1. Cardiorespiratory Endurance

Written By Jennifer Pishko

A Clinical Vital Sign for Chronic Disease Prevention

Prepared by the CrossFit Medical Society; with contributions from Dr. Tom McCoy, Dr. Janette Watkins, and Jennifer Pishko, MSNE

Key Takeaways:

  • CRF Strongly Predicts Health Outcomes: High cardiorespiratory fitness is one of the most powerful predictors of all-cause and disease-specific mortality.

  • Each MET Matters: Metabolic Equivalent of Task (MET) is a standardized unit used to express the energy cost of physical activities, defined as the amount of oxygen consumed at rest, equivalent to 3.5 mL of oxygen per kilogram of body weight per minute (3.5 mL/kg/min). It provides a simple way to describe exercise intensity and compare cardiorespiratory fitness (CRF) levels across individuals. Fitness improvements are dose-responsive. For every 1-MET increase in work capacity an individual can perform, there is an associated reduction in all-cause mortality.

  • Aesthetics don’t matter: Unfit individuals, regardless of BMI, face 2–3 times higher mortality risk.

A 2025 umbrella review by Lang et al. published in the British Journal of Sports Medicine reinforces the critical role of fitness in long-term health and disease prevention [1]. This comprehensive analysis synthesized data from 26 meta-analyses, incorporating 199 cohort studies and over 20.9 million observations. The conclusion is clear: cardiorespiratory fitness (CRF) is among the most robust predictors of morbidity and mortality, exceeding the predictive power of conventional risk factors such as smoking, high blood pressure, and high cholesterol. For CrossFit coaches, this presents a vital opportunity to redefine success—not just in terms of physical performance, but as a means of promoting health and reducing the risk of chronic disease [2].

The findings are compelling. Individuals with high CRF had a 53% lower relative risk of all-cause mortality compared to those with low CRF [3]. High CRF is defined when an individual can complete greater than 12-METs in one bout of exercise. Comparatively, low CRF is less than 8-METs [4]. The ability to perform 1 additional MET (Metabolic Equivalent of Task) was linked to an 11–17% reduction in relative risk of mortality. These benefits extend well beyond heart health: high CRF is associated with substantially lower relative risk of developing heart failure (by as much as 69%), as well as hypertension, type 2 diabetes, stroke, atrial fibrillation, chronic kidney disease, dementia, depression, and several cancers [5]. 

Crucially, these associations exist regardless of body mass index. In the study by Lang et al. those who are overweight or obese and physically fit do not face higher mortality or cardiovascular risk than their leaner counterparts [6]. Conversely, being unfit—regardless of weight—was linked to 2–3 times higher relative risk of death [7].  This underscores a critical shift in how we discuss health and body composition in the gym. It’s not about appearance—it’s about physiological function. And as the data shows, CRF is one of the most powerful and modifiable indicators of that function.

Programming that emphasizes constantly varied, functional movements executed at high intensity has been shown to increase CRF. Participation in structured CrossFit training has been shown to produce meaningful gains in cardiorespiratory fitness. VO₂max is a key measure of cardiorespiratory fitness (CRF) and can be expressed in METs, where 1 MET equals 3.5 mL/kg/min of oxygen uptake. In the StartXFit study, CrossFit beginners improved their average VO₂max by approximately 6 mL/kg/min (about 1.7 METs) over nine months of training [8]. Notably, participants in the lowest baseline fitness tertile showed the greatest gains, likely achieving improvements of roughly 2.5–3 METs. Such substantial increases in cardiorespiratory fitness are especially meaningful, as each 1 -MET gain is associated with an estimated 11–17% relative reduction in all-cause mortality risk. This highlights the particular value of structured, high-intensity functional training for those starting with low fitness levels [9]. This principle is central to the CrossFit methodology. The Level 1 Guide identifies cardiorespiratory endurance as one of the “10 General Physical Skills” essential for fitness and health. It is defined as the “ability of body systems to gather, process, and deliver oxygen.” CrossFit training—through repeated efforts of dynamic, full-body movements—maximizes this ability, improving VO₂ max [10].

So how can CrossFit coaches translate this science into practice?

First, we are improving a metric that predicts members’ longevity and quality of life. We don’t need medical equipment to make an impact. The CrossFit stimulus itself, when appropriately scaled and consistently applied, drives the kind of systemic adaptations that build CRF. Coaches can use time domains, pacing, and thoughtful progressions to ensure athletes are hitting the intensities required for cardiovascular adaptation, while also respecting individual capacity and recovery needs.

Second, we can educate athletes about why CRF matters. Many still believe health is about body weight or blood pressure alone. But helping them understand that their ability to move, breathe, and recover is a more powerful indicator of health outcomes than their appearance can be liberating—and motivating. It reorients the conversation away from aesthetics and toward functionality. This aligns closely with CrossFit’s core philosophy: "fitness as work capacity across broad time and modal domains [11]."

Third, we can work to make CRF visible. While most gyms don’t have metabolic carts or lactate threshold testing equipment, we can still track fitness in practical ways. Benchmark workouts can serve as proxy measures of aerobic and anaerobic capacity. Coaches can create repeatable test-retest opportunities, giving athletes tangible evidence of their improving CRF over time. It’s not just about PRs—it’s about long-term progress in performance and health.

It’s also worth noting the mental health benefits. The Lang et al. review found consistent evidence that higher CRF is associated with reduced risk of depression and cognitive decline [12]. In addition, a 2020 article in the BMD Medicine Journal looking at a 7-year prospective cohort study with 152,978 adults showed that low and medium CRF were associated with 49% and 14% higher odds of depression or anxiety, respectively, compared to high CRF. Individuals with both low CRF and low grip strength had nearly 2x the odds of developing such disorders [13].   This intersects with growing awareness in the CrossFit community of the links between movement, mood, and mental resilience. High-intensity training supports not only physical, but emotional well-being—a dual benefit that coaches should continue to preach.

This work contributes to a broader movement advocating for fitness to be recognized as a medical necessity rather than a luxury by clinicians and fitness professionals. With increasing chronic disease rates and growing pressure on healthcare systems, effective preventative measures like CrossFit training are becoming crucial. CrossFit coaches are uniquely positioned to drive this change, thanks to our combination of technical expertise, motivational abilities, and community engagement.

The message is clear: cardiorespiratory fitness improves healthspan and lifespan. And that makes CrossFit affiliates more important than ever.

Whether we’re training seasoned athletes or beginners, we’re contributing to one of the most powerful health interventions available. CRF is the new vital sign—and it’s one we can help improve every single day. In a world where chronic disease is often seen as inevitable, CrossFit stands out as a method that not only builds better athletes—but builds better lives.

Action Steps for Coaches:

  • Educate athletes: Shift the conversation from weight loss to functional fitness and long-term health.

  • Track improvements: Use benchmark workouts to measure improvements in work capacity over time.

  • Scale with purpose: The largest reductions in mortality risk occur when those with the lowest fitness improve even modestly..

 1. Lang, J. J., Prince, S. A., Merucci, K., Cadenas-Sanchez, C., Chaput, J. P., Fraser, B. J., … & Tomkinson, G. R. (2024). Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults: an overview of meta‑analyses representing over 20.9 million observations from 199 unique cohort studies. British Journal of Sports Medicine, 58(10), 556–566. https://bjsm.bmj.com/content/58/10/556#ref-24

2. CrossFit, Inc. (n.d.). CrossFit Level 1 Training Guide [PDF]. CrossFit Library. https://library.crossfit.com/free/pdf/CFJ_English_Level1_TrainingGuide.pdf

3. Lang, J. J., Prince, S. A., Merucci, K., Cadenas-Sanchez, C., Chaput, J. P., Fraser, B. J., … & Tomkinson, G. R. (2024). Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults: an overview of meta‑analyses representing over 20.9 million observations from 199 unique cohort studies. British Journal of Sports Medicine, 58(10), 556–566.

4. Kodama, S., Saito, K., Tanaka, S., Maki, M., Yachi, Y., Asumi, M., … Sone, H. (2009). Cardiorespiratory fitness as a quantitative predictor of all‑cause mortality and cardiovascular events in healthy men and women: A meta‑analysis. JAMA, 301(19), 2024–2035. https://doi.org/10.1001/jama.2009.681.

5. Lang, J. J., Prince, S. A., Merucci, K., Cadenas-Sanchez, C., Chaput, J. P., Fraser, B. J., … & Tomkinson, G. R. (2024). Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults: an overview of meta‑analyses representing over 20.9 million observations from 199 unique cohort studies. British Journal of Sports Medicine, 58(10), 556–566.

6. Ibid.

7. Ibid.

8.  Schlie, J., Brandt, T., & Schmidt, A. (2023). StartXFit—Nine Months of CrossFit® Intervention Enhance Cardiorespiratory Fitness and Well-Being in CrossFit Beginners. Physiologia, 3(4), 494–509. https://doi.org/10.3390/physiologia3040036.

9. Ibid.

10.  CrossFit, Inc. (n.d.). CrossFit Level 1 Training Guide [PDF]. CrossFit Library. https://library.crossfit.com/free/pdf/CFJ_English_Level1_TrainingGuide.pdf.

11. Ibid.

12.  Lang, J. J., Prince, S. A., Merucci, K., Cadenas-Sanchez, C., Chaput, J. P., Fraser, B. J., … & Tomkinson, G. R. (2024). Cardiorespiratory fitness is a strong and consistent predictor of morbidity and mortality among adults: an overview of meta‑analyses representing over 20.9 million observations from 199 unique cohort studies. British Journal of Sports Medicine, 58(10), 556–566.

13.  Kandola, A., Ashdown-Franks, G., Hendrikse, J., Sabiston, C. M., & Stubbs, B. (2020). Physical fitness and the risk of depression and anxiety: A prospective cohort study. BMC Medicine, 18(1), 168. https://doi.org/10.1186/s12916-020-01782-9.

Jennifer Pishko