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Analysis of human motor unit discharge variability: Changes with aging and motor learning

Christopher Anson Knight, University of Massachusetts Amherst


Variability in the discharge of human motor neurons was investigated in the context of aging and skill acquisition. In the aging experiment, young and older adults performed constant-force and complex sinusoidal isometric force-matching tasks while motor unit action potentials were recorded from the first dorsal interosseous (FDI) muscle. In three manuscript chapters, the characteristics of motor unit inter-spike interval (ISI) distributions were compared, patterns in the time series of ISIs were investigated with two experimental measures of complexity, and the extent of motor unit synchronization and common drive was compared between young and older adults. The sinusoidal force-matching conditions required substantial motor unit rate coding compared to the constant-force task. In both tasks, older adults utilized a greater proportion of motor units that were discharging at lower rates and with greater discharge variability. Positive skewness of ISI distributions from older adults indicated that motor unit discharge rates varied more freely towards lower rates and de-recruitment. That skewness was greater in older adults indicates that disruption of consistent discharge through de-recruitment and subsequent recruitment may be one reason why force fluctuations are greater and force-matching performance was poor in older adults. The complexity measures indicated that the fluctuations in motor unit discharge during constant-force contractions were random. Detrended fluctuation analysis, but not approximate entropy, was able to distinguish between the constant-force and sinusoidal force-matching tasks. It was demonstrated with simulated data that these techniques are sensitive to the manner in which motor unit data are pre-conditioned. Indeed, different methods of creating firing rate time series resulted in opposite conclusions regarding complexity. These complexity measures should be further evaluated with discharge data from patient populations in which patterns might be expected due to altered neural function. Motor unit synchronization and common drive were similar in young and older adults and greater synchronization was associated with greater discharge variability. (Abstract shortened by UMI.)

Subject Area

Sports medicine|Cellular biology|Neurology

Recommended Citation

Knight, Christopher Anson, "Analysis of human motor unit discharge variability: Changes with aging and motor learning" (2003). Doctoral Dissertations Available from Proquest. AAI3078699.