## 4 Discussion  
The depression of the amplitude of the ST segment during the stress exercise is a traditional biomarker for the diagnosis of coronary heart disease (CHD). However, the prognostic information of this variable has not yet been specifically studied and compared with more traditional exercise ECG variables such as a simple end-exercise ST-segment depression, or heart rate-adjusted ST/HR index [19; 20] and ST/HR slope [21; 22]. Since ST-segment depression has limited diagnostic characterization at exercise electrocardiography (ECG), ST-segment depression/heart rate (ST/HR) hysteresis and cardiopulmonary exercise test (CPET)-derived parameters have been proposed as alternatives for the diagnosis of the exercise-induced myocardial ischemia [23]. Recently, the prognostic information of other exercises, i.e., ECG variables based on the combined ST/HR data sampled during exercise and recovery phases (the so-called stress recovery index (SRI) [24]) has been demonstrated by [25; 26]. The ST/HR hysteresis takes also into account a recovery phase but is not so susceptible to the reached maximum heart rate during the exercise. To obtain more precise information about an individual patient's exercise capacity, the measurements of metabolic equivalents and oxygen consumption should be analyzed accurately [27]. This paper introduces a novel approach that provides valuable information about individual metabolic performance from the variation of the algebraic JT/ST relationship during the load and the recovery processes. The JT interval corresponds to the cardiac electric systole in the repolarization phase, and its changes are associated with the intensity of the myocardial metabolism. The alterations of the JT interval are influenced by the regulatory nervous system. Metabolic changes in the organism are closely associated with repolarization changes. ECG leads, where the JT interval is shorter, show that the repolarization processes happen faster in those myocardium areas. A longer duration of the JT interval indicates slower repolarization (and slower metabolic reactions) [28]. The limitations of this study are, first of all, related to the fact that this study included only men (in order to avoid higher dispersion of the obtained data). All data were recorded  
Figure 11: The dynamics of the JT/ST relationship during the load and recovery processes for person #10. The \(x\)-axis in parts (**A**,**B**) represents the time (measured in minutes) from the beginning of the cycle ergometry exercise. The left y-axis in part (**B**) represents the power of the load in Watts. The right y-axis in part (**B**) represents the variation of the RR interval in ms. The algebraic JT/ST relationship is represented by the thin dashed black line in part A. The thick black solid vertical line in part A marks the end of the load. during the bicycle ergometry tests, and the functional state evaluation was performed only on these data. Every assessed parameter reflects only specific physiological information of the human body. Our hypothesis is that all circuits of the organ control systems that have been activated during the load return to the baseline levels during the recovery process (for healthy people). In fact, the minimum level of the JT/ST relationship was in the range between 0.3 and 0.5 for all observed persons. We assume that when the human organism reaches this limit, it can no longer fall below this range, and then the reorganization of the cardiovascular system takes place. There are two possible options during the repolarization process--the compensatory mechanisms are turned on for healthy persons or are completely imbalanced for persons with ischemic heart disease. It can be observed that healthy persons are submitted to protocols with rather high loads. The possible transit through the anaerobic threshold could be a factor affecting the JT/ST relationship.