  • Publication
    Fatigue-Related Alterations to Intra-Muscular Coherence
    Oscillations in the alpha (8-12 Hz), beta (15-35 Hz) and gamma (35-60 Hz) frequency bands are commonly observed in recordings from the primary motor cortex. Coherence analysis based on motor unit spike trains is commonly used to quantify the degree of shared cortical input and the common modulation of motor unit discharge rates between muscles. In this study, intra-muscular coherence is used to investigate the alterations in the neural drive to the First Dorsal Interosseous muscle directly after a fatiguing contraction and following a rest period. An increase in coherence was observed for all frequency bands examined, which was statistically significant within the alpha and beta frequency ranges. There was no consistent difference between the coherence estimates obtained pre-fatigue and those reported after the recovery period. The increase in beta band coherence post-fatigue may indicate increased levels of cortical drive to the motor unit pool. Although the functional significance behind the increase in beta frequency coherence is unclear, it may aid in the coordination of muscle activity to compensate for the decline in the force generating capacity after fatigue.
    Scopus© Citations 3  309
  • Publication
    Alpha Band Cortico-Muscular Coherence Occurs in Healthy Individuals during Mechanically-Induced Tremor
    The present work aimed at investigating the effects of mechanically amplified tremor on cortico-muscular coherence (CMC) in the alpha band. The study of CMC in this specific band is of particular interest because this coherence is usually absent in healthy individuals and it is an aberrant feature in patients affected by pathological tremors; understanding its mechanisms is therefore important. Thirteen healthy volunteers (23±4 years) performed elbow flexor sustained contractions both against a spring load and in isometric conditions at 20% of maximal voluntary isometric contraction (MVC). Spring stiffness was selected to induce instability in the stretch reflex servo loop. 64 EEG channels, surface EMG from the biceps brachii muscle and force were simultaneously recorded. Contractions against the spring resulted in greater fluctuations of the force signal and EMG amplitude compared to isometric conditions (p<.05). During isometric contractions CMC was systematically found in the beta band and sporadically observed in the alpha band. However, during the contractions against the spring load, CMC in the alpha band was observed in 12 out of 13 volunteers. Partial directed coherence (PDC) revealed an increased information flow in the EMG to EEG direction in the alpha band (p<.05). Therefore, coherence in the alpha band between the sensory-motor cortex and the biceps brachii muscle can be systematically induced in healthy individuals by mechanically amplifying tremor. The increased information flow in the EMG to EEG direction may reflect enhanced afferent activity from the muscle spindles. These results may contribute to the understanding of the presence of alpha band CMC in tremor related pathologies by suggesting that the origin of this phenomenon may not only be at cortical level but may also be affected by spinal circuit loops.
    Scopus© Citations 21  305