The Problem With Using the Heart Rate to Measure Exercise Intensity

If you have read my blog on my latest fitness gadget, the Fitbit Charge 2, you know that it is measuring your heart rate. The heart rate is an important indicator of how intensively you exercise. And for a Personal Trainer, exercise intensity is one of the most important elements of an exercise program.

Using the heart rate for measuring exercise intensity is probably one of the most widely used methods. There are others, such as maximal oxygen uptake (VO2 max) and ventilatory threshold (talk test).


The method of measuring the heart rate to determine exercise intensity is only accurate, however, if we know an individual’s maximum heart rate. This can only be measured with a maximal-effort exercise test, which is not appropriate for most people. As a result, the maximum heart rate is often calculated via a mathematical formula, for example 220 minus age. This formula is widely used as it is easy to compute, but it is not very accurate as there are various factors influencing an individual’s maximum heart rate, such as:

  • Genetics
  • Type of exercise (lower body, upper body, both)
  • Medication
  • Body size (a smaller heart generally pumps less blood volume so the resting heart rate is higher. This is why women often have a higher resting heart rate than men).
  • Altitude can lower the maximum heart rate as most individuals cannot train at a higher intensity.
  • Age does not necessarily drop by 1 bpm per year in all individuals.

The formula 220 minus age, which is used in my Fitbit Charge 2 for example, tends to overestimate the maximum heart rate in young adults and underestimate it in older individuals. Based on this over- or underestimation of the maximum heart rate, one may end up over- or undertraining. But overtraining is very dangerous as it increases the risk for injury, while undertraining is not challenging enough for the body to be effective.

As a result, I would recommend not relying on maximum heart rate numbers, and the training zones derived from them. Instead, I would use the talk test: if you cannot comfortably talk anymore, but can sustain the activity for about 20 to 30 minutes, you are training at the highest sustainable intensity. This higher-intensity training is suitable for individuals that have performance goals. For individuals looking to lose weight or develop their aerobic efficiency, can use the talk test as follows: Training at an intensity where the answer to the question “Can you speak comfortably?” becomes less than an unequivocal “yes” is probably the ideal training intensity marker. Training at this or near this intensity increases the likelihood of a better training experience (i.e. you enjoy working out at this intensity as it is not too strenuous or hard).

This doesn’t mean that a fitness tracker is totally useless. I still love using mine. Because even if the values are not very accurate for one day or one exercise, it still gives you an idea of how you are progressing over time. I like to observe if my resting heart rate decreases over time or to see if my heart rate goes back to normal faster after a workout. As I said in my blog about my fitness tracker, you don’t really need a heart rate monitor, but for me it’s another motivating tool. And you can never have enough of those!

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