Heart rate training makes use of the fact that the demand for oxygen rises with exercise intensity. As would be expected heart rate has a close relationship to oxygen consumption, especially at exercise intensities between 50 and 90% VO2 max (1).
Heart rate is easy to monitor and for the majority of athletes it offers a practical measure for assessing exercise intensity, which is why it is so popular.
It's important to monitor exercise intensity for a number of reasons. Firstly, the specific physiological adaptations to training change depending on what relative work load is employed. It's fundamental that the athlete or coach understands which type of endurance training (as a reflection of intensity) is best for their sport or event.
Secondly, monitoring the intensity of individual sessions allows the coach or athlete to manipulate the overall program, helping to prevent over training and in order to reach a physical peak for competition.
While heart rate is convenient and practical for most athletes, for many it can be inaccurate in determining the best exercise intensity (2)...
Most heart rate training programs are devised around an estimation of the maximum heart rate. The are two problems with this approach. The first is that maximum heart rate is estimated with the basic formula 220-age. For a significant number of athletes however, this estimation maybe out by as much as 25 beats per minute (2).
The only way to accurately determine maximum heart rate is perform a short, maximal stress test (to exhaustion). During the test heart rate will rise steadily until a plateau is reached despite the exercise intensity continuing to rise (assuming the individual is fit enough to last until such a time). This is a direct marker that the heart is beating as fast as possible.
The second problem is that, even if maximum heart rate is estimated accurately, prescribing exercise on the back of standardized zones makes no allowances for individual differences. For example, endurance performance improves when lactate threshold as a percentage of VO2 max is increased and it can be improved with training (3,4). A standard heart rate zone of 85-90% of the age-predicted maximum is commonly prescribed to improve lactate threshold but this may not be accurate. As with maximal heart rate, the only way to determine the correct heart rate training zone for improvement of lactate threshold is to measure it during laboratory testing.
Despite these limitations, heart rate training still offers a more objective method for determining exercise intensity than nothing at all.
Different exercise intensities tax the body's energy systems in different ways.
Exercising at 60% of maximum heart rate for example, is said to predominantly tax the aerobic system in most people. If exercise duration is long enough, the major source of fuel will be fat.
This type of intensity is often favoured by people who want to lose weight and are generally de-conditioned.
A heart rate training zone of 70-80% maximum will still predominantly tax the aerobic system in fitter individuals but the main source of fuel will be carbohydrate, or more specifically, glycogen. This is the heart rate training zone that endurance athletes typically aim for.
Here is a quick example of calculating a heart rate training zone using the age-predicted maximum of 220-age:
Rachel is 35 years old and wants to train for a 10km run.
Simply using 220-age makes no allowances for individual differences. All 35-year olds will have the same heart rate training zones according to this formula.
The Karvonen formula takes into account resting heart rate making it a slightly more specific to the individual. Because resting heart rate decreases with conditioning it also makes allowances for differing degrees of fitness to some extent.
Keeping with the example above, here's how Rachel (who has a resting heart rate of 65bpm) would use the Karvonen formula to achieve a more accurate heart rate training range for aerobic endurance conditioning...
Maximum heart rate - resting heart rate x heart rate zone + resting heart rate
Now, let's do the same again for the upper limit...
Target heart rate zone = 149 - 161bpm (exercising at 70-80% effort)
You can see that the Karvonen formula calculates a higher training zone than just using 220-age and this is often the case.
It's often a good idea to use a rate of perceived exertion along side heart rate to make the intensity more specific to the individual. Rate of perceived exertion, although subjective, has been shown to correlate with heart rate (5). Essentially, it is a scale of difficulty that ranges from 6 (no exertion at all) to 20 (maximal exertion). It is often called the Borg Scale after its creator (6).
Swimming is a Little Different Maximum heart rate while swimming tends to be lower than for running events. To adjust for this subtract 13 from your maximum heart rate i.e. use 207-age rather than 220 - age. Use this adjustment for the Karvonen formula also.
As mentioned earlier, the simplest method for determining the lactate threshold is to assume it occurs at 85-90% of the maximum heart rate. An alternative is to use the Conconi test...
In 1982 Conconi et al, stated that the lactate threshold was linked to a deflection point in heart rate data. Heart rate plateaus briefly before rising sharply again and this is said to correspond with a sudden rise in blood lactate concentrations. There are various protocols used to elicit the plateau Conconi and co-workers refer to. Here is an example:
You can now plot a simple heart rate graph like the one below and read off lactate threshold:
You can see from the graph above the obvious plateau and deflection in heart rate. It seems to correspond with a heart rate of 172bpm. In theory, then an athlete could train at or just above this heart rate training zone and improve their lactate threshold. However, caution is required when using this test as subsequent research has questioned its validity (7,8). It has been argued that the deflection point occurrs only in a certain number of those tested and that it underestimates the lactate threshold exercise intensity.
Here's a simple heart rate training program to increase lactate threshold...