Fatigue and Perceived Energy in Healthy Ageing

Feeling energetic is part of how people experience healthspan, yet it is difficult to reduce to one organ, pathway, or laboratory value. Research separates several related constructs: current fatigue, recalled fatigue over a period, perceived energy, and the fatigue produced by a defined task. These distinctions matter because the same fatigue report can arise from very different activity levels and physiological demands. [2] [3] [4]

Who This Is Useful For

This page is useful for readers interpreting fatigue, vitality, exhaustion, energy, or fatigability in studies of healthy ageing, frailty, mobility, and cognitive function. It is especially relevant when two studies appear to examine the same symptom but use different time windows, activities, or scales. [1] [3]

Related but Distinct Constructs

Why Activity Context Matters

A person can preserve a low level of reported fatigue by walking more slowly, taking more rests, or avoiding demanding activities. A global fatigue question may therefore classify a highly active person and a person who self-limits activity similarly. Fatigability measures reduce this ambiguity by anchoring the response to a defined demand. [3] [4]

The Pittsburgh Fatigability Scale, for example, asks respondents to estimate physical and mental fatigue after activities with specified intensity and duration. Performance approaches instead use a standardized walking or cognitive task and measure perceived exertion or deterioration during it. These methods describe complementary, not interchangeable, aspects of function. [3] [9]

Energy Demand and Physiological Capacity

One mechanistic model treats fatigue as emerging partly from the relationship between the energy cost of an activity and the physiological capacity available to perform it. In a small study of well-functioning older adults, greater walking energetic cost relative to aerobic capacity was linked with greater fatigability. The result is consistent with an energetic model, but the pilot design and sample size limit broad causal conclusions. [5]

This relationship is not solely muscular. Cardiovascular and respiratory capacity, motor control, skeletal-muscle efficiency, pain, body composition, and the brain's regulation of effort can all alter the perceived cost of activity. Reviews therefore describe fatigue in later life as multidimensional, with central and peripheral processes interacting rather than one universal fatigue pathway. [1] [5] [6]

Perceived Energy Is Not a Metabolic Assay

Everyday language about having energy can sound biochemical, but self-reported energy does not measure ATP production or mitochondrial function directly. It is a whole-person perception shaped by mood, physical capacity, symptoms, expectations, sleep, and context. In the Health, Aging and Body Composition cohort, higher reported energy was cross-sectionally associated with fewer depressive symptoms, faster walking performance, and more intense activity. [2]

Because that analysis was cross-sectional, it cannot establish whether higher energy supports activity, activity supports higher energy, or both reflect other aspects of health. It does show why perceived energy can be informative without being a direct readout of one biological mechanism. [2]

Physical and Mental Dimensions

Fatigue can be described as physical, mental, motivational, emotional, or mixed. Mental fatigability concerns the fatigue associated with sustained cognitive activity, whereas physical fatigability concerns whole-body or motor activity. The dimensions can correlate, and task performance also depends on attention, motivation, sensory processing, and prior expectations. [3] [6]

A recent systematic review estimated that fatigue and perceived fatigability are common among older adults, but estimates varied greatly across studies. Population selection, definitions, measurement tools, and cut-off values contributed to this heterogeneity, so a single prevalence figure should not be treated as universal. [10]

Relation to Function and Frailty

Fatigue appears in some frailty definitions as exhaustion, but fatigue and frailty are not synonyms. An older adult can have greater fatigability before meeting a frailty classification, and fatigue can also occur for reasons not captured by frailty models. [7] [8]

Longitudinal studies have linked fatigue or greater perceived fatigability with later mobility decline, functional limitation, disability, and mortality. These associations make fatigue-related measures useful risk signals. They do not show that fatigue itself is always the causal driver, because underlying disease, low reserve, mood, sleep disruption, pain, inflammation, and activity restriction can influence both fatigue and later outcomes. [7] [8]

Evidence Quality and Interpretation

Confidence is strong that fatigue is multidimensional and that activity-normalized fatigability adds information beyond an unanchored fatigue question. Validated scales and standardized tasks support comparisons, while cohort studies consistently connect greater fatigability with poorer physical function. [3] [4] [7]

Confidence is weaker about how much any single mechanism explains an individual's perceived energy or fatigue. Much of the evidence is observational, constructs and cut-offs differ, and studies often include participants with varying disease burdens. Fatigue should therefore be interpreted as a meaningful but nonspecific feature of healthspan. [1] [2] [10]

What This Does Not Mean

• It does not mean persistent or new fatigue is an inevitable consequence of chronological ageing; fatigue can reflect many health, psychological, behavioral, and environmental factors. [1] [6]
• It does not mean feeling energetic proves that mitochondrial or metabolic function is high; perceived energy is a subjective, multidimensional report. [2]
• It does not mean a low fatigue score always indicates high capacity, because activity avoidance and self-pacing can keep fatigue within a tolerable range. [3] [4]
• It does not mean physical and mental fatigue can always be cleanly separated; they may share determinants and influence one another. [3] [6]

Practical Interpretation Examples

• If two people report equal fatigue: the person who reaches that level after a lighter or shorter activity has greater fatigability for that task. [3]
• If fatigue stays stable while activity falls: self-pacing may conceal a change in functional capacity, so activity context is necessary. [3] [4]
• If perceived energy correlates with walking speed: the finding may reflect physical capacity, mood, activity, disease burden, or bidirectional relationships rather than one metabolic cause. [2]

Related Reading

• Physical Function Trajectories Across the Lifespan
• Functional Decline and Ageing
• Frailty Phenotype vs Frailty Index
• Homeostatic Resilience in Healthy Ageing
• Intrinsic Capacity and Healthy Ageing

Summary

Fatigue and perceived energy describe lived aspects of healthspan, while fatigability connects those perceptions to the demands that produce them. Their value lies in capturing how physiological capacity, activity, brain and body function, mood, symptoms, and context converge in everyday experience. They are informative outcomes and risk signals, but neither is a single-mechanism biomarker of ageing. [1] [2] [3]

References

1. Zengarini, E., Ruggiero, C., Pérez-Zepeda, M. U., et al. (2015). Fatigue: relevance and implications in the aging population. Experimental Gerontology. https://pubmed.ncbi.nlm.nih.gov/26215825/
2. Ehrenkranz, R., Rosso, A. L., Sprague, B. N., et al. (2021). Functional correlates of self-reported energy levels in the Health, Aging and Body Composition Study. Aging Clinical and Experimental Research. https://pmc.ncbi.nlm.nih.gov/articles/PMC8531104/
3. Glynn, N. W., & Qiao, Y. (2023). Measuring and understanding the health impact of greater fatigability in older adults: a call to action and opportunities. Fatigue: Biomedicine, Health & Behavior. https://pmc.ncbi.nlm.nih.gov/articles/PMC10707490/
4. Eldadah, B. A. (2010). Fatigue and fatigability in older adults. PM&R. https://pubmed.ncbi.nlm.nih.gov/20656622/
5. Richardson, C. A., Glynn, N. W., Ferrucci, L. G., & Mackey, D. C. (2015). Walking energetics, fatigability, and fatigue in older adults: the Study of Energy and Aging Pilot. The Journals of Gerontology: Series A. https://pmc.ncbi.nlm.nih.gov/articles/PMC4447797/
6. Lin, F. V. (2023). A multi-dimensional model of fatigue in old age: implications for brain aging. The American Journal of Geriatric Psychiatry. https://pmc.ncbi.nlm.nih.gov/articles/PMC10653728/
7. Simonsick, E. M., Glynn, N. W., Jerome, G. J., et al. (2016). Fatigued, but not frail: perceived fatigability as a marker of impending decline in mobility-intact older adults. Journal of the American Geriatrics Society. https://pmc.ncbi.nlm.nih.gov/articles/PMC4914474/
8. Schrack, J. A., Simonsick, E. M., & Glynn, N. W. (2020). Fatigability: a prognostic indicator of phenotypic aging. The Journals of Gerontology: Series A. https://pmc.ncbi.nlm.nih.gov/articles/PMC7749190/
9. Glynn, N. W., Santanasto, A. J., Simonsick, E. M., et al. (2015). The Pittsburgh Fatigability Scale for older adults: development and validation. Journal of the American Geriatrics Society. https://pmc.ncbi.nlm.nih.gov/articles/PMC4971882/
10. Tian, H., Xin, Z., Lin, Z., et al. (2025). Prevalence of fatigue and perceived fatigability in older adults: a systematic review and meta-analysis. Scientific Reports. https://www.nature.com/articles/s41598-025-88961-x