Understanding Cardiorespiratory Conditioning : VO to the max?

I apologize for the corny title… exercise physiology jokes are hard to pull off. 

Cardiorespiratory conditioning refers to the body’s ability to utilize oxygen for exercise, however, it involves both aerobic and anaerobic metabolism. Efficient use of oxygen relies on several key physiological factors: pulmonary ventilation; oxygen diffusion across the lungs into the blood; the frequency with which the heart beats and volume of blood pumped with each beat; the ability of skeletal muscle to absorb oxygen from the blood; the metabolic processes to convert oxygen in to energy (ATP). 

Training to improve cardiorespiratory conditioning is important from the perspective of basic health, but also for athletic development (especially for athletes with a large endurance component to their sport, or for “tactical” athletes such as military and fire safety personnel who need substantial endurance to perform their jobs). Like any other form of exercise, training cardiorespiratory conditioning is based on the principles of progressive overload (progressively introducing more intense stimuli and then allowing the body to adapt through super-compensation).  To gauge improvement and exertion during exercise there are two important variables to consider with respect to cardiorespiratory conditioning: (1) maximal oxygen uptake and (2) energy expenditure. 

Maximal oxygen uptake is represented by the VO_2max statistic. VO_2max  is a measure of maximum aerobic capacity representing the amount of oxygen (in mL) that the body takes in per minute. Often, VO_2max  is normalized to body weight, making the units mL/kg/min (mL of oxygen per kg of body weight per minute). For unfit individuals, the VO_2max  might be around 30 mL/kg/min, whereas 60mL/kg/min is a more common level for fit athletes in non-endurance sports. In sports that have substantial endurance components (distance cycling, running, rowing, and cross-country skiing), VO_2max  tend to be much higher, around 80 mL/kg/min. The highest recorded VO_2max  belongs to Bjørn Dæhlie, 8-time Olympic gold medalist in cross country skiing.  Dæhlie’s VO_2max  was recorded to be an uncanny 96 mL/kg/min, and even this figure was taken out of season!

Energy expenditure is often expressed as kilocalories (kcals; or Calories in the US) per hour or per minute and represents the amount of energy an individual is burning over a given period of time. 1,000 mL of oxygen per minute is roughly equivalent to burning 5 kcal/min. At rest, the average person takes in about 3.5 ml/kg/min (… I usually ignore statistics that refer to a non-existent “average person”, but this figure makes a useful point). From this, you can roughly estimate your resting energy expenditure by multiplying 3.5 * (your body weight in kg) to roughly estimate your resting energy expenditure.

I weigh about 88 kg, so:

88 * 3.5 = 308 ml/min /(1,000mL) = .308 * 5kcal = 1.54 kcal/min  

Thus, over the course of a day I burn about 2,218 kcals if I’m just sitting around.

1.54 kcal/min * 60 min/hr * 24 hr/day = 2,217.6 kcal/day

So, how does this relate to training? Both oxygen consumption and energy expenditure can be used as quick tools to determine your work rate. For example, you could work a low level (say, 50%) of your VO_2max  which would be fairly easy, or at 70% were things start to be difficult, or at 90%, which you can probably only sustain for a short period of time unless you are very well trained. You might be thinking, “Okay, but how am I supposed to measure my VO_2max?” The simple answer is that you’re not, unless you have convenient access to an exercise physiologist and really like exercising in a lab. What you can do, however, is estimate your VO_2max by measuring correlated variables like heart rate (in beats per minute; BPM).

Estimating Your Maximal Aerobic Capacity

You can establish the relationship between heart rate and VO_2max  yourself by using a number of estimation techniques. Two protocols that I use when working with athletes are (1) an incremental work-rate test on a stationary bicycle and (2) the Cooper Test on a track (or treadmill).  Incremental work rate on the stationary bike is a little tricky because you need more detailed information, but the calculations are simple. First, you want to have a bicycle that can give your energy expenditure in kcal/hr (or per min) based on your age and weight (being able to input these variables will greatly improve the accuracy of the estimation). Next, you will progress through 4 stages of intensity keeping your RPMs around 60 (again, the closer you can stay to 60 RPMs, the more accurate the estimation will be). For each stage you should cycle for 2 minutes at 450 kcal/hour, 550 kcal/hour, 650 kcal/hour, and 750 kcal/hour, while recording your heart rate at the end of each 2-minute stage. 

Plot these four data points with heart rate (in bpm) on the y-axis and kcal/hr on the x-axis.  You can plot an actual regression line (using a program like Excel, or a real stats program) or just draw a straight line through these data points by eye. Using my own data as an example, it should look something like this:

The red dot is the estimated point of my maximum heart rate (MaxHR). MaxHR can be estimated by subtracting your age from 220; my estimated MHR is thus 195 bpm. This graph is a little hard to interpret because the x-axis is still in terms of energy expenditure. To understand how heart rate relates to VO_2max  we need to convert this axis into oxygen consumption (L/min). Remember that 1,000 mL/min is approximately 5 kcal/min and then we can change the units to make our new graph look like this:

Now my estimated VO_2max  is more interpretable. My estimated MaxHR (195 bpm) corresponds to about 5.52 L of oxygen consumed per minute, which, when adjusted for my mass (88 kg) is about 64.54 mL/kg/min.

5.52 * 1,000 = (5,520 mL/min) / 88kg = 64.54 mL/kg/min

We can corroborate this estimation by doing the Cooper Test. The cooper test is slightly easier to calculate and can be done on a running track or a treadmill. The goal in the Cooper Test is to run as far as you can in 12 minutes. Measure that distance in meters and then subtract 505. Divide what remains by 45 and you will have an estimate of your VO_2max in mL/kg/min. By the Cooper Test, my most recent VO2max is 69.244 mL/kg/min or 6,093 mL/min.

(3620m – 505)/45 = 69.224 mL/kg/min * 88kg = 6,091 mL/min

Looking at the results of both tests, we can see that there is pretty good agreement on my maximum oxygen consumption (and I’m inclined to believe that the Cooper Test is over estimating a little bit, but I do not know the research on any potential bias in this test). My Cooper Test estimate is shown as the purple triangle in the figure:

I’m going to try and establish a little continuity is these posts and in my next post I will talk about training at different levels or zones of intensity based on heart rate and VO_2max. By training in specific zones you can work on improving very specific dimensions of performance (such as your improving your anabolic threshold versus burning fatty acids). In the post after that, I want to address applying these metrics to recreational/novice running specifically. Stay tuned ^_^