• Matthew A Schrager, Jennifer A Schrack, Eleanor M Simonsick, and Luigi Ferrucci.
• From the Department of Integrative Health Science, Stetson University, DeLand, Florida (MAS); Division of Geriatric Medicine and Gerontology, Johns Hopkins School of Medicine, Baltimore, Maryland (JAS); and Longitudinal Studies Section, Clinical Research Branch, National Institute on Aging, National Institutes of Health, Baltimore, Maryland (EMS, LF).
• Am J Phys Med Rehabil. 2014 Oct 1; 93 (10): 876-83.

ObjectiveAge-related declines in physical activity are commonly observed in human and animal populations, but their physiologic bases are not fully understood. The authors hypothesize that a lack of available energy contributes to low levels of activity in older persons.DesignCross-sectional analyses of relationships between physical activity level and energy availability were performed in 602 community-dwelling volunteers aged 45-91 yrs from the Baltimore Longitudinal Study of Aging. Energy expenditure was measured at rest and during a maximal 400-m walk for calculation of "available energy." Overall and vigorous physical activity levels were assessed using standardized questionnaires. General linear regression models were used to assess the relationships between available energy and general and vigorous physical activity, and stratified analyses were used to analyze the possible differential association between available energy and physical activity across high and low (peak sustained walking oxygen consumption per unit time, <18.3 ml of oxygen per kilogram per minute) levels of aerobic fitness.ResultsLow available energy was associated with low levels of total physical activity (β = 64.678, P = 0.015) and vigorous activity (β = 9.123, P < 0.0001). The direct relationship between available energy and physical activity was particularly strong in persons categorized as having low aerobic fitness between available energy and physical activity with both total (β = 119.783, P = 0.022) and vigorous activity (β = 10.246, P = 0.015) and was independent of body composition and age.ConclusionsThe findings from this study support the hypothesis that available energy promotes the maintenance of physical activity in older persons. The findings also run counter to the perception that age-related declines in physical activity are primarily societally or behaviorally driven.

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