After completing the exercise, the reoxygenation indicator informs about the level of muscle fatigue, which allows us to adjust the length of breaks and the intensity of training. The analysis of such parameters by means of the heart rate is either impossible or burdened with a significant error.
The heart rate value depends on many external factors, such as air temperature, diet, stress, hydration and the quality of sleep. Eliminating these variables at once is practically impossible or very difficult to achieve. Looking directly into the muscles we eliminate all external factors.