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In general, a training impulse intends to result in a positive training adaptation and improved physical performance. However, it is known that excessive training may lead to reduced performance as well as to the development of overtraining syndrome (Jeukendrup, Hesselink, Snyder, Kuipers, & Keizer, 1992; Lehmann, Dickhuth, Gendrisch, Lazar, Thum, Kaminski, Aramendi, Peterke, Wieland, & Keul, 1992; Lehmann, Dickhuth, Gendrisch, Lazar, Thum, Kaminski, Aramendi, Peterke, Wieland, & Keul, 1991). Often there is a fine distinction between optimal training impulses and a training impulse that deteriorates performance. Hence, it is believed that it is important to carefully monitor three components of a training program: duration, frequency and intensity. Training duration and frequency are not difficult to monitor. Regarding intensity, there are many methods for measuring it (A. Jeukendrup & VanDiemen 1998). For the optimal way to monitor intensity of exercise, a balance needs to be struck between the validity of the measured parameter and the ease of that parameter’s measurement process.
It is common knowledge that heart rate is correlated with physical exercise both in short term, as heart rate variability (HRV) i.e. the speed with which the heart rate adapts to the energy needs of the body, as well as, in long term, i.e. mean heart rate (HR). (Achten & Jeukendrup, 2003). It is also established that HR and oxygen volume (VO2 – a direct measure of energy expenditure) are linearly related over a wide range of submaximal exercise intensities. By determining the quantitative relationship between HR and VO2, HR can then be utilized to estimate VO2, which gives a fair reflection of the intensity of work that is being performed. With the development of portable, wireless Heart Rate Monitors (HRMs), HR has become the most commonly used method to get an indication of the exercise intensity in the field. HR is easy to monitor and shows a very stable pattern during exercise. As a consequence, HR data aid athletes to immediately adjust the intensity of a workout when and if necessary (Achten & Jeukendrup, 2003)
These conclusions have been around for quite some time. For that reason they have been utilized in formal guidelines and standards. In a recent American College of Sports Medicine position, a general classification of physical activity intensity was given. Utilising the percentage of HR- reserve (HRmax – HRrest) and the percentage of the HRmax the intensity of exercise is expressed. That intensity was divided into six different categories ranging from very light to maximal. This classification makes it possible to estimate the intensity of an exercise bout expressed as % VO2max or metabolic equivalents, without determining the individual relationship between HR and VO2. It is important to note that the intensity obtained this way will only give an indication of the true intensity and the individual relationship between HR and VO2 needs to be determined for a more accurate estimation. (Achten & Jeukendrup 2003; American College of Sports Medicine and others 2013).