## 3 Results
In Study B, dBP at baseline was 80.0 \(\pm\) 2.4 mmHg in the under-45 group and 82.0 \(\pm\) 2.5 mmHg in the over-45 group. Throughout the intermittent exercise protocol, dBP decreased during repetitions and increased during rest in both groups. Notably, during  
Figure 2: The HR dynamic during (**A**) the bicycle ergometer workload while the workload increased stepwise every minute and (**B**) the intermittent workloads applied to back muscles. _Note. HR, heart rate; bpm, beats per minute; W, watts; min, minute; REP, repetition._*_p < 0.05._  
the final repetition, dBP decreased more in the over-45 group (70.0 \(\pm\) 2.3 mmHg) compared with the under-45 group (75.8 \(\pm\) 3.2 mmHg). During the third rest period, a statistically significant difference emerged: dBP increased to 87.0 \(\pm\) 2.0 mmHg in the younger group and only to 74.7 \(\pm\) 2.9 mmHg in the older group (\(p\) < 0.05). Age has a moderate effect on diastolic blood pressure during and after exercise, with the over-45 group showing slightly higher values. Effect sizes range from 0.25 to 0.67, indicating small to moderate differences between age groups. Figure 4 illustrates that in Study A, ST-segment depression increased significantly in both age groups as the workload approached 200 W. At this intensity, the ST-segment depression was measured at 0.38 \(\pm\) 0.09 mV in participants aged under 45 years of age and at 0.43 \(\pm\) 0.1 mV in those over 45 years of age. However, the difference between the groups was not statistically significant. In Study B, during localised repeated loading of the back muscles, ST-segment depression values progressively increased in response to rising workload and were further elevated during the recovery phase. Higher ST-segment depression values were recorded in the group aged over 45 years, particularly during recovery. Following the third repetition, ST-segment depression during recovery was 0.17 \(\pm\) 0.06 mV in the older group and 0.10 \(\pm\) 0.02 mV in the younger group; however, this difference also did not reach statistical significance. Figure 3: The sBP (**A**) and dBP (**C**) dynamic during the bicycle ergometer workload while the workload increased stepwise every minute and sBP (**B**) and dBP (**D**) dynamic during the the intermittent workloads applied to back muscles. _Note. sBP, systolic blood pressure, dBP, diastolic blood pressure, mmHg, millimetres of mercury; W, watts; min, minute; REP, repetition. \({}^{*}\)p < 0.05_. Figure 5 presents the dynamics of \(\mathrm{StO_{2}}\) during both the exercise and recovery phases. In Study A, as the workload on the bicycle ergometer increased incrementally each minute, a decline in \(\mathrm{StO_{2}}\) was observed in both age groups. Upon reaching a workload of 150 W, the mean \(\mathrm{StO_{2}}\) value in participants under 45 years of age was 77.8 \(\pm\) 5.3%, whereas in those over 45 years of age it was 54.4 \(\pm\) 7.7%, with a statistically significant difference between the groups (\(p\) < 0.05). During the recovery phase, \(\mathrm{StO_{2}}\) values in both groups exceeded baseline levels. In the younger group (<45 years), the peak value was recorded in the second minute post-exercise (120.7 \(\pm\) 4.6%), while in the older group (\(\geq\)45 years), the highest value occurred in the third minute of recovery (135.1 \(\pm\) 7.1%). The study results showed that \(\mathrm{StO_{2}}\) levels were higher in participants over the age of 45 compared to younger individuals. The calculated effect sizes indicate a strong age-related impact on muscle oxygenation dynamics both during physical exertion and in the recovery period. In Study B, involving intermittent loading of the back muscles, \(\mathrm{StO_{2}}\) levels decreased during exercise repetitions and increased during recovery periods. However, the difference in \(\mathrm{StO_{2}}\) values between the age groups was not statistically significant. The study revealed differences in \(\mathrm{StO_{2}}\) dynamics between age groups during exercise and recovery.