Cardiorespiratory Fitness Decline After Short-Term Experimental Bed Rest in Older Adults and Ageing-Focused Cohorts Aged 55–65 Years: A Systematic Review

Background/Objectives: Short-term bed rest may reduce cardiorespiratory capacity for mobility and rehabilitation, but gas-exchange-assessed fitness has not been synthesised in older adults. We characterised immediate V.O2peak/V.O2max change, recovery, and countermeasures. Methods: This PROSPERO-registered review (CRD420261431685) searched seven databases and two trial registries through 24 May 2026, with Embase and SPORTDiscus through 26 May. Eligible studies imposed 5–14 days of horizontal bed rest (HBR) or 6° head-down tilt bed rest (HDBR) in cohorts with mean age ≥ 60 years or ageing-focused programmes enrolling adults aged 55–65 years. Reports sharing participants and protocol formed one cohort/protocol unit. Body-mass-normalised V.O2 was primary; absolute V.O2 was separate. Risk of bias was assessed with RoB 2 or the revised JBI tool, and certainty with GRADE. Meta-analysis was not undertaken because three independent, comparable effects were unavailable. Results: Of 1964 records, 15 reports formed six units representing four to six independent participant groups. Across four HBR units (n = 9–19), V.O2peak/V.O2max declined by 4.9–15.3%; two percentages were directly reported, one was calculated from a reported change, and one from unit-adjudicated means (low certainty for direction; very low for magnitude). In one HDBR trial, changes were −13.4% ± 12.5% SD in controls (n = 9) and −3.5% ± 5.2% SD with multimodal exercise (n = 10). Single leucine, approximately 2000 steps/day, and whey comparisons provided insufficient evidence of V.O2 preservation. Recovery evidence from three programmes was heterogeneous and very uncertain; only one small HDBR cohort had approximately four-week follow-up. Conclusions: V.O2peak/V.O2max consistently declined after short-term HBR, but a precise decline, duration–response relationship, specific countermeasure prescription, and universal recovery time are not established. Minimising avoidable immobility is a plausible clinical implication, not a directly tested intervention. No external funding.