Now showing 1 - 4 of 4
  • Publication
    Alterations in Motor Unit Firing Rate and Action Potential Properties during Isometric Fatigue in Stroke Survivors
    The limited number of studies that have investigated fatigue in chronic stroke survivors during voluntary contr actions to the endurance limit have reported relatively higher central fatigue and lower peripher al fatigue on the affected side when compared to the less-affected side and healthy controls (Riley and Bilodeau, 2002; Knorr et al., 2011). Although these changes have been investigated using global indices of motor unit (MU) activation, alterations at th e level of the single motor unit have not yet been examined.
  • Publication
    Muscle fatigue increases beta-band coherence between the firing times of simultaneously active motor units in the first dorsal interosseous muscle
    Synchronization between the firing times of simultaneously active motor units (MUs) is generally assumed to increase during fatiguing contractions. To date, however, estimates of MU synchronization have relied on indirect measures, derived from surface electromyographic (EMG) interference signals. This study used intramuscular coherence to investigate the correlation between MU discharges in the first dorsal interosseous muscle during and immediately following a submaximal fatiguing contraction, and after rest. Coherence between composite MU spike trains, derived from decomposed surface EMG, were examined in the delta (14 Hz), alpha (812 Hz), beta (1530 Hz), and gamma (3060 Hz) frequency band ranges. A significant increase in MU coherence was observed in the delta, alpha, and beta frequency bands postfatigue. In addition, wavelet coherence revealed a tendency for delta-, alpha-, and beta-band coherence to increase during the fatiguing contraction, with subjects exhibiting low initial coherence values displaying the greatest relative increase. This was accompanied by an increase in MU short-term synchronization and a decline in mean firing rate of the majority of MUs detected during the sustained contraction. A model of the motoneuron pool and surface EMG was used to investigate factors influencing the coherence estimate. Simulation results indicated that changes in motoneuron inhibition and firing rates alone could not directly account for increased beta-band coherence postfatigue. The observed increase is, therefore, more likely to arise from an increase in the strength of correlated inputs to MUs as the muscle fatigues.
      322Scopus© Citations 36
  • Publication
    Changes in motor unit behavior following isometric fatigue of the first dorsal interosseous muscle
    The neuromuscular strategies employed to compensate for fatigue-induced muscle force deficits are not clearly understood. This study utilizes surface electromyography (sEMG) together with recordings of a population of individual motor unit action potentials (MUAPs) to investigate potential compensatory alterations in motor unit (MU) behavior immediately following a sustained fatiguing contraction and after a recovery period. EMG activity was recorded during abduction of the first dorsal interosseous in 12 subjects at 20% maximum voluntary contraction (MVC), before and directly after a 30% MVC fatiguing contraction to task failure, with additional 20% MVC contractions following a 10-min rest. The amplitude, duration and mean firing rate (MFR) of MUAPs extracted with a sEMG decomposition system were analyzed, together with sEMG root-mean-square (RMS) amplitude and median frequency (MPF). MUAP duration and amplitude increased immediately postfatigue and were correlated with changes to sEMG MPF and RMS, respectively. After 10 min, MUAP duration and sEMG MPF recovered to prefatigue values but MUAP amplitude and sEMG RMS remained elevated. MU MFR and recruitment thresholds decreased postfatigue and recovered following rest. The increase in MUAP and sEMG amplitude likely reflects recruitment of larger MUs, while recruitment compression is an additional compensatory strategy directly postfatigue. Recovery of MU MFR in parallel with MUAP duration suggests a possible role for metabolically sensitive afferents in MFR depression postfatigue. This study provides insight into fatigue-induced neuromuscular changes by examining the properties of a large population of concurrently recorded single MUs and outlines possible compensatory strategies involving alterations in MU recruitment and MFR.
      280Scopus© Citations 40
  • 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.
      256Scopus© Citations 3