title: Neurophysiological correlates of aging-related muscle weakness.
abstract: Muscle weakness associated with aging implicates central neural degeneration. However, role of the primary motor cortex (M1) is poorly understood, despite evidence that gains in strength in younger adults are associated with its adaptations. We investigated whether weakness of biceps brachii in aging analogously relates to processes in M1. We enrolled 20 young (22.6 ± 0.87 yr) and 28 old (74.79 ± 1.37 yr) right-handed participants. Using transcranial magnetic stimulation, representation of biceps in M1 was identified. We examined the effect of age and sex on strength of left elbow flexion, voluntary activation of biceps, corticospinal excitability and output, and short-interval intracortical and interhemispheric inhibition. Interhemispheric inhibition was significantly exaggerated in the old (P = 0.047), while strength tended to be lower (P = 0.075). Overall, women were weaker (P < 0.001). Processes of M1 related to strength or voluntary activation of biceps, but only in older adults. Corticospinal excitability was lower in weaker individuals (r = 0.38), and corticospinal output, intracortical inhibition and interhemispheric inhibition were reduced too in individuals who poorly activated biceps (r = 0.43, 0.54 and 0.38). Lower intracortical inhibition may reflect compensation for reduced corticospinal excitability, allowing weaker older adults to spread activity in M1 to recruit synergists and attempt to sustain motor output. Exaggerated interhemispheric inhibition, however, conflicts with previous evidence, potentially related to greater callosal damage in our older sample, our choice of proximal vs. distal muscle and differing influence of measurement of inhibition in rest vs. active states of muscle. Overall, age-specific relation of M1 to strength and muscle activation emphasizes that its adaptations only emerge when necessitated, as in a weakening neuromuscular system in aging.
authors.affiliation: Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio;
authors.forename: Ela B
authors.initials: EB
authors.lastname: Plow
authors.forename: David A
authors.initials: DA
authors.lastname: Cunningham
authors.forename: Corin
authors.initials: C
authors.lastname: Bonnett
authors.forename: Dina
authors.initials: D
authors.lastname: Gohar
authors.forename: Mehmed
authors.initials: M
authors.lastname: Bayram
authors.forename: Alexandria
authors.initials: A
authors.lastname: Wyant
authors.forename: Nicole
authors.initials: N
authors.lastname: Varnerin
authors.forename: Bernadett
authors.initials: B
authors.lastname: Mamone
authors.forename: Vlodek
authors.initials: V
authors.lastname: Siemionow
authors.forename: Juliet
authors.initials: J
authors.lastname: Hou
authors.forename: Andre
authors.initials: A
authors.lastname: Machado
authors.forename: Guang H
authors.initials: GH
authors.lastname: Yue
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009119:Muscle Contraction; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009433:Neural Inhibition; D011712:Pyramidal Tracts / Q000502:physiology; D012737:Sex Factors; D050781:Transcranial Magnetic Stimulation
keywords: aging; muscle weakness; primary motor cortex; strength; transcranial magnetic stimulation
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