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<.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.