Deconditioning
Loss of cardiovascular and muscular fitness from inactivity — sets in within weeks, faster than people expect.
Physical deconditioning develops within a few weeks of reduced activity and produces breathlessness, exercise intolerance, and fatigue that can masquerade as ongoing disease. Where post-exertional malaise (PEM) is absent, cautious, symptom-titrated reconditioning — strength work, inspiratory muscle training, then gentle aerobic activity — genuinely helps capacity (though often not fatigue). The critical safety caveat: if PEM is present, graded or fixed-incremental exercise can cause lasting harm, and pacing within an energy envelope is required instead. Screen for PEM before any exercise advice.
How it's tested
Cardiopulmonary exercise test (CPET)
Measures cardiovascular and pulmonary capacity during exercise; a deconditioning pattern supports reversible fitness loss. Caution: maximal CPET can trigger post-exertional malaise.
2-day CPET (post-exertional malaise biomarker)
Repeating CPET on consecutive days; a reproducible drop in capacity on day 2 objectively documents post-exertional malaise. Caution: can itself trigger a crash — submaximal protocols are preferred.
Treatment options & their evidence
Graded honestly — including treatments that failed in good trials, which is worth knowing.
Resistance / strength training
Mixed evidenceCautionpromise 2 · 1 RCTA 3-month personalized resistance program improved walk distance and grip strength (JAMA Netw Open 2025) — but did NOT improve fatigue, and the population was not PEM-screened.
Caution: If post-exertional malaise is present, graded/fixed-incremental exercise can cause lasting harm — pace instead.
Inspiratory / respiratory muscle training
Mixed evidencepromise 2.6 · 3 RCTsLow-exertion breathing-muscle training improved exercise capacity and quality of life in several trials (though the largest IMT RCT missed its primary QoL endpoint). Low PEM risk.
Cardiopulmonary / pulmonary rehabilitation
Mixed evidenceCautionBenefits breathlessness and capacity in post-hospital, deconditioned (non-PEM) patients. RECOVER-ENERGIZE deliberately routes PEM patients AWAY from rehab into pacing.
Caution: Contraindicated as graded exercise where post-exertional malaise is present.
Structured pacing (energy-envelope management)
Weak evidenceThe only approach recommended when post-exertional malaise is present: manage activity within an energy envelope to avoid triggering crashes. The large RECOVER-ENERGIZE pacing RCT is due to report.