Abstract
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.
Highlights
Sarcopenia [1,2,3], the age-related loss of muscle mass, and dynapenia, [2,4], the age-related loss of muscle strength, are increasingly recognized as significant adverse detriments often associated with advanced age
We propose that age-related strength loss, e.g., sarcopenia, and the resulting increase in relative task intensity for any given task with age, may be sufficient to offset the age-related physiological improvements in fatigueresistance
Age-adjusted joint-specific endurance time models The old adult endurance time-intensity curves for the knee and elbow shifted to the right by approximately 16% and 23%, respectively (Figure 1)
Summary
Sarcopenia [1,2,3], the age-related loss of muscle mass, and dynapenia, [2,4], the age-related loss of muscle strength, are increasingly recognized as significant adverse detriments often associated with advanced age. Two recent meta-analyses have supported an increase in age-related fatigue resistance based on controlled laboratory conditions; reporting overall moderate effect sizes on the order of 0.49 [8] and 0.56 [9] based on 46 and 37 studies, respectively. These controlled laboratory conditions rely largely on normalized static workloads, i.e., isometric contractions performed at a percentage of each individual’s maximum strength, to assess for differences in muscle fatigue properties. 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
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