## 4 Discussion
ST-segment depression is a widely discussed ECG parameter, which, for many decades, has been recognised as an indicator of myocardial ischaemia [33]. Deviation in this parameter is usually measured at its junction with the end of the QRS complex [34]. In our study, ST-segment depression increased during the bicycle test and decreased at recovery. During intermittent workloads applied to the back muscles, ST-segment depression reacted to workload in a different way from that most commonly described in the literature [35], with the values increasing more during recovery than during exercise. We attribute this observation to the reasons described above. This increasing dynamic is not significant in the study group over 45 years of age. In this case, ST-segment depression increased in every exercise repetition and also during recovery. Studies have shown that an increase in ST-segment depression of more than 0.1 mV during or after exercise might predict coronary artery disease, based on an ST-segment depression of more than 0.2 mV as the super-positive standard [36]. In our study, the highest value during exercises to the back muscles reached \(-\)0.16 \(\pm\) 0.1 mV. The observed dynamics in these indicators could be related to the recovery period being too short to allow a return to baseline values. Previous studies reported that return of ST-segment depression to baseline after workload required a prolonged recovery phase of at least 5 min [37]. Similar dynamics were observed in the group aged under 45, but in this case, changes in ST-segment depression values were smaller and were not significant. Thus, the emergence of these functional ischaemic episodes during exercise can determine the capacity of the functional state of the CVS in older age. Therefore, analysing the ST segment during back muscle training can help to identify early signs of ischemia [38]. Changes in StO\({}_{2}\) in muscle tissue are associated with changes in arterial blood flow intensity [39]. In the event of fatigue in the working muscles, the oxygen saturation begins to decrease, which is especially noticeable in the case of the highest workloads. During exercise, these changes determine muscle blood flow intensity and StO\({}_{2}\) for muscle work [40]. In our study, during the bicycle ergometer exercise when workload reached 150 W, the muscles received a lower supply of oxygen in participants over 45 years age. At recovery, the StO\({}_{2}\) of these participants was higher than that seen for participants under 45 years age, which may be caused by vasodilation of the peripheral blood vessels and severe intensification of blood flow in the calf. These phenomena are widely described in the literature [41]. During the intermittent workloads to the back muscles, \(\text{StO}_{2}\) monitored in the muscle tissue was not significantly different between the groups, which may result from the intensity and specific nature of the workload. Age-related alterations in the autonomic nervous system play a crucial role in explaining the differences observed between various age groups. Older adults typically show decreased parasympathetic activity alongside an increased influence of the sympathetic nervous system [42]. This shift results in a higher resting heart rate, a stronger heart rate response during physical exercise, and a slower recovery afterward. Furthermore, vascular ageing--marked by reduced arterial elasticity and endothelial dysfunction--impairs the blood vessels' ability to dilate (vasodilation) and adjust to the elevated cardiac output required during physical activity [43]. Thus, these physiological changes together affect not only heart rate dynamics and blood pressure regulation but also muscle oxygenation and myocardial oxygen demand. Therefore, older participants in the study showed a more pronounced increase in heart rate, a more stable but higher diastolic blood pressure, and a slower recovery phase of muscle oxygen saturation and ST-segment changes.