This blog entry is written in response to the questions I received from a couple of people. (Thanks to Curious Joe and Bunny2 for your questions).
In my previous blog, Exercise intensity, I described 3 methods of exercise intensity that are easy to use and do not require specialized testing.
However, there are situations when more sophisticated testing is needed to assess the fitness of individuals for sports (for example, elite athletes or mountain climbers), as well as certain professions (such as firefighters). In the last several decades, though, it has become apparent that the study of exercise physiology is also relevant in clinical settings. This realization has emerged from our understanding of how exercise can be used in the treatment and prevention of a number of diseases, including cardiovascular diseases, lung diseases, type 2 diabetes and several types of cancer (more on this issue will follow in a future blog entry).
VO2 max: the gold standard of aerobic fitness
VO2 max (maximal oxygen consumption, maximal oxygen uptake) is the maximum capacity of an individual's body to transport and use oxygen during incremental exercise, and reflects the aerobic or cardiopulmonary fitness of the individual. The name is derived from the following: V = volume; O2 = oxygen; and max = maximum. VO2 max is expressed in millilitres of oxygen used per kilogram of body weight per minute (mL/kg/min).
Measuring VO2 max
The most accurate and universally recognized way to measure an individual’s fitness level is by direct measurements of inspired and expired air during different stages of increasing intensity of exercise while on a treadmill or exercise bike. The volume of oxygen your body is able to use at the peak of effort on the test reflects the following:
- the capacity of the lungs to transfer oxygen to the blood;
- the ability of the heart to deliver the oxygenated blood to the working muscles; and
- the capacity of the muscles to extract the oxygen from the blood to use for energy to pedal a bicycle, or walk or jog.
You can see why the more oxygen you are able to use reflects your general cardiovascular health; as well, higher levels reflect better survival rates in individuals and fewer chronic diseases. The caveat is that this test requires expensive, sophisticated hardware and software, including a specialized computer program, face mask, gas analyzer, etc. You can watch a VO2 max test being conducted on an exercise bike by clicking here.
Heart rate, blood pressure, rate of perceived exertion and electrocardiogram are also recorded during the test.
Here is an example of a patient’s VO2 max test result from the Toronto Rehabilitation Institute:
Peak effort levels
Heart rate, beats/min
Blood pressure, mm Hg
Work load, kilopounds/meter
*From normative tables, we determine that this individual’s fitness is 60% of those who are the same age and gender with no known chronic disease (i.e. otherwise healthy individuals).
A MET is defined as the metabolic equivalent of a task. A MET can also be defined as oxygen uptake in mL/kg/min with one MET being equal to the oxygen cost of sitting quietly, which is equivalent to 3.5 mL/kg/min. Two METs means that you need twice the amount of oxygen or energy to do a particular activity than you need at rest, and so on.
In the example above, you divide the VO2 max score by 3.5. This is the patient’s maximum MET capacity: 20.3 / 3.5 = 5.8 METs
In this patient attending the rehabilitation program, his walking pace was set at 50% of VO2: 20.3 x 50% = 10 mL/kg/min = 2.85 METs
There are charts available that translate these METs into the walking pace at which oxygen consumption will be 10mL/kg/min. In this case, that is approximately one mile in 23 minutes. You can access a compendium of physical activities here; simply click on the activity you are interested in. However, a word of caution: if you are not using the exercise stress test to determine the intensity of your activity by METs, remember that METs are not relative to your individual fitness level. For example, if a person who is not fit and a person who is very fit are both jogging at an intensity equivalent to 3 METs (i.e. at the same pace), it will be very easy for the fit person, but very difficult for the unfit person.
Continuing with our patient example from the table above, here is the exercise prescription summary over the 6-month duration of the program, as the patient was able to do more:
- 1 mile in 22 minutes, 5 times per week
- 1.5 miles in 33 minutes (increased distance)
- 2 miles in 44 minutes (increased distance)
- 2.5 miles in 55 minutes (increased distance)
- 2.5 miles in 52.5 minutes (increased walking speed)
- 2.5 miles in 50 minutes (increased walking speed)
- plus resistance training
After attending the 6-month Diabetes, Exercise and Healthy Lifestyle program, here is the patient’s final report:
Fasting blood glucose: mmol/L
5.4 (decreased by 45%)
6.5 (decreased by 11%)
VO2 max, mL/kg/min
33.5 (increased by 67%)
VO2 max and type 2 diabetes
VO2 max testing is not performed routinely by endocrinologists. However, a number of rehabilitation programs perform this test, for the following reasons: to design an exercise prescription; follow up with a repeat test to see improvement in aerobic fitness; or as part of a research study.
VO2 max and METs recent research has concluded that low aerobic fitness is a hallmark of insulin resistance in people at high risk of developing type 2 diabetes. Read more about it here.