Limitations Of Measurement
So--do you recommend using vo2max or hrmax? It seems that to get an accurate HRmax number is very difficult and could be dangerous for someone who is deconditioned. However, VO2max testing isn't exactly readily available. If done by a non-experienced person, the numbers could be all over the place. Fortunately, many gyms are offering VO2max testing out here. But how reliable are the people giving the test?
Good question Tea. As usual I don't think the answer is as simple as just picking one over the other.
First things first, anyone that says they can test your VO2max with a test that doesn't involve gas analysis is wrong. To explain this I'll delve into the definition of maximal oxygen consumption, beyond it simply being the highest rate of oxygen that can be used by the exercising body.
Oxygen consumtion (VO2) is the product of cardiact output (Q) and arteriovenous oxygen difference (CaO2 - CvO2):
VO2 = Q x (CaO2 - CvO2)
This is reflected by both central and peripheral physiological variables and can be calculated by the Fick method:
Q = Heart Rate x Stroke Volume
therefore: VO2 = (Heart Rate x Stroke Volume) x (CaO2 - CvO2)
Actually measuring cardiac stroke volume and artriovenous oxygen difference are highly invasive and both practically and ethically diffulty to justify for routine measurement of VO2max. Oxygen consumption can be determined by open-circuit indirect calorimetry, involving measurement of pulmonary ventilation and comparing insired versus expired cardon dioxide and oxygen concentrations.
An incremental exercise test (usually on a cycle ergometer or treadmill) sees a linear relationship between increasing power output and VO2. As power output is increased, so does oxygen consumption. There comes a point when despite an increase in power output, oxygen consumption will no longer increase. This indicates VO2max has been attained. After this point VO2 will plateau or even decline.
The accepted criteria for VO2max is:
- A plateau in VO2 depsite increases in workload
- An R value > 1.10 (where R is the Respiratory Exchange Ratio: CO2/O2)
- A 5 min postexercise blood lactate of > 8.0mmol/L
So any test that doesn't involve the above is guesswork based on some big assumptions. The typically available at most gyms is a three-step protocol that has you either ride or run at three set intensities. These are usually a relatively low power output to play it safe from the gyms legal stand point. Heart rate at the end of each of the three steps are then plotted on a graph and a straight line is drawn through them. Then they see where the line intersects with your predicted HRmax (usually based on 220-age). Based this is then compared to a chart of VO2max values and you are given your predicted VO2max.
This can be extremely inaccurate. Personally when I was having regular laboratory VO2max evaluations (with results of 65-68ml/kg/min) the three-step gym test was giving me results of 120ml/kg/min. Which is an impossible value.
Heart Rate versus Oxygen Consumption
In my next post I'll get into how exercise intensity can be monitored, and the pros and cons of different methods.