runner-training-zoneIn order to put together a training programme, we need to divide our exercises into different intensity levels.

Each intensity is aimed at stressing the body in different ways, and hence lead to different kinds of adaptation. These different exercise intensities are often called “training zones”.

Confusingly, there are three different (but related) tools that are commonly used to define training zones:

  • you can define your exercise intensity as a percentage of VO2 max – that is, as a proportion of maximum capacity to turn oxygen into energy (see below); for example, easy running is 65% to 75% of VO2 max; and your threshold run effort level is about 75-90% of VO2 max;
  • or you can define intensity according to your heart rate; because of the availability of relatively cheap heart rate monitors, this is a convenient way to specify different levels of exertion.  The relationship between percentage of working heart rate and percentage of VO2 max is complicated, as we shall see;
  • or you can judge your exercise intensity by considering your pace or stroke rate; for example, the easy running zone is slower than marathon race pace; the threshold zone is about your half marathon race pace.



Personally, I reckon the most useful way to define and monitor training intensity is to use pace.  This seems to me to be both convenient and have the strongest scientific and empirical underpinning.  But since there are lots of athletes who use VO2 max or heart rates to describe their exercise intensity, we shall look at all three, and show how they fit together.

Using VO2 max

One approach to training, which is used by many elite athletes, is to base your training zones on a measure of the body’s ability to use oxygen to generate energy.

What is VO2 max?

As you increase your effort when you exercise, the amount of oxygen you consume to produce energy (and hence the rate at which you exhale carbon dioxide) increases.  However, there is a maximum level of oxygen consumption, beyond which increases in exercise intensity don’t lead to further increases in oxygen consumption. This level of oxygen consumption is called the VO2 max.  (The initials simply stand for volume of oxygen.)

Some experts believe that VO2 max is a key physiological determinant of an athlete’s performance, and that it is an important objective of a training programme to improve it. Other sports scientists argue that the limits to an athlete’s performance are determined by a range of factors – such as adaptation of muscles, running efficiency, metabolism – and that VO2 max is simply a measure of the oxygen that the athlete consumes at his or her maximum level of energy output.  On this view, which I find more likely, VO2 max is not the critical factor which determines maximum performance, but is rather a consequence of a combination of other limiting factors.  Whichever way you look at it there is a measurable level of exercise intensity at which the athlete’s consumption of oxygen reaches a plateau and does not increase further.

How to measure VO2 max

To measure VO2 max accurately, the athlete has to exercise in a laboratory (for example, on a treadmill or exercise bike) to the point of exhaustion. At the point when the athlete is working at peak power output, a sample of exhaled air is captured for analysis.  The amount of carbon dioxide in the sample provides a measure of the amount of oxygen the athlete is burning (that is, converting into energy) at maximum energy output – which is the athlete’s VO2 max.

All this involves a gas analysis by a laboratory. Fortunately, there are a number of less complicated ways of approximating the measurement of VO2 max.. These involve measuring an athlete’s performance (e.g. how far they can run in 15 minutes) and then using known correlations to predict the VO2 max.  When done properly, these approximations are good predictors of an accurately measured VO2 max.

The two most common tests for VO2 max are the Balke Test, described in the box, and the Bleep Test which involves running backwards and forwards between two lines 20m apart, getting faster and faster, until the athlete is unable to reach the other line. Both tests are maximal in that they involve testing the athlete to exhaustion (ie beyond the exercise intensity of VO2 max).  Alternative measures of VO2 max (often used in high street gyms) work by extrapolating from sub-maximal performances; these are less reliable.

Your VO2 max is to a large extent determined by your genes; but it can be increased by training. Most people can increase their VO2 max by between 5% and 20% (increases of up to 60% have been reported); but there is a small proportion of the population for whom training seems to make little difference.

VO2 max and training zones

Having established the VO2 max as the maximum rate at which the body can consume oxygen, the various other levels of exercise intensity can be defined in relation to this maximum, as follows:

Intensity % VO2 max
Repeats > 100%
Interval training 98-100%
Anaerobic threshold 86-88%
Easy run / LSD 65%-75%

But this rather begs the question of how we can use this information in practice. While some of us have measured our VO2 max, few of us have any reliable way of measuring our oxygen consumption as we workout, nor do we have a reliable way of knowing what our oxygen consumption is at a given pace. So we need some way relate our actual exercise intensity to these target training intensities.

Training with heart rate information

As exercise intensity increases, so does your heart rate.  Your heart rate, which is conventionally measured in beats per minute (bpm) can therefore be used as an indicator of exercise intensity.

Determining your maximum heart rate

Unlike your resting heart rate, which falls as you get fitter, your maximum heart rate does not vary much with fitness. It does fall as you get older.

Rules of thumb for estimating your maximum heart rate based on your age are shown in the box.  But be aware that these are very rough estimates, and each individual’s maximum heart rate might be quite different from the estimate provided by this formula (in some cases, the formula is out by more than 20 beats a minute).

A simple prediction of your maximum heart rate

Men:  MHR = 214 – (0.8 x age)
Women: MHR = 209 – (0.9 x age)

The best way to test your maximum heart rate is to do a running test which you can do on a track, in a park or on a treadmill. After warming up, run at an even pace for three minutes, as fast as you can.  Jog for two minutes; then run again for three minutes as fast as you can.  Your maximum heart rate is the maximum level reached during the second 3 minute run.

If you are significantly overweight, however, you should get medical advice before undertaking a test designed to increase your heart rate to its maximum.

Resting heart rate

The best way to measure your resting heart rate is to use a watch or a heart rate monitor before you get out of bed in the morning (and before your first cup of tea or coffee, since caffeine stimulates the heart rate).  Some athletes even sleep with their heart rate monitor strap on.

As you get fitter, your resting heart rate should gradually reduce. If you notice a blip up in your resting heart rate one morning, this is a sign of overtraining or impending illness.  (You can also use the increase your heart rate when you first get out of bed as a good early warning indicator of overtraining.)

Using heart rate to estimate exercise intensity

Heart rates are often used to define and monitor training zones. For example, the low intensity zone or “fat burning zone” is usually defined as 60-70% of maximum heart rate, and the “threshold zone” is said to be 80‑90% of maximum heart rate.

You can base your training zones either on a percentage of your maximum heart rate or on a percentage of your working heart rate.  Your working heart rate is the gap between your resting heart rate and your maximum heart rate.

training_zone_diagram

For example, my maximum heart rate is 191bpm; and my resting heart rate is 48bpm.  This means my working heart rate is:

WHR = 191 – 48 = 143 bpm

To get the heart rate corresponding to the aerobic threshold, which is at 85% of my working heart rate, I have to add 85% of my working heart rate to my resting heart rate.  So the heart rate corresponding to my aerobic threshold is estimated to be:

AT = 48 + (0.85 x 143) = 170 bpm.

However, for most runners, the heart rate is not an especially good way to calibrate effort. There are many factors which affect the heart rate, including your level of dehydration, temperature, blood sugar, and excitement (e.g. adrenalin on race day). Because the heart rate depends on all these factors it is an unreliable guide to your intensity of effort.

However, heart rate measurements can be useful as a proxy for intensity if you know your body well, and you know how your heart rate responds to different conditions.  It is also useful for monitoring trends in your fitness over time.

In the days before GPS running watches which give you an accurate indication of pace were widely available and reasonably cheap, the heart rate was a reasonable proxy indicator for effort which could be measured.  Now that we can measure pace directly, it is generally more convenient to use your pace as a good measure of your effort.

Comparing different measures of intensity

We have seen that we can judge running intensity by percentage of VO2 max, percentage of working heart rate, or by pace.  The trouble with using percentage of VO2 max to regulate our training intensity is that we don’t have any direct way of checking our oxygen consumption; so day-to-day we have to fall back on pace or heart rate as a proxy measure to enable us to estimate our oxygen uptake.  Nonetheless, many training guides are calibrated on the basis of percentage of VO2 max.

We can measure heart rates directly, cheaply and while we are running, so this might be a more valuable tool for assessing training intensity.  But heart rates can vary quite considerably depending on a variety of conditions, and so they are not an especially good guide to actual exercise intensity on a particular day.

For these reasons, I believe that the simplest and most logical way to judge exercise intensity is to look at the pace.  This can also be measured directly (for example, using GPS watches described in Chapter 3, or by running on a track), and appears to be a good indicator of exercise intensity.

The following table gives an approximate guide to correlation between the different levels of VO2 max, the percent of working heart rate, and the corresponding race pace.

Comparison of heart rate, VO2 max and race pace

Training zone Equivalent
race pace
%
Maximum
heart rate
%
Working
heart rate
%
VO2 max
Maximum 400m 100% 100% > 100%
VO2 max 3km 96% 94% 100%
Interval pace 5km 94% 92% 98%
Anaerobic threshold
or Lactate threshold
Half M’thon to 10 mile 85–92% 80-90% 80-90%
Aerobic threshold Marathon, or slower 78–85% 70–80% 70–80%
Long, slow, distance 20% slower than M’thon 70-78% 60-70% 60-70%

Relationship between maximum heart rate and VO2 max

Your VO2 max generally occurs between 95% and 98% of your maximum heart rate, so your maximum heart rate is at a higher level of effort than VO2 max.  You may be wondering how it is possible that you can exercise at a higher intensity than your VO2 max.  Recall that your VO2 max is the most rapid rate at which you can convert oxygen into energy. But very high intensity exercise (such as sprinting) is sustained by anaerobic metabolism – that is, without oxygen. For short periods, your body can produce energy at a higher rate than you can burn oxygen.  That is why your maximum energy output can exceed your VO2 max.