No abstract

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24-37 yr of age) and 10 elderly (65-88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.

1. The purpose of this study was 1) to characterize the decrease observed in mean firing rates of motor units in the first 8-15 s of isometric constant-force contractions and 2) to investigate possible mechanisms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mean firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the first dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 19 healthy subjects during a total of 85 constant-force isometric contractions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of computer algorithms to decompose the signal into the constituent motor unit action potentials. Time-varying mean firing rates and recruitment thresholds were also calculated. 3. Motor units detected from the TA muscle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at force values ranging from 30 to 80% of maximal effort and a duration of 8-15 s. Likewise, motor units detected in the FDI muscle displayed a decrease in firing rate in 32 of 41 trials performed during constant-force isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing rates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitment thresholds and lower firing rates tended to display the greater decreases in firing rate over the constant-force interval, whereas motor units with lower recruitment thresholds and higher firing rates had lesser rates of decrease. Furthermore, increasing contraction levels tended to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the maintaining of force output while motor units decreased their firing rates: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first because none was observed during the 8-15 s duration of any of the 85 contractions. Furthermore, contractions outside the physiological range of motor unit recruitment (at 80% of maximal effort) revealed the same decreasing trend in firing rates, ruling out recruitment as the means of sustaining force output. 6. The role of agonist or antagonist muscle interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar needle electrodes from the wrist extensor muscles while myoelectric activity in the wrist flexor muscles was concurrently monitored with surface electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extensor muscles simultaneously decreased while the flexor muscl...

Daily preferential use was shown to alter physiological and mechanical properties of skeletal muscle. This study was aimed at revealing differences in the control strategy of muscle pairs in humans who show a clear preference for one hand. We compared the motor unit (MU) recruitment and firing behavior in the first dorsal interosseous (FDI) muscle of both hands in eight male volunteers whose hand preference was evaluated with the use of a standard questionnaire. Myoelectric signals were recorded while subjects isometrically abducted the index finger at 30% of the maximal voluntary contraction (MVC) force. A myoelectric signal decomposition technique was used to accurately identify MU firing times from the myoelectric signal. In MUs of the dominant hand, mean values for recruitment threshold, initial firing rate, average firing rate at target force, and discharge variability were lower when compared with the nondominant hand. Analysis of the cross-correlation between mean firing rate and muscle force revealed cross-correlation peaks of longer latency in the dominant hand than in the nondominant side. This lag of the force output with respect to fluctuations in the firing behavior of MUs is indicative of a greater mechanical delay in the dominant FDI muscle. MVC force was not significantly different across muscle pairs, but the variability of force at the submaximal target level was higher in the nondominant side. The presence of lower average firing rates, lower recruitment thresholds, and greater firing rate/force delay in the dominant hand is consistent with the notion of an increased percentage of slow twitch fibers in the preferentially used muscle, allowing twitch fusion and force buildup to occur at lower firing rates. It is suggested that a lifetime of preferred use may cause adaptations in the fiber composition of the dominant muscle such that the mechanical effectiveness of its MUs increased.

Myoelectric signals were detected from the tibialis anterior muscle of 5 subjects with a quadrifilar needle electrode while the subjects generated isometric forces that increased linearly with time (10% of maximal voluntary contraction/s) up to maximal voluntary level. Motor unit firing rates were studied as a function of force throughout the full range of muscle force output. The relationship between force and firing rate was found to contain three distinct regions. At recruitment and near maximal force levels, firing rates increased more rapidly with force than in the intermediate region. Furthermore, in the regions with rapid increases, the rate of change of firing rate was correlated to the recruitment threshold, with higher recruitment threshold motor units displaying greater rates of change. In the intermediate region, all motor units had similar rates of change of firing rate. A weak positive correlation was found between initial firing rate and recruitment threshold. Firing rates of motor units at any instant were found to be ordered according to the recruitment order: at any given time in the contraction motor units with lower recruitment thresholds had higher firing rates than units with higher recruitment thresholds. Firing rates of all motor units were observed to converge to the same value at maximal forces. Mechanisms underlying motor unit recruitment and firing rate modulation are discussed in the context of a conceptual model. © 1996 John Wiley & Sons, Inc.