title: Fitness: a new look at an old term (measurements of human aerobic performance).
abstract: Despite the increased public interest in being fit, there is no universally accepted definition of "fitness." Various reports equate fitness to oxygen consumption with exercise. This genetically determined V02 maximum is the limiting factor in high-intensity, short duration maximal aerobic exercise. For the nonathelete, however, this capacity may not be an accurate assessment of fitness. Instead, the ability of this individual to do less vigorous work of greater duration may be the best estimate of his fitness. Energy expenditure, a requirement of work, is obtained from the metabolism of carbohydrates and lipids. A measure of the relative utilization of these substrates is the respiratory quotient (R.Q.). Initial phases of exercise depend primarily on carbohydrate metabolism which is reflected by an RQ of 1.0. As exercise continues, substrate utilization relies upon an increasing percentage of free fatty acid metabolism and the R.Q. will approach 0.7. As nonathletes are trained, there is more rapid shift to free fatty acid metabolism which is reflected by a lower R.Q. in the early phases of exercise. The serial measurement of R.Q. as an individual steadily exercises below his anaerobic threshold will reflect his ability to utilize lipids as an energy source. The greater the rate of change of R.Q. with time, the greater is his ability to metabolize lipids. Increased utilization of lipids during exercise indicates his augmented capacity to do submaximal long-duration work, therefore, fitness.
mesh_terms: D003574:Cytochrome c Group / Q000378:metabolism; D004734:Energy Metabolism; D006801:Humans; D008954:Models, Biological; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism; D010101:Oxygen Consumption; D005082:Physical Exertion; D010809:Physical Fitness; D012119:Respiration
chemical_list: D003574:Cytochrome c Group