Effects of aging and sex on voluntary activation and peak relaxation rate of human elbow flexors studied with motor cortical stimulation

Abstract

Data are equivocal on whether voluntary activation is preserved or decreased in old compared to young adults. Further, data are scant on the effect of age on the rate of muscle relaxation when the muscle is contracting voluntarily. Assessment of both measures with transcranial magnetic stimulation (TMS) yields information which cannot be obtained with traditional peripheral nerve stimulation. Hence, voluntary activation and peak relaxation rate of the elbow flexors were assessed with TMS during repeated maximal efforts in 30 men and 28 women between the ages of 22-84 years. Voluntary activation was similar for the two sexes (P00.154) and was not affected by age in men (96.2±2.7 %; P00.887) or women (95.1±3.0 %; P00.546). Men had a significantly faster peak rate of relaxation than women in absolute units (−880.0±223.2 vs. −360.2±78.5 Nm/s, respectively; P<0.001) and when normalized to subject strength (−12.5 ± 2.1 vs. −8.7±1.0 s −1 , respectively; P<0.001). Absolute and normalized relaxation rates slowed with age in men (P00.002 and P00.006, respectively), but not women (P00.142 and P00.950, respectively). Across the age range studied, all subjects, regardless of age or sex, were able to achieve high voluntary activation scores for the elbow flexors (~95 %). In contrast, peak relaxation rate was markedly faster in men than women and slowed with age in men but not women. Normalization of relaxation rates to strength did not affect the influence of age or sex.

Skeletal muscle performance and ageing

“…These studies largely report discrepant findings, but a critical examination indicates several notable observations. First, many older people, particularly those who are healthy and physically active, do not exhibit impairments in voluntary activation . Second, weaker older people, as well as the oldest old, do exhibit impairments in voluntary activation .…”

Sex differences in human fatigability: mechanisms and insight to physiological responses

“…Any observed increase in the increment in force evoked by a superimposed stimulation at the muscle or cortex during the voluntary contraction implies a failure of voluntary drive at one or more sites proximal to the site of stimulation. During brief maximal efforts, men and women are able to similarly activate and drive their motor cortex during upper limb maximal efforts before fatiguing exercise (Hunter et al., 2006a, Keller et al, 2011, Molenaar et al, 2013). Similarly, there is no sex difference in voluntary activation of the elbow flexor muscles (Miller et al, 1993, Yoon et al., 2007) and dorsiflexor muscles (Russ and Kent-Braun, 2003).…”

Innervation and neuromuscular control in ageing skeletal muscle

“…Despite lower maximal firing rates of MUs with ageing, the exact mechanism(s) contributing to this feature are not known, but it does not seem to be due to impaired voluntary activation from spinal (Jakobi & Rice, ) or supraspinal levels (Molenaar et al . ).…”