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Factors Affecting Maximal Motor Unit Discharge Rate In Young And Older Individuals

Purpose: The aim of this investigation was: (i) to examine age-related differences in maximal motor unit firing rates, cortical excitability and inhibition, spinal excitability and inhibition, and motoneuron afterhyperpolarization (AHP) duration, in the tibialis anterior; (ii) to examine which of these factors limit maximal motor unit firing rates in young and older adults; and (iii) to determine if these factors are adaptable in young and older males and females, in response to resistance training. A host of neuromuscular measures were made from young and older subjects before and after a 2-week isometric resistance training protocol. Methods: Sixty subjects, 30 young (mean age 21.9±3.1 years) and 30 older (mean age 72.9±4.6 years) individuals were tested. Fifteen subjects from each age group were randomly assigned to either a control or training group. All subjects underwent baseline testing, including measures of: maximal voluntary contraction (MVC) force of the dorsiflexors, central activation, maximal motor unit firing rates, duration of the motoneuron AHP, cortical excitability and inhibition, spinal excitability and presynaptic and reciprocal inhibition. The control group was then asked to carry out their normal daily activities for two weeks, and the training group participated in isometric training of the dorsiflexors. Training consisted of three sets of ten 5-second MVCs, three times per week, for two weeks. Following the two-week period, all subjects returned to the laboratory for reassessment of the baseline measures. During testing sessions, a needle electrode was inserted into the tibialis anterior (TA) to monitor muscle activity while subjects perform 3 maximal voluntary contractions, each lasting 5 s. Subjects also performed 3 contractions, at approximately 2% MVC, each held for approximately 7-8 minutes, and the duration of the motoneuron AHP was estimated from the motor unit firings. Spinal excitability was assessed with the slope of the ascending limb of the stimulus-response curve of the TA H-reflex, evoked at various stimulus intensities. Presynaptic and reciprocal inhibition were assessed by conditioning the TA H-reflex with a stimulus applied to the tibial nerve. The slope of the ascending limb of the TA motor-evoked potential (MEP) stimulus-response curve was used as an indication of cortical excitability. MEPs were also evoked while subjects perform isometric dorsiflexion to 50% MVC, and the duration of the silent period was used as an indication of cortical inhibition. Results: Young subjects had a higher MVC force, central activation ratio, maximal motor unit firing rate, spinal excitability and cortical excitability than older subjects. Older subjects also had longer duration AHP time constants, and shorter duration cortical silent periods than young subjects. The only gender differences observed were higher MVC force and higher maximal motor unit firing rates in males compared to females. Isometric strength training of the dorsiflexors resulted in increases in MVC force and maximal motor unit firing rate, and decreases in the AHP time constant and presynaptic inhibition. With respect to predictors of maximal motor unit firing rate, the baseline data suggested that, of all of the neuromuscular measures, presynaptic inhibition was the best predictor of maximal firing rate. The change in the maximal motor unit firing rate from Day 1 to Day 2, however, was best predicted by changes in presynaptic inhibition and changes in the duration of the AHP time constant. Conclusions: The results of this study suggest significant age-related changes in the ability to produce force, maximal motor unit firing rates, and associated changes at multiple levels of the neuromuscular system. Many of the neuromuscular measures, however, showed changes in response to isometric strength training in both young and older adults. These results suggest that there is some plasticity in the neuromuscular system, regardless of age. The regression analysis suggests that the properties of the motoneurons, as well as the level of spinal inhibition may be important factors contributing to the upper limit on maximal motor unit firing rate.
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