Cardiorespiratory fitness

See also: VO2 max and Cardiovascular fitness

Cardiorespiratory fitness (CRF) refers to the ability of the circulatory and respiratory systems to supply oxygen to skeletal muscles during sustained physical activity. Scientists and researchers use CRF to assess the functional capacity of the respiratory and cardiovascular systems. These functions include ventilation, perfusion, gas exchange, vasodilation, and delivery of oxygen to the body's tissues. As these body's functions are vital to an individual's health, CRF allows observers to quantify an individual's morbidity and mortality risk as a function of cardiorespiratory health.

In 2016, the American Heart Association published an official scientific statement advocating that CRF, quantifiable as V̇O₂ max/peak, be categorized as a clinical vital sign and should be routinely assessed as part of clinical practice.^[1] Low levels of CRF have been shown to increase the risk of cardiovascular disease (CVD) and all-cause mortality.^[1][2] Some medical researchers claim that CRF is an even stronger predictor of mortality than smoking, hypertension, high cholesterol, type 2 diabetes mellitus, or other common risk factors.^[1]

Regular physical activity and exercise can improve CRF, thus decreasing risk of CVD and other conditions while improving overall health.^[3][4]

History and etymology

The emergence of a method to quantify CRF began in the 1920s when Archibald Hill, a British physiologist, proposed a multifactorial relationship between the maximum rate of oxygen uptake by body tissues and intensity of physical activity.^[5] This measure was found to be dependent upon functional capacities of an individual's cardiovascular and respiratory systems.^[5] He coined the term VO₂ max, or maximal oxygen consumption, the numerical result of exercise testing that represents the maximum rate of oxygen consumed per kilogram of body mass per minute during exercise which now serves as the primary measure of CRF. This proposal ignited a multitude of studies demonstrating a relationship between VO₂ max and cardiovascular disease and all-cause mortality.

In 2016, the American Heart Association published an official scientific statement advocating that CRF be categorized as a clinical vital sign and should be routinely assessed as part of clinical practice.^[6]

The prefix "cardio-" refers to the heart while "-respiratory" links the heart and respiratory system, which includes organs that contribute to gas exchange in plants and animals, especially the lungs (animals). Fitness refers to an individual's state of health.

Exercise

Cardiorespiratory fitness can be increased by means of regular physical activity and exercise. The medical community agrees that regular physical activity plays an important role in reducing risk of cardiovascular disease, stroke, hypertension, diabetes, and a variety of other morbid conditions.^[3][4] A 2005 Cochrane review demonstrated that physical activity interventions are effective for increasing CRF,^[7] while other studies have determined that improved CRF is associated with lower risk of CVD and all-cause mortality.^{[8][9][2][10]}

Multiple forms of exercise exist and are all generally beneficial to an individual's health (endurance running, weightlifting, sports activity, etc.), but studies show that high intensity interval training (HIIT) is highly effective in increasing CRF and VO₂ max in people of all ages.^[11][12][13] A 2020 review of the literature by Wu et al. concluded that HIIT is effective in increasing CRF, physical fitness, muscle power, cardiac contractile function, and reducing blood triglycerides in older individuals.^[11]

Measurement

A method of estimating CRF entails using formulas, derived from extrapolated regressive analyses, to predict a theoretical level of CRF. These formulas take into consideration an individual's age, sex, BMI, substance use, relative levels of physical activity, and pathologic co-morbidites. In 2016, Nauman and Nes et al. demonstrated the added and unique utility of estimated cardiorespiratory fitness (eCRF) in predicting risk of cardiovascular disease and all-cause mortality.^[14]

Various methods of measurement exist for determining an individual's cardiorespiratory fitness. VO₂ max is the most commonly accepted indicator of CRF and has been since the 1960s.^[15] Cardiopulmonary exercise testing (CPET) with spirometry is the gold standard for determining VO₂ max. It requires the individual to perform exercise with analysis of gas exchange usually until maximal exertion is achieved. The use of electrocardiography is often used to examine heart response to exercise and exertion.^[16] CPET is performed on a treadmill or a cycle ergometer. The method of test administration is based on the abilities of the test subject, as the cycle ergometer is generally less taxing on the body and often better suited for elderly populations, although is shown to sometimes produce results 10% - 20% lower in individuals not accustomed to cycling due to leg fatigue.^[16]

In many cases, children or the elderly are not subjected to the vigor of cardiopulmonary exercise testing. There are other methods used to mathematically estimate the VO₂ max of a test subject by having the subject walk or jog a certain distance in as little time as possible, complete the maximum number of repetitions of a short-distance run (commonly known as the PACER test in the United States), or walk on a treadmill at increasing incline until a sub-maximal goal is achieved, along with others.^[17]

Cardiovascular system adaptations

The cardiovascular system responds to changing demands on the body by adjusting cardiac output, blood flow, and blood pressure. Cardiac output is defined as the product of heart rate and stroke volume which represents the volume of blood being pumped by the heart each minute. Cardiac output increases during physical activity due to an increase in both the heart rate and stroke volume.^[18]

See also

• Aerobic conditioning
• Central governor
• Physical fitness
• Exercise physiology
• VO₂ max

References

1. Ross, Robert; Blair, Steven N.; Arena, Ross; Church, Timothy S.; Després, Jean-Pierre; Franklin, Barry A.; Haskell, William L.; Kaminsky, Leonard A.; Levine, Benjamin D.; Lavie, Carl J.; Myers, Jonathan; Niebauer, Josef; Sallis, Robert; Sawada, Susumu S.; Sui, Xuemei (2016-12-13). "Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association". Circulation. 134 (24): e653–e699. doi:10.1161/CIR.0000000000000461. ISSN 0009-7322. PMID 27881567. S2CID 3372949.
2. Kodama, Satoru (2009-05-20). "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. doi:10.1001/jama.2009.681. ISSN 0098-7484. PMID 19454641.
3. Foster, Charles; Hillsdon, Melvyn; Thorogood, Margaret; Kaur, Asha; Wedatilake, Thamindu (2005-01-24). Cochrane Heart Group (ed.). "Interventions for promoting physical activity". Cochrane Database of Systematic Reviews (1): CD003180. doi:10.1002/14651858.CD003180.pub2. PMC 4164373. PMID 15674903.
4. Lee, Duck-chul; Sui, Xuemei; Artero, Enrique G.; Lee, I-Min; Church, Timothy S.; McAuley, Paul A.; Stanford, Fatima C.; Kohl, Harold W.; Blair, Steven N. (2011-12-06). "Long-Term Effects of Changes in Cardiorespiratory Fitness and Body Mass Index on All-Cause and Cardiovascular Disease Mortality in Men: The Aerobics Center Longitudinal Study". Circulation. 124 (23): 2483–2490. doi:10.1161/CIRCULATIONAHA.111.038422. ISSN 0009-7322. PMC 3238382. PMID 22144631.
5. "Muscular exercise, lactic acid and the supply and utilisation of oxygen.— Parts VII–VIII". Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character. 97 (682): 155–176. December 1924. doi:10.1098/rspb.1924.0048. ISSN 0950-1193.
6. Ross, Robert; Blair, Steven N.; Arena, Ross; Church, Timothy S.; Després, Jean-Pierre; Franklin, Barry A.; Haskell, William L.; Kaminsky, Leonard A.; Levine, Benjamin D. (2016-12-13). "Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association". Circulation. 134 (24): e653–e699. doi:10.1161/CIR.0000000000000461. ISSN 0009-7322. PMID 27881567. S2CID 3372949.
7. Hillsdon, M.; Foster, C.; Thorogood, M. (2005-01-25). "Interventions for promoting physical activity". The Cochrane Database of Systematic Reviews (1): CD003180. doi:10.1002/14651858.CD003180.pub2. ISSN 1469-493X. PMC 4164373. PMID 15674903.
8. Myers, Jonathan; Prakash, Manish; Froelicher, Victor; Do, Dat; Partington, Sara; Atwood, J. Edwin (2002-03-14). "Exercise Capacity and Mortality among Men Referred for Exercise Testing". New England Journal of Medicine. 346 (11): 793–801. doi:10.1056/NEJMoa011858. ISSN 0028-4793. PMID 11893790.
9. Blair, Steven N.; Brodney, Suzanne (1999-11-01). "Effects of physical inactivity and obesity on morbidity and mortality: current evidence and research issues". Medicine & Science in Sports & Exercise. 31 (Supplement 1): S646-62. doi:10.1097/00005768-199911001-00025. ISSN 0195-9131. PMID 10593541.
10. Cao, Chao; Yang, Lin; Cade, W. Todd; Racette, Susan B.; Park, Yikyung; Cao, Yin; Friedenreich, Christine M.; Hamer, Mark; Stamatakis, Emmanuel; Smith, Lee (2020-01-30). "Cardiorespiratory Fitness Is Associated with Early Death Among Healthy Young and Middle-aged Baby Boomers and Generation Xers". The American Journal of Medicine. 133 (8): 961–968.e3. doi:10.1016/j.amjmed.2019.12.041. ISSN 0002-9343. PMID 32006474.
11. Wu, Zhi-Jian; Wang, Zhu-Ying; Gao, Hao-En; Zhou, Xian-Feng; Li, Fang-Hui (2021-07-15). "Impact of high-intensity interval training on cardiorespiratory fitness, body composition, physical fitness, and metabolic parameters in older adults: A meta-analysis of randomized controlled trials". Experimental Gerontology. 150: 111345. doi:10.1016/j.exger.2021.111345. ISSN 0531-5565. PMID 33836261. S2CID 233131437.
12. Knowles, Ann-Marie; Herbert, Peter; Easton, Chris; Sculthorpe, Nicholas; Grace, Fergal M. (2015-03-14). "Impact of low-volume, high-intensity interval training on maximal aerobic capacity, health-related quality of life and motivation to exercise in ageing men". AGE. 37 (2): 25. doi:10.1007/s11357-015-9763-3. ISSN 1574-4647. PMC 4359174. PMID 25773069.
13. Martin-Smith, Rhona; Cox, Ashley; Buchan, Duncan S.; Baker, Julien S.; Grace, Fergal; Sculthorpe, Nicholas (January 2020). "High Intensity Interval Training (HIIT) Improves Cardiorespiratory Fitness (CRF) in Healthy, Overweight and Obese Adolescents: A Systematic Review and Meta-Analysis of Controlled Studies". International Journal of Environmental Research and Public Health. 17 (8): 2955. doi:10.3390/ijerph17082955. ISSN 1660-4601. PMC 7215828. PMID 32344773.
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16. Fletcher, Gerald F.; Ades, Philip A.; Kligfield, Paul; Arena, Ross; Balady, Gary J.; Bittner, Vera A.; Coke, Lola A.; Fleg, Jerome L.; Forman, Daniel E.; Gerber, Thomas C.; Gulati, Martha; Madan, Kushal; Rhodes, Jonathan; Thompson, Paul D.; Williams, Mark A. (2013-08-20). "Exercise Standards for Testing and Training: A Scientific Statement From the American Heart Association". Circulation. 128 (8): 873–934. doi:10.1161/CIR.0b013e31829b5b44. ISSN 0009-7322. PMID 23877260.
17. Buttar, Karampreet Kour; Kacker, Sudhanshu; Saboo, Neha (2022). "Normative Data of Maximal Oxygen Consumption (VO2 Max) among Healthy Young Adults: A Cross-sectional Study". Journal of Clinical and Diagnostic Research. doi:10.7860/jcdr/2022/53660.16672. ISSN 2249-782X. S2CID 251203633.
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