Heart rate monitoring is one of the most popular methods of monitoring the internal load of team sport players. One of the reasons for this, is that it is a very intuitive method: we all know that when we exercise, our heart rate is going up. Therefore, analyzing the heart rate of players can help us in monitoring the intensity of an exercise. However, one might question whether heart rate data is as intuitive as most people think it is. Can a raise in heart rate only be caused by intense activity? Or are we also measuring something else? In this blog, we will provide background information about heart rate to answer these questions.
Before we can go into detail about the use of heart rate data, we first have to go back to the basics. The function of the heart is to pump oxygen through our body. This oxygen is taken up by, among others, the muscles to perform activities. When we are performing intense exercise, our muscles are using more oxygen than normal and therefore our heart rate has to go up to fulfill this increased oxygen demand. So far, this story should not be new. However, what most people do not realize, is that oxygen is also used by the body to recover from intense exercise: this can be recognized by the few minutes of elevated heart rate after exercise. This elevated heart rate after exercise is, of course, not indicative of the activities that we perform at that moment, it is rather indicative of the restoration of the balance within our bodies. Therefore, heart rate is not solely measuring the load of the activities that we perform, but also of the recovery process of the body afterward.
We should be aware that the energy production of the muscles via the use of oxygen is a relatively slow process. Would our bodies solely rely on the energy production via the use of oxygen, Usain Bolt would not have been able to sprint as fast as he did (his body would not be able to produce that amount of energy in such a short period of time). This indicates that our body has other ways to produce high amounts of energy very fast. Indeed, our body has two other systems that can produce energy without using oxygen: the phosphocreatine system and the anaerobic glycolysis. Since our muscles are also able to produce energy without the use of oxygen, heart rate is not able to measure all the load that is put on the player.
Even though heart rate might not be able to measure the load on the body as produced via these two systems, it should be noted that the capacity of these systems is rather low. They can only be used for short, maximal activities of 10-15 seconds (phosphocreatine system) or an almost maximal activity of 90-120 seconds (anaerobic glycolysis). Furthermore, refilling of the phosphocreatine system and removing the byproducts of the anaerobic glycolysis (e.g. lactate) requires oxygen. Taken this together it means that heart rate cannot measure the load of the short explosive activity itself, but it rather measures the load on the body after the activity. However, if the time to recover between the repeated explosive actions is limited, there will be an incomplete recovery of these systems. This decreases the production of energy via these systems and increases the reliance on the aerobic (oxygen) system. This leads to a better agreement between heart rate and the energy produced by the body (meaning: heart rate is a better indicator of the load on the body).
Heart Rate in Practice
But how does this knowledge translate to the use of heart rate monitoring in practice? Let’s illustrate this with an example of two sprint exercises: 10 x 15m maximal sprint with 30 seconds recovery in between and 10 x 15m maximal sprint with 10 seconds recovery in between. In the first maximal sprint of 15m the energy needed to perform the sprint will mainly be produced by the phosphocreatine system. During the 30 seconds recovery afterward, this system will be refilled (with the use of oxygen). This indicates that in the next sprint most energy will again be produced via this system. Therefore, during a short maximal sprint with maximal recovery, heart rate will mostly be indicative of the load on the body during the recovery process, rather than of the sprint itself. In the same sprints with 10 seconds recovery in between, the system will not be refilled in time. Since there will be less energy produced via these systems the reliance on energy production via the oxygen system will increase with every sprint. Therefore, during a sprinting exercise with little time to recover, heart rate is (more) indicative of the load of the activity itself as well as of the recovery phase.
Heart rate is a very popular tracking method because it feels very intuitive. Based on what we have talked about, we can conclude that heart rate will provide an indication of how hard the body has to work to perform activities as well as to recover. However, an increase in heart rate does not automatically mean that the muscles are producing energy for running activities. Nor is the other way around true: explosive acceleration will not always result in an increase in heart rate, but the recovery process afterward will. Only when you have little time to recover (as is often the case in team sports), the activity itself will also cause an increase in heart rate. Therefore, when working with heart rate, you need to be aware of the type of activities that the players perform in order to know what kind of information heart rate is going to give you.
JOHAN’s Tip of the week
Because of the fact that heart rate might not be able to measure the total load on the body, especially during exercises where the focus is on explosiveness, heart rate might not provide you with a complete picture of the load on the players. If you plan an exercise in which the focus is on explosive power, and thus you plan a maximal exercise with long recovery periods in between, heart rate will mostly reflect the work the body has to do in the recovery phase. If you want to get a complete picture of the load on the players, using Rating of Perceived Exertion will provide a more comprehensive picture of the internal load (GPS can be used to measure the external load).
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