Fatigue vs Fatigability in Healthy Ageing Research

Fatigue and fatigability answer different research questions. Fatigue asks how tired or lacking in energy a person feels. Fatigability asks how readily fatigue appears, or performance changes, under a defined demand. The distinction is especially important in ageing studies because participants can adapt their pace and activity exposure as physical or cognitive capacity changes. [1] [2] [3]

Who This Is Useful For

This page is useful for readers comparing studies that use terms such as tiredness, exhaustion, perceived exertion, performance decline, physical fatigability, or mental fatigability. These labels are not interchangeable, and differences in task, time window, intensity, and outcome can change what a result means. [2] [4] [9]

The Core Distinction

Why the Activity Denominator Matters

An unanchored fatigue score contains no direct information about the demand preceding the report. Two people can describe the same fatigue level even if one remained active and the other reduced walking, shortened tasks, or took more rests. This self-pacing can make global fatigue appear stable while functional reserve is changing. [1] [2] [3]

Fatigability measures add an activity denominator by holding some aspect of demand constant. The demand may be described in a questionnaire, imposed through a treadmill or corridor walk, or quantified from performance over time. Anchoring improves comparison, but results remain dependent on the chosen activity and protocol. [2] [4]

Common Measurement Approaches

Global fatigue is usually measured with a direct symptom question or a multi-item questionnaire over a stated time window. The Pittsburgh Fatigability Scale instead asks respondents to estimate physical and mental fatigue after ten activities with specified intensity and duration, producing separate physical and mental scores. [3] [9]

Laboratory or clinic approaches can measure perceived exertion after a standardized walk, deterioration in lap time or speed during a longer walk, or other task-specific changes. A five-minute slow treadmill walk followed by a Borg rating and a 400-m corridor walk have been studied in mobility-intact older adults, but they capture different expressions of fatigability. [4] [6]

Perception and Performance Can Diverge

A participant can report high effort without showing marked performance deterioration, or slow during a task without reporting severe fatigue. Perceived and performance fatigability should therefore be treated as complementary outcomes rather than alternative labels for the same measurement. [2] [4]

This divergence is not necessarily measurement failure. Subjective effort integrates bodily signals, expectations, mood, motivation, and prior experience, whereas performance also depends on task design, pacing strategy, motor control, and the output selected by the study. [4] [8]

Physical and Mental Fatigability

Physical fatigability concerns susceptibility to whole-body fatigue during physical demands. Mental fatigability concerns fatigue linked to sustained cognitive or mentally demanding activity. The two dimensions can overlap, but validation work supports reporting them separately because their correlates and roles in functional decline may differ. [2] [9]

A cognitive task may also show performance decline without matching the participant's fatigue report. As with physical testing, researchers need to state whether the outcome is perceived fatigue after the task, change in objective performance, or both. [8] [9]

Physiological Capacity and Task Cost

One model relates fatigability to the fraction of available capacity required by an activity. In a small pilot study of well-functioning adults aged 70 to 89, slower walkers used a larger proportion of their aerobic capacity during walking and showed greater fatigability, while global fatigue measures did not distinguish slow and fast walkers. [5]

This result is consistent with an energetic explanation but does not establish a single mechanism. Cardiorespiratory fitness, muscle function, motor control, pain, sleep, mood, inflammation, disease burden, cognition, and activity patterns can all contribute to perceived or performance fatigability. [2] [5] [8]

Relation to Functional Ageing

In mobility-intact older adults, greater perceived exertion after a standardized slow walk predicted subsequent decline in gait speed, physical performance, and reported walking ability over approximately two years. Global tiredness and energy reports did not predict the objective performance outcomes in the same analysis. [6]

Reviews and cohort studies also associate greater fatigability with mobility limitation, disability, and mortality. These are prognostic associations, not proof that fatigability independently causes the outcomes, because baseline health, fitness, comorbidity, and behavior can influence both the measure and later risk. [2] [7]

Evidence Quality and Interpretation

Confidence is strong that global fatigue and fatigability are distinct constructs and that anchoring a report to standardized demand reduces ambiguity from unequal activity exposure. Several perceived and performance measures have been validated in older cohorts. [1] [3] [4]

Confidence is lower when comparing prevalence or effect sizes across studies. Definitions, cut-offs, tasks, time windows, and study populations vary, and much of the outcome evidence is observational. Recent review evidence reports substantial heterogeneity and warns against treating one prevalence estimate as universal. [2] [7] [10]

What This Does Not Mean

• It does not mean fatigue questions are uninformative; they measure a meaningful subjective symptom, but not the demand that produced it. [1] [8]
• It does not mean fatigability is fully objective; perceived fatigability is self-reported even when the activity is standardized. [2] [3]
• It does not mean performance fatigability identifies one biological cause; task performance reflects multiple physiological and behavioral processes. [4] [8]
• It does not mean greater fatigability is inevitable with age or equivalent to frailty, disability, or disease. [6] [7]

Practical Interpretation Examples

• If fatigue is unchanged but activity falls: the stable symptom score may conceal greater fatigability because the person is reaching the same fatigue at a lower demand. [1] [2]
• If exertion is high after a standardized slow walk: the result indicates greater perceived fatigability for that protocol, not necessarily poor performance on every task. [4] [6]
• If walking speed deteriorates but perceived effort stays low: performance and perception have diverged, so both outcomes should be reported rather than collapsed into one fatigue label. [2] [4]

Related Reading

• Fatigue and Perceived Energy in Healthy Ageing
• Physical Function Trajectories Across the Lifespan
• Frailty Phenotype vs Frailty Index
• Six-Minute Walk Test and Functional Capacity in Older Adults
• Intrinsic Capacity and Healthy Ageing

Summary

Fatigue describes how a person feels; fatigability describes that feeling or a performance change in relation to a defined demand. This distinction makes fatigability useful when ageing-related self-pacing could hide changing capacity. Perceived and performance fatigability remain separate, task-dependent measures, and neither identifies a single mechanism of ageing. [1] [2] [4]

References

1. Eldadah, B. A. (2010). Fatigue and fatigability in older adults. PM&R. https://pubmed.ncbi.nlm.nih.gov/20656622/
2. Glynn, N. W., & Qiao, Y. S. (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/
3. 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/
4. Simonsick, E. M., Schrack, J. A., Glynn, N. W., & Ferrucci, L. (2014). Assessing fatigability in mobility-intact older adults. Journal of the American Geriatrics Society. https://pmc.ncbi.nlm.nih.gov/articles/PMC3947865/
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. 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/
7. 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/
8. 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/
9. Renner, S. W., Bear, T. M., Brown, P. J., et al. (2021). Validation of perceived mental fatigability using the Pittsburgh Fatigability Scale. Journal of the American Geriatrics Society. https://pmc.ncbi.nlm.nih.gov/articles/PMC8127403/
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