Time to complete two tests of manual dexterity, the 9-hole Peg Test and Grooved Pegboard Test, increases with advancing age. However, the adaptations responsible for the differences in pegboard times between middle-aged and older adults are largely unknown. Potential mechanisms include neuromuscular characteristics, cognitive function, and cutaneous sensation. To provide a tractable framework to address these gaps in knowledge, the purpose of the current study was to identify the latent variables underlying age-associated differences in time to complete the 9-hole and grooved pegboard tests. The approach involved an independent component analysis that identified associations between the two pegboard times for the two groups of participants with two to six secondary outcomes. The common association across three of the four conditions (two groups and two pegboard tests) was features derived from force-matching tasks requiring submaximal isometric contraction. In addition, there were significant associations for older adults between age, measures of cognitive function, and pegboard times. Nonetheless, the significant associations were unique for each age group and pegboard test. The results provide a framework for subsequent mechanistic studies to identify the adaptations underlying age-associated declines in manual dexterity.
The purpose of our study was to examine the associations between the performance of older adults on four tests of mobility and the physical capabilities of the lower leg muscles. The assessments included measures of muscle strength, muscle activation, and perceived fatigability. Muscle activation was quantified as the force fluctuations-a measure of force steadiness-and motor unit discharge characteristics of lower leg muscles during submaximal isometric contractions. Perceived fatigability was measured as the rating of perceived exertion achieved during a test of walking endurance. Twenty participants (73 ± 4 yr) completed one to four evaluation sessions that were separated by at least 3 wk. The protocol included a 400-m walk, a 10-m walk at maximal and preferred speeds, a chair-rise test, and the strength, force steadiness, and discharge characteristics of motor units detected by high-density electromyography of lower leg muscles. Multiple-regression analyses yielded statistically significant models that explained modest amounts of the variance in the four mobility tests. The variance explained by the regression models was 39% for 400-m walk time, 33% for maximal walk time, 42% for preferred walk time, and 27% for chair-rise time. The findings indicate that differences in mobility among healthy older adults were partially associated with the level of perceived fatigability (willingness of individuals to exert themselves) achieved during the test of walking endurance and the discharge characteristics of soleus, medial gastrocnemius, and tibialis anterior motor units during steady submaximal contractions with the plantar flexor and dorsiflexor muscles. NEW & NOTEWORTHY Differences among healthy older adults in walking endurance, walking speed, and ability to rise from a chair can be partially explained by the performance capabilities of lower leg muscles. Assessments comprised the willingness to exert effort (perceived fatigability) and the discharge times of action potentials by motor units in calf muscles during submaximal isometric contractions. These findings indicate that the nervous system contributes significantly to differences in mobility among healthy older adults.
The purpose of our study was to compare the influence of five types of electrical nerve stimulation delivered through electrodes placed over the right biceps brachii on motor unit activity in the left biceps brachii during an ongoing steady isometric contraction. The electrical stimulation protocols comprised different combinations of pulse duration (0.2 and 1.0 ms), stimulus frequency (50 and 90 Hz), and stimulus current (greater or less than motor threshold). The electrical nerve stimulation protocols were applied over the muscle of the right elbow flexors of 13 participants (26 ± 3 yrs) while they performed voluntary contractions with the left elbow flexors to match a target force set at 10% of maximum. All five types of electrical nerve stimulation increased the absolute amplitude of the electromyographic (EMG) signal recorded from the left biceps brachii with high-density electrodes. Moreover, one stimulation condition (1 ms, 90 Hz) had a consistent influence on the centroid location of the EMG amplitude distribution and the average force exerted by the left elbow flexors. Another stimulation condition (0.2 ms, 90 Hz) reduced the coefficient of variation for force during the voluntary contraction and both low-frequency conditions (50 Hz) increased the duration of the mean interspike interval of motor unit action potentials after the stimulation had ended. The findings indicate that the contralateral effects of electrical nerve stimulation on the motor neuron pool innervating the homologous muscle can be influenced by both stimulus pulse duration and stimulus frequency.
Goal-directed movements that involve greater motor variability are performed with an increased risk that the intended goal will not be achieved. The ability to estimate motor variability during such actions varies across individuals and influences how people decide to move about their environment. The purpose of our study was to identify the decision-making strategies used by middle-aged and older adults when performing two goal-directed motor tasks and to determine if these strategies were associated with the time to complete the grooved pegboard test. Twenty-one middle-aged (48 ± 6 yr; range 40–59 yr, 15 women) and 20 older adults (73 ± 4 yr; range 65–79 yr, 8 women) performed two targeted tasks, each with two normalized target options. Decision-making characteristics were not associated with time to complete the test of manual dexterity when the analysis included all participants, but slower pegboard times were associated with measures of greater movement variability during the target-directed actions. When the data were clustered on the basis of pegboard time rather than age, relatively longer times for the faster group were associated with greater motor variability during the prescribed tasks, whereas longer times for the slower group were associated with increased risk-seeking behavior (α) and greater variability in the targeted actions. NEW & NOTEWORTHY This study was the first to examine the association between decision-making choices and an NIH Toolbox test of manual dexterity (grooved pegboard test) performed by middle-aged and older adults. Significant associations were observed between decision-making choices and time to complete the test when the analyses were based on pegboard times rather than chronological age. This result indicates that decision-making choices of middle-aged and older adults, independent of age, were associated with time to complete a test of manual dexterity.
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