Gerontology & Geriatric Research

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Strength Loss Counteracts Age-related Improvements in Muscle Fatigue Properties when Considering Functional Task Endurance A Modeling Study

Avin Keith G, Tumuluri Aruna, Looft John MSE and Frey-Law Laura A

Background: Older adults may fatigue more easily than their younger counterparts during functional activities, yet controlled laboratory studies have demonstrated the opposite; muscle becomes more fatigue-resistant with age. We hypothesized that increased task intensities during daily activities due to strength loss may explain this apparent dichotomy. Accordingly, the purpose of this study was to model the potentially opposing influences of age-related improvements in muscle fatigue properties coupled with declining strength on predicted task endurance. Methods: We developed two mathematical models to simulate the relative influences of age-related changes in muscle fatigue and strength: 1) We adapted known endurance time-intensity fatigue models for young adults for older adults based on previously published data; and 2) We modeled changes in functional task intensity for varying levels of strength loss. Using these models, we predicted endurance times for functional tasks requiring from 10– 50% maximum strength. Results: Even small declines in strength (i.e., 10%) more than offset these age-related improvements in fatigue resistance when modeling endurance time for functional tasks. For example, a 30% decline in strength resulted in an approximately 50% reduction in functional task endurance time, particularly for low intensity tasks. Conclusions: This study provides a plausible explanation for the apparent dichotomy between laboratory fatigue studies, showing greater fatigue-resistance with advancing age, and anecdotal observations that older adults fatigue more rapidly with advancing age. Our findings suggest that declines in strength will ultimately have a larger effect on fatigue for functional tasks despite the known age-related improvements in fatigue-resistance with age. This information suggests targeted strengthening interventions may be effective in reducing muscle fatigue associated with functional activities, but future controlled trials are needed to validate these findings.