Maximal Oxygen Consumption (VO2)
Oxygen consumption (VO2) is linearly related to the workload. As the exercise intensity increases, VO2 increases proportionally. However, there comes a point at which the VO2 ceases to rise even though the exercise intensity continues to rise. This point is referred to as the maximal oxygen uptake (VO2max) and is considered to be the benchmark of maximal aerobic power. In general, the VO2max is related to the amount of muscle mass used during the test in that maximal leg protocols result in a larger VO2max than maximal arm protocols.
The VO2max assesses the maximal ability of the body to deliver and utilize oxygen and is related to the ability to perform prolonged exercise. Genetic factors and training regulate the various physiological factors that contribute to the body's ability to transport oxygen and which include
a) pulmonary ventilation,
b) diffusion of oxygen from the alveoli to blood,
c) cardiac performance,
d) redistribution of blood to working skeletal muscle, and
e) extraction and utilization of oxygen by working skeletal muscle.
Two separate schools of thought have emerged to explain the primary limitation on VO2: an argument for central limitation and one for peripheral limitation. The "central" part of the oxygen delivery system depends upon the maximal cardiac output (Q) and maximal arterial oxygen content; the "peripheral" part is the extraction of oxygen from the blood (a-v O2 diff). When combined, these factors describe the VO2max which is expressed as the Fick equation: VO2max = Qmax x a-v O2max diff.
Historically, VO2max was thought to be limited by the pumping capacity of the heart. Early studies observed a relationship with heart size and stroke volume (SV) to VO2max, and that an elevated Q was thought to be the critical factor for an improved VO2max. However, others argued that the failure of SV to continue increasing at higher heart rates resulted from too short a filling time, thus if VO2 continued to increase while SV was not, Q would not be the limiting factor. Later studies showed this not to be the case.
In the late 1960s, a new era began in exercise physiology with the availability of improved techniques to study muscle blood flow and the oxidative enzymes. Subsequent studies sparked a debate that the ability of the mitochondria to utilize oxygen might be the real limitation of VO2max. There is the potential for limitation of oxygen utilization in the Kreb's cycle as a consequence of the rate-limiting enzymes.
Opinions were to swing back in the 1980s in support of the central limitation argument as more sophisticated techniques for studying blood flow concluded that the more oxygen available to the muscle, the higher the oxygen uptake. Although still somewhat controversial, the central limitation argument is held by most researchers today. Another possibility, however, is that VO2max in trained individuals is limited by oxygen availability but limited by insufficient mitochondria in untrained individuals. Regardless, it should be understood that there are many links that control the oxygen delivery system and that they each can affect VO2max.
By far, the best method to assess aerobic capacity is to measure VO2 directly in the laboratory while maximally exercising the subject. Various protocols can be used which usually yield similar results. However, treadmill tests generally give higher values than cycle protocols. This is probably due to the fact that most individuals are accustomed to walking or running but not to cycling.
The accuracy of a VO2max test can be maximized by achievement of several criteria: (1) a plateau of VO2 with an increase in workload; (2) an RER that exceeds 1.05; (3) a plateau of heart, rate and being within ±15 bpm of the subject's predicted HR max; and (4) exhaustion of the subject. Thus, the results of each test should be evaluated to whether or not a true VO2max was achieved.
While laboratory testing using indirect calorimetry is the most accurate method to determine maximal aerobic capacity, the procedure is expensive and time-consuming. Field tests were developed in order to test large numbers of subjects more quickly and easily and were based on their correlation with laboratory data. Cooper's 12-minute and 1.5-mile runs are two of the most widely known and used field tests. However, these tests also require a highly motivated subject exercising to voluntary exhaustion in order to maximize their predication ability. Not all individuals have the motivation to perform a maximal test and certain contraindications prohibit maximal testing of an individual. Consequently, tests to estimate VO2max were devised based on the heart rate (HR) response at a submaximal workload. These methods commonly utilize bench stepping, cycle ergometry, and walking/running protocols, and are able to quickly test large groups of individuals. Some of the more well-known prediction tests include the Harvard Step Test and the Åstrand-Rhyming nomogram.