DISRUPTION OF TONIC DISCHARGE IS A SOURCE OF MOTOR UNIT DISCHARGE VARIABILITY IN OLDER ADULTS

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During sustained muscular contractions, motor units discharge about a mean rate with variability (c.f. Galganski et al., 1993; Laidlaw et al. 2000; Patten & Kamen, 2000; Vaillancourt et al., 2002). The nature of the variability may be linked to age-related changes in motor unit morphology and impaired control of muscular force. Typically, only stable segments of motor unit discharge time series are used in variability computations and this selection may result in the loss of information. PURPOSE The purpose was to compare motor unit discharge variability among young and older adults with specific interest in the momentary disruption of tonic discharge. METHODS Young (n = 12, mean 18.8 years) and older (n = 12, mean 72.6 years) adults performed each of two different isometric force-matching tasks involving the first dorsal interosseous muscle (FDI). Motor unit discharge behavior was recorded using a quadrifilar needle electrode. A linear, 20% maximal voluntary contraction (MVC) force-matching task was used to investigate the fluctuations that occur during the maintenance of constant force. A complex sinusoidal force-matching task (20% +/− 3.65%MVC) was used to investigate motor unit discharge during force control requiring substantial modulation of motor unit discharge rates. RESULTS Older adults were less proficient in both force-matching conditions (F= 8.58, p= 0.008) and there was a trend towards greater motor unit discharge variability in this group (F= 2.36, p= 0.118). The most notable difference between the two groups was a greater positive skewness of motor unit inter-spike interval (ISI) distributions in the older adults (F= 11.63, p= 0.003). In both conditions, these skewness results were driven by atypically long ISIs. CONCLUSION Momentary disruptions in tonic motor unit discharge occur more often in older adults. These disruptions could occur due to either the inherent discharge variability associated with activity around the recruitment threshold or some other control mechanism. One of these mechanisms may account for part of the impaired force control.