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. Author manuscript; available in PMC: 2021 Aug 1.
Published in final edited form as: Med Hypotheses. 2020 Apr 9;141:109732. doi: 10.1016/j.mehy.2020.109732

Fig. 3.

Fig. 3.

Proposed model describing how age-related changes in aerobic fitness and muscle contractility often occur in tandem such that increased fatigue/fatigability would be expected to reduce the likelihood of spontaneous participation in free-living physical activity.