Motor unit activity, force steadiness, and perceived fatigability are correlated with mobility in older adults

Abstract

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.

Influence of sex on the age‐related adaptations of neuromuscular function and motor unit properties in elite masters athletes

“…Despite the MAs demonstrating exceptionally high ability as evidenced by their high AGP and regular physical activity (our cohort trained for an average of 5-7 h per week), functionality still declined into older age. These age-related decrements of strength (Wilkinson et al 2018) and control (Castronovo et al 2018;Mani et al 2018) have been well described, and lifelong exercisers are no exception to this, although it is probable they have attenuated balance declines when compared to age-matched controls (Leightley et al 2017).…”

“…2018; Mani et al . 2018) have been well described, and lifelong exercisers are no exception to this, although it is probable they have attenuated balance declines when compared to age‐matched controls (Leightley et al . 2017).…”

The Accurate Assessment of Muscle Excitation Requires the Detection of Multiple Surface Electromyograms

“…For readers seeking additional, compelling arguments, the selectivity of bipolar electrodes may be well substantiated by visually inspecting the amplitude of action potentials in high-density surface EMGs. Considering (27)(28)(29), a dramatic decrease in the amplitude of action potential detected by any consecutive channels covering the same target muscle can be observed consistently, if not always, in high-density recordings. Phrasing this statement interrogatively, why should the amplitude of an action potential decrease dramatically between consecutive channels if their pick-up volume was sufficiently large to include the whole target muscle, with all target fibers contributing equally to the surface EMGs?…”

Neurophysiological responses and adaptation following repeated bouts of maximal lengthening contractions in young and older adults

“…The age-related alterations in CNS also result in impairments in motor function, including the ability to control force output (for review see Ref. 23), which has important implications in performance of functional daily tasks (24,32,58) and incidence of falls (43,108). For example, older adults typically exhibit greater torque variability, particularly during dynamic contractions (45,46,93).…”