Age-associated changes in motor unit physiology: observations from the Baltimore Longitudinal Study of Aging.

Abstract Ling SM, Conwit RA, Ferrucci L, Metter EJ. Age-associated changes in motor unit physiology: observations from the Baltimore Longitudinal Study of Aging. Objective To examine motor unit characteristics (size and firing rate) associated with aging. Design Cross-sectional, observational. Setting Community. Participants Baltimore Longitudinal Study of Aging participants (N=102), aged 22.2 to 94.1 years, were studied. Interventions Not applicable. Main Outcome Measures Surface-represented motor unit size and firing rate were collected from the vastus medialis during knee extension at 10%, 20%, 30%, and 50% of each subject's maximum isometric voluntary contraction (MVC). Results MVC declined with older age ( P P =.04). Relative to force generated, the average firing rate began increasing at 57.8±3.4 years and between 50.2 and 56.4 years (±4y) for motor unit size. Conclusions The size of motor units and firing rates used to achieve a given force changes with age, particularly after middle age. Whether these changes precede, follow, or occur concurrent to age-related modifications in muscle structure and contractile properties or sarcopenia is not known.