|Fig. 1: Lt. Col. David Hamilton, testing for VO2max on the treadmill administered by the Army Wellness Center. (Source: Wikimedia Commons)|
VO2max is the maximal volume of oxygen that the body can deliver to the working muscles per minute. VO2max is essentially:
This is measured in milliliters of oxygen per minute delivered to the muscles, mL/kg/min.  The amount of oxygen consumed by the muscles as an individual runs at a steadily increasing pace continues to climb until an upper limit is reached. This plateau represents the body's "maximal oxygen uptake".  Found by comparing the level of O2 inhaled to O2 exhaled using a Douglas bag and oxygen analyzers.  A VO2max test is conducted by increasing the pace of a runner on a treadmill and measuring the body's maximal oxygen intake at exhaustion, as portrayed in the figure above. Oxygen consumption calculation is achieved by evaluating the change in nitrogen content or a nomogram to compare the data. 
VO2max can often be affiliated with athletic performance yet it is in fact the Lactate Threshold and vV02 which predicts how long and effectively exercise can be performed. Lactate threshold is the intensity of exercise, or percentage of VO2max exertion at which lactate begins to be produced by the individual and enters the blood at a faster rate than the body can remove it.  This is the body's limit at which lactic acid begins to build up triggering a defensive response often immediately felt in nausea. Lactic acid spills into the blood as a result of "oxygen debt" in the muscles and glycogen reverting to lactate through glycolysis to produce ATP (energy).  Individuals with a higher lactate threshold can reach a higher percentage of their VO2max before feeling the side effects of lactate build up and being forced to stop. This is often tested through having an individual run on a treadmill and testing their blood every few minutes with a lactate analyzer to measure at which percentage point of their VO2max lactate begins to build up in their blood, as shown in the figure above. This anaerobic process begins at roughly 80 percent of VO2max for the highest performance high-endurance athletes. 
The above mentioned vVO2 signifies the speed at which an athlete is running or cycling, amount lifting, etc. when a specific point at VO2max is reached. vVO2 is essentially the actual output and work done per level of VO2max being exerted. This attributes the athletes efficiency, power, and technique as the driving force behind their performance in high intensity exercise. 
Athletes can indeed surpass their VO2max for short periods of time as the body enters an anaerobic (oxygen debt) stage. This cannot be sustained however and the time depends on the athlete's mental strength and pain tolerance. It is not healthy to operate above the VO2max threshold for long periods of time. Much of the debate surrounding VO2max and maximizing performance in athletes is what exactly controls and sets one's VO2max. Recent research points to the brain as the organ responsible for putting the brakes on exercise to avoid these dangerous limits of VO2max and Lactate Threshold which cannot be safely exceeded for a prolonged period of time.  VO2max is dynamic and can be improved with training. The test is used as a mark of physical fitness, other more common and effective tests include the 20-m shuttle run, gradually increasing speed intervals but not distance, giving an end score to mark one's fitness. This test is less scientifically accurate since it can vary with pain tolerance, while using the oxygen mask and analyzers can pinpoint one's maximal oxygen intake at exhaustion.
© Amir Bashti. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 B. Smirmaul, D. R. Bertucci, and I. P. Teixereira, "Is the VO2max That We Measure Really Maximal?" Front. Physiol. 4, 203 (2013).