Article

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<.0001). Adjusting for differences in MVC, both firing
rate and motor unit size per newton force generated began to increase in the 6th
decade of life. Motor unit size increased per newton force to a greater extent
than firing rate. Those over the age of 75 years also activated significantly
larger motor units per unit force (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.

Langue : ANGLAIS