Exercise Physiology Studies

1 Cardiopulmonary Exercise Testing (CPET)

The two-day CPET protocol is the most important methodological contribution to ME/CFS exercise physiology research. By repeating maximal exercise testing on consecutive days, it objectively documents post-exertional malaise—the hallmark feature of ME/CFS that cannot be captured by single-day testing. Keller et al. (2024) published the largest rigorous two-day CPET study to date (n=84 ME/CFS, n=71 sedentary controls) (Keller et al. 2024). However, a 2026 null replication by Mancini, Natelson et al. (NIH-funded, n=58 ME/CFS + 25 sedentary controls) found no significant group-average Day 2 VO₂ decline (Mancini et al. 2026); elevated RPE and chronotropic incompetence were the consistent findings across both studies. The CPET decline evidence is currently at equipoise — one positive study and one equivalently powered null — and should not be presented as settled. Key findings from Keller et al. include:

  • Day 2 performance decline: ME/CFS patients show significant reductions in VO2peak (\(-\) 5–8%), peak work rate, and ventilatory anaerobic threshold on Day 2. Healthy controls and sedentary individuals show stable or improved Day 2 performance
  • Worsening impairment classification: The proportion of patients classified as severely impaired nearly doubles from Day 1 to Day 2 (14% to 27%)
  • Prolonged recovery: ME/CFS patients require 13+ days for physiological recovery versus approximately 2 days for controls
  • Earlier anaerobic threshold: ME/CFS patients reach anaerobic threshold at lower absolute and relative workloads, indicating premature shift to anaerobic metabolism
  • Independence from baseline fitness: Day 2 decrements persist when matched for baseline aerobic capacity, ruling out deconditioning as the sole explanation Lim et al. (2020) performed an earlier systematic review confirming significant workload decline at ventilatory threshold in ME/CFS patients (\(-\) 14.6 W) while controls showed improvement (+6.5 W, p=0.01) (Lim et al. 2020). The Workwell Foundation has been instrumental in standardizing CPET methodology for ME/CFS. Their work has contributed to demonstrating that ME/CFS exercise intolerance is measurable — though the specific VO₂ decline claim is contested and the group’s declared COI (fee-for-service CPET) should be weighed alongside the null replication when evaluating the evidence.

2 Muscle Studies

Skeletal muscle studies have revealed both functional and structural abnormalities in ME/CFS:

  • Lactate accumulation: ME/CFS patients show earlier and greater lactate accumulation during submaximal exercise, consistent with premature reliance on anaerobic glycolysis. This is compatible with impaired oxidative phosphorylation (Chapter Energy Metabolism and Mitochondrial Function)
  • Oxidative capacity: Muscle biopsy studies have reported reduced mitochondrial enzyme activity and decreased oxidative capacity in some ME/CFS cohorts, though results are heterogeneous across studies
  • WASF3 and ER stress: Wang et al. (2023) identified elevated WASF3 protein in ME/CFS muscle biopsies, linking endoplasmic reticulum stress to mitochondrial respiratory supercomplex disruption (Wang et al. 2023). This provides a specific molecular mechanism connecting cellular stress to exercise intolerance (Section The NIH Deep Phenotyping Study (Walitt et al. 2024))
  • Fiber type distribution: Some studies report a shift from oxidative Type I fibers to glycolytic Type II fibers, consistent with chronic disuse but also potentially reflecting metabolic reprogramming
  • Small fiber neuropathy: Reduced intraepidermal nerve fiber density has been documented in ME/CFS muscle and skin biopsies, providing a structural basis for pain and sensory symptoms

References

Keller, Betsy A, Candace N Receno, Carl J Franconi, Sebastian Harenberg, Jared Stevens, Xiangling Mao, Staci R Stevens, et al. 2024. “Cardiopulmonary and Metabolic Responses During a 2-Day CPET in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Translating Reduced Oxygen Consumption to Impairment Status to Treatment Considerations.” Journal of Translational Medicine 22 (1): 627. https://doi.org/10.1186/s12967-024-05410-5.
Lim, Eun-Jin, Eun-Bum Kang, Eun-Su Jang, and Chang-Gue Son. 2020. “Systematic Review of the Two-Day Cardiopulmonary Exercise Test as an Objective Assessment Tool for Post-Exertional Malaise in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome.” Journal of Clinical Medicine 9 (12): 4040. https://doi.org/10.3390/jcm9124040.
Mancini, Donna M., Dane B. Cook, Danielle L. Brunjes, Tiffany Soto, Michelle Blate, Patrick Quan, Tadahiro Yamazaki, Anna Norweg, and Benjamin H. Natelson. 2026. “Cardiopulmonary Exercise Test Results Do Not Change over Two Sequential Days in Patients with Chronic Fatigue Syndrome.” Frontiers in Physiology 17: 1816082. https://doi.org/10.3389/fphys.2026.1816082.
Wang, Ping-yuan, Jin Ma, Young-Chae Kim, et al. 2023. “WASF3 Disrupts Mitochondrial Respiration and May Mediate Exercise Intolerance in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome.” Proceedings of the National Academy of Sciences 120 (34): e2302738120. https://doi.org/10.1073/pnas.2302738120.