being able to use a high volume of oxygen with a high degree of efficiency is one of the best indicators of endurance fitness out there! vo2 refers to a volume of oxygen consumed, and vo2 max refers to the greatest amount of oxygen that someone can consume, which usually occurs at maximum exercise intensity. the maximal rate at which an individual can process oxygen is usually measured in milliliters of oxygen consumed in a minute per kilogram of body weight (mL/kg/min).
to give you an idea of average vo2 broken down by sex, age, and fitness level,
the most accurate way to have vo2 max tested is in a laboratory. vo2 max is considered the gold standard for measuring intensity and work capacity, but this is not easily available to the average person. however, some fitness watches have a feature to estimate vo2 max through a formula that looks at heart rate variability. the most basic formula you can use on your own to calculate your vo2 max is, 15 x (max heart rate/resting heart rate). there are also a number of other field tests and formulas that can be used to estimate vo2 max, but if you ever have the opportunity to be tested in a laboratory, this would be the most optimal data!
so while we won’t be able to see vo2 in real time during exercise, the goal of this post will be to understand the relationship between heart rate and vo2, so we can use the more convenient measure of heart rate to monitor training sessions while understanding how this affects vo2 and exercise intensity as well as overall fitness.
while heart rate and vo2 are both ways to measure the intensity of a workout, heart rate is an indirect measurement of how hard the cardiovascular system is working to distribute oxygen. whereas vo2 measures include heart rate as well as the work of the respiratory and muscular systems to directly measure the work of the cardiovascular and cardiorespiratory systems.
as intensity increases, so does energy expenditure, and vo2 reflects energy expenditure better than heart rate. energy expenditure is a function of oxygen consumption, and not necessarily of heart rate, because the heart rate responds to exercise changes as a function of many factors, whereas vo2 tends to stay the same at rest or at any given workload.
as a workout session increases in intensity and durtation, both heart rate and vo2 will increase, but their responses are not linear. at lower intensities, they may increase in a linear manner, but at higher intensities, the heart rate response will level off, while vo2 response continues to climb.
as an eample, an exerciser may climb to a 92% max heart rate, but only be at 70% vo2 max. this may be a factor of the point in the exercise session that the individual is at, as mentioned in the previos paragraph, vo2 has a linear relationship to work intensity, but heart rate response is only linear until around 75% intensity. this relationship is important, because heart rate in this case is a more reliable indicator of fitness at a given workload, whereas vo2 still remains the better indicator of energy expenditure.
another important response pattern that differs between vo2 and heart rate, is that vo2 responds slowly at the start of exercise, whereas there may be a dramatic and rapid increase in heart rate at the start of exercise. even with abrupt changes in intensity, vo2 responds slowly whereas heart rate responds abruptly at the lower intensities, and then generally changes more slowly at higher intensities. this relationship is important to understand because it helps explain certain aspects of performance such as warm up and recovery.
two major factors contribute to a high vo2 max: the amount of oxygen you can transport and your muscle physiology. better oxygen transport will lead to a higher vo2 max. muscle physiology means how many muscle fibers you have, the cross-sectional area of these muscle fibers, how many mitochondria there are, and how responsive the muscles are to the demands of exercise. more aerobic, oxygen guzzling muscles equals a higher vo2 max.
just like if you want to increase your anaerobic fitness you must train your heart rate at an anaerobic threshold, if you want to increase the capacity of your vo2 max you must also challenge it at maximal intensity, also known as vo2 max training.
the outcome that we would like to see after a period of training is, a lower heart rate at a fixed workload and the same vo2 max at a fixed workload, but the vo2 max is a lower percentage of maximum capacity, because maximum capacity has increased. these outcomes have to do with efficiency. the vo2 max increases because of an increase in the demands to the cardio respiratory network of heart, lung, and skeletal muscles; improved oxygen extraction; and improved fat metabolism. the heart rate at the fixed load decreases because the heart muscle is now stronger and can move more blood with each beat, meaning it has a higher stroke volume.
vo2 has been highly correlated to longevity. as vo2 max improves mortality risk tends to decline. as we age, especially in the later decades, fitness qualities can decline precipitously especially if an effort is not being made to maintain some level of fitness. the american heart association has called vo2 max the most important correlate of health and the strongest predictor of all cause mortality!
because oxygen is so vital to muscle function, it is important to be adept at using it efficiently!