Heart Rate versus Oxygen Consumption

Monitoring exercise intensity is a mixture of art and science. Using a only single number (no matter which variable) to fully quantify training doesn't cut it. The body is a complex and changing organism that doesn't always respond as expected. Training can be simple, complex or everything in between. In order to get the most out of yourself it may help to have a good understanding of the scientific concepts. From this grounding, the art of training can be further developed.

To begin with I need to define a few things. Yes the first part is heavy on jargon, but I have tried to stay away from too much detail and focus on the key concepts. Hopefully this provides a good enough base to see where I am coming from and heading to.

Oxygen consumption (also termed VO2) is how much oxygen the body can utilise for exercise. Oxygen is essential in the breakdown of fats and carbohydrates to the usable form of energy for cells, adenosine-tri-phosphate (ATP), without the resultant waste and fatigue inducing byproducts of anaerobic metabolism.


The following formula is a good representation of aspects that influence oxygen consumption:

VO2 = (Heart Rate x Stroke Volume) x (CaO2 - CvO2)


CaO2: is the concentration of O2 in the arteries (the blood vessels carrying oxygenated blood from the heart to the muscles.)

CvO2: is the concentration of O2 in the venous sytsem (the blood vessels carrying deoxygenated blood from the muscles back to the heart)

CaO2 - CvO2: is the arteriovenous oxygen difference. An indication of how much oxygen is used by the supplied muscles.


The arteriovenous oxygen difference is reliant on a number of factors, which include: muscle fibre types, capillary density of the muscles, mitochondrial density, aerobic enzyme concentration, rate of ATP demand, accumulation of waste by-products, muscle and blood acidity. This part of the VO2 equation reflects the ability of the muscle to utilise the oxygen that is delivered to it.


The first part of the equation: Heart Rate x Stroke Volume, which can be termed Cardiac Output reflects the body's ability to pump oxygenated blood towards the working muscles. It wouldn't matter how good the muscles are at using O2, if there wasn't enough being pumped to it in the first place. So to meet the bigger demands of higher intensity exercise, the heart increases it's cardiac output by both increasing the stroke volume (amount of blood pumped with each beat) and also the by increasing the heart rate.


Therefore the total oxygen consumption (VO2) of any intensity is the product of Cardiac Output and Oxygen Utilised At A Cellular Level.


Thresholds, thresholds and more thresholds. The sports science literature has so many different definitions of thresholds that it can be very confusing. My take on the concept is it essentially amounts to two key thresholds or transition periods where there a specific physiological changes. I'll simply things and call them Threshold 1 and Threshold 2.

Threshold 1 is when there is a noticable rise in blood lactate concentrations above resting level. This is low intensity. Resting may seen [lactate] of 1.0mmol/L and Threshold 1 may see a rise to about 2.0-3.0mmol/L that is then sustained at a steady state. At this level there is mild, but noticeable increasing in respiratory effort.

Threshold 2 is when then is a shift from a steady state intensity where blood lactate concentration can no longer be held in check and continues to rise. At this point respiratory effort will increase significantly and a large portion of energy production will be from anaerobic sources.


Putting It All Together



There are likely to be arguments about the terminology or definitions I have used. But there has been for more than 30 years over these concepts, so what I present is my interpretation, and what I believe to work.


Measurement of VO2 and VO2max in the laboratory has been shown to be very accurate across multiple tests. For example, if a runner is shown to consume 50.0ml/kg/min of O2 while running at a pace of 3:50/km in one test, then the results are very likely to be same if tested within the same week. Oxygen consumption is well accepted as being a very reliable and repeatable measure of exercise intensity. Unfortunately testing is not only a very expensive process, but requires extensive facilities to conduct the test.

The biggest issue is you can't measure your VO2 while out on a training run through some trails. This is almost impossible. Plus who would want to wear a mask, headpiece and gas analyser, on each run? Not to mention making sure everything is calibrated for ambient temperate, barometric pressures, changes in these variables throughout a training session in an uncontrolled environment (ie. outside a laboratory).

Heart Rate is much easier and cheaper to measure with the various available monitors. However, it isn't such an accurate guide of exercise intensity. So many factors other than the actual exercise intensity can vary the heart rate. These include heat, humidity, time of day, stress, hydration status, caffeine intake, rate of change of exercise intensity, time exercising and plenty of others.

For example, a person could be asked to cycle on an ergometer at a set power output of 280watts and exactly 90rpm. This may elicit a VO2 of 60ml/kg/min. If this test is repeated three times over the next week, it is more than likely the same VO2 will be elicited each time (give or take an error margin of about 2%). Heart rate (HR) may average 162bpm the first time, 178bpm and 164bpm on each occasion. Also the HR at 3, 5, 10 and 20min during each test will likely so some variation of a few beats.

What Do I Do?

I believe that VO2 testing, coupled with HRmax, blood lactate measures and power output is the most accurate way of developing a physiological profile. However, this is probably of no use if there isn't regular testing to see how the thresholds, and other aspects change in response to training. Gone are the days when I had access to this testing.

Now I base my training intensity on my experience as an athlete, past results of test and most importantly using some common sense combined with a few checks.

When redeveloping a base, I know my Threshold 1 will be relatively low, usually approaching 60%HRmax. As I develop my base this should rise to close to 70%HRmax. Threshold 2 will usually have dropped to about 82%HRmax and should climb to 90%HRmax with appropriate training. For everything else using HR is a relatively arbitrary exercise. I know what easy aerobic feels like. I know what working at a rate that gradually accumulates lactic acid feels like. I know what it feels like to be breathing at a rate and depth near my maximal oxygen uptake. Each level can be described with one word, easy, steady, hard. How I feel during training is important, sometime I do just ignore the monitor. Other times there is good reason why the number don't match.

To sum up my current training plan is a short paragraph would be: start at an easy aerobic intensity, gradually build volume then gradually increase the intensity of training. Peak with a few select sessions of high intensity training.

Comments

  1. Very interesting ... need to read a couple of time to take it all in :)

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