Endurance athletes frequently use Heart Rate (HR) to define their intensity zones. Although heart rate is a good indicator of exercise intensity, there are several limitations to using this index correctly for your training.
Various factors influence the cardiac response
Under controlled conditions, the cardiac response is relatively stable (≠2-3 bpm). However, not everyone can train in the laboratory, so it is not easy to perform your training under controlled conditions because for this you must perform an identical session under similar conditions of achievement: temperature, humidity, training schedule, level of fatigue, diet, digestion, stress…… As you can see different factors (non-exhaustive list) influence your heart rate response which severely limits the use of HR to define your intensity zones. Let’s go a little further in the exploration of HR to exercise.
Attention to cardiac drift over a long period of exercise
It has been clearly identified, in various scientific studies, an increase in heart rate with increased exercise duration despite stable intensity.
As can be seen below, on a concrete case of an athlete trained by GUTAÏ who had to perform 3*15′ @210W, despite a stable intensity (symbolized by power), the HR drifts on each 15′ block for a total increase of +22 bpm.
If this athlete had used his heart rate to define his intensity zones, he would have had to reduce his power to keep his heart rate stable.
Heart rate and heat do not mix well
This phenomenon of cardiac drift is mainly caused by an increase in endogenous heat. Indoor training that does not allow air cooling, as can be the case during an outdoor session, increases endogenous heat and therefore cardiac drift. In a warm environment (33° | 50% humidity) with water input, a study by Mountain & Coyle (1992) showed an increase in HR (+13 bpm) for a constant intensity (62-67% VO2max) over a 2-hour exercise. During this same study, they were able to highlight the effects of heat coupled with dehydration, subjects who performed the 2-hour session without water intake had an increase of +40 bpm after 1h40′ of exercise.
These elements show once again the limits of the heart rate to define its intensity zones.
Exercise intensity and cardiac response
One might think that the relationship between HR and VO2max is linear, which would facilitate the use of HR to prescribe training intensities, but unfortunately this is not the case! During a prolonged effort at constant intensity (85% VO2max), the HR can reach its maximum value, which means that the HR cannot be used on high intensity sessions (>85% VO2max).
Progress or fatigue… what a reflection of HR
When an athlete sees his heart rate decrease for a similar intensity level, he may think that he has improved his performance while it can be totally the opposite since an advanced state of fatigue can produce a decrease in heart rate for the same intensity. To know whether it is an improvement in your performance or a period of fatigue, you should monitor your heart rate response on a daily basis by comparing it with similar sessions in the same context (heat, humidity, altitude difference…), all crossed with the perceived difficulty of your effort. For example: you do a high intensity session on Monday with a good state of freshness and a good cardiac response then the next day during the low intensity session you feel tired (Welnness down), your heart rate may have 5 to 10 beats less than usual for a close or similar intensity and feel the effort (RPE scale of 1 to 10) much more difficult. This example shows how difficult it is to routinely use HR to define intensity zones when training daily with varying levels of fatigue.
What can we conclude from this
HR is easily accessible thanks to cardiofrequencemeters but the limits are numerous to make it a reliable indicator to define training intensity zones. On the other hand, the HR is a good witness to follow the positive or negative adaptation (progression-stabilization-fatigue) of his body on a daily basis within the training process.