Determination of age- and sex-related differences in motor unit activity

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Muscle activity has traditionally been measured in laboratory studies, and results have subsequently been extrapolated to provide an indication of functional decline during daily life. However, recent work in my laboratory is the first to use portable electromyography (EMG) to measure muscle activity during daily life. These recordings over a typical 8-hour day in upper and lower limb muscles indicate that low threshold EMG increases in women compared with men, and that this difference becomes greater with age. Low-threshold EMG is quantified through assessing bursts of muscle activity (>2% maximum voluntary contraction (MVC), >0.1s duration) and gaps in activity (<1% of maximal >0.1s duration). The underlying cause of sex- and age-related increases in low threshold EMG remains to be determined. Surface EMG recordings detect only activity from muscle fibres of motor units (MU) active within the recording area of the electrode. Thus, the IMMEDIATE OBJECTIVE of my research is to determine whether changes in MU recruitment and discharge rates contribute to the age- and sex-related increase in low threshold EMG. My WORKING HYPOTHESIS is that as a consequence of changes in muscle length during movement sensory feedback to the motor neuron pool is altered and MU recruitment and discharge rates are modulated and this influences the low-threshold EMG recordings. Shorter muscle fascicle length and greater tendon compliance in old adults and women will influence the position dependent effect. These studies will contribute to understanding the basic physiological role of sensory feedback in modulating MU activity and elucidate the cause of age- and sex-related increases in low threshold EMG activity.