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. There are a multitude of ways scientists have developed to measure and estimate an individual's cardiovascular and respiratory fitness. One such way is using an exercise stress test, either treadmill or cycling, that entails using a graded-intensity aerobic stress to assess whether or not an individual can maintain physical exertion up to a heart rate of 85% of their age-predicted maximum. Another 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. The emergence of a gold-standard method to quantify CRF began in 1923-1924. A.V. Hill et al. proposed a multifactorial relationship between the maximum rate of oxygen uptake by body tissues, termed VO2 max, and intensity of physical activity dependent upon and limited by functional capacities of an individual's cardiovascular and respiratory systems. This proposal served as an impetus for a multitude of studies demonstrating a relationship between VO2 max and cardiovascular disease (CVD) and all-cause mortality. While many methods of estimating CRF exist, each has been validated as a vital tool for predicting morbidity and mortality risk. In fact, 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. Regular exercise makes these systems more efficient by enlarging the heart muscle, enabling more blood to be pumped with each stroke, and increasing the number of small arteries in trained skeletal muscles, which supply more blood to working muscles. Exercise improves not just the respiratory system but the heart by increasing the amount of oxygen that is inhaled and distributed