Adaptations in motor unit activity with age and physical activity
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The hypothesis that age-related alterations to the morphological properties of a motor unit (MU) are accompanied by modifications in their control properties has been supported by data that compared young (24-37 y.o.) and elderly (65-88 y.o.) adults (Erim et al. J. Neurophys., 1999). The objective of the present dissertation was to characterize whether such modifications in MU control properties are progressive across a continuum of ages from childhood to senescence, and whether such adaptations are muscle and usage dependent. Multiple concurrently active MUs were assayed from the first dorsal interosseous (FDI) and vastus lateralis (VL) muscles in healthy subjects from 8-86 years of age. Surface EMG (sEMG) signals were acquired while the participants isometrically tracked a trapezoidal force trajectory at 20%, 50% and 80% of their maximal voluntary force capacity. Data were decomposed into MU trains using a recently developed sEMG decomposition procedure (De Luca et al. J. Neurophys., 2006; Nawab et al. J Clin. Neurophys., 2010) that provides a much greater yield (typically 3- 6 X) than that of prev1ous needle sensor based technologies. Results from n=65 subjects (representing approximately 5307 analyzed MUs) indicate that the average firing rates of the earliest recruited MU trains were significantly reduced with increasing age (p<0.05)) for both muscles and the three normalized force levels tested. Characteristics of MU behavior in young children were reported for the first time, and demonstrated unique properties compared to findings in adults and the elderly. Additionally, those elderly adults who scored at the high end of a physical activity scale deviated least in their firing rate properties from young adults, demonstrating that habitual physical activity can modify the effects of ageing. These findings indicate an age and usage-dependency to MU control properties that is progressive.
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