OXYGEN CONSUMPTION AND EXERCISE

By Nenad Rodic,
04/05

The amount of oxygen used by muscles is directly proportional to the intensity of exercise until the maximum heart rate is reached. If the intensity of exercise is further increased after that, the oxygen consumption can decrease due to high-energy demands that can only be met through anaerobic systems engagement.
Maximum values for oxygen consumption (VO2 max) are measured by Conconi test*. In well-trained endurance athletes VO2 max usually varies between 4-6 L of oxygen per minute. In sports like triathlon, the relative value of VO2max (absolute value divided by weight of an athlete) represents a more useful parameter than the absolute value. This is due to the power-to-weight ratio nature of our sport. The smaller you are, the higher your relative VO2max will be, meaning that you’ll be able to exercise and race at a higher rate. This explains why pro cyclists are so thin. Pure power is simply overrated. Fitness is everything in our sport and VO2max is a true measure of fitness. The highest VO2max ever recorded was 94 mlO2/kg/min (cross country skier). World-class triathletes’ record values fall between 70-85 ml/kg/min.

*The Conconi test is a simple method for measuring the values of an individual's maximum, anaerobic and aerobic threshold rates. In the Conconi test the athlete increases their speed gradually (by 0.5 km/h) every 200 meters and the heart rate at each 200 meter point, is recorded. This gradual increase in speed every 200 meters is maintained until the athlete is unable to maintain the pace. Other protocols can be used instead of 200 m one. The test is often performed on a bike ergo meter in which case power is increased every minute or less often by 20-50 W. Oxygen uptake is measured by the gas analyzer. Data gathered from this machine is used to determine anaerobic threshold and maximum oxygen uptake. If gas analyzer is not available a simple heart rate monitor can be used to determine the approximate value of the anaerobic threshold easily. Determining your maximum oxygen uptake with a heart rate monitor is not very accurate and therefore not very useful.

During exercise, it is possible that oxygen requirements may not be met. This will create oxygen “debt” that can and must be “repaid” during the remainder of the exercise or during the resting period. You’ve probably experienced going into oxygen debt during the first few hundred yards of the swim of a triathlon. It sometimes may be necessary to push a little harder in order to secure a good position.
VO2 relates directly to the intensity of exercise, and the correlation between perceived effort and actual effort (oxygen consumption) is approximately as follows:

  • 50-60% of VO2max=30-40% of max perceived;
  • 70-90 % of VO2max=60-80 % of max perceived;
  • 100 % of VO2max= 80-90% of max perceived;
  • 90-100% of perceived maximum are probably equal to 110-130 % of VO2max.

This would depend largely on your anaerobic capacity. In case you are not conditioned to do much anaerobic work, a 100% perceived can come close to 100% of VO2max.

By Nenad Rodic, founder of TriathlonPlace.com