## 4 Discussion  
To our knowledge, this is the first study to assess the normative values of the regional oxygen saturation (rSO\({}_{2}\)) in selected areas of the body, in healthy people, taking into account age and gender. For this purpose, 18 ROIs on the front and back surfaces of the body were determined independently. The research was conducted in two groups of healthy volunteers, young and old, in the age range from 20 to 30 years and from 60 to \begin{table}
*** \(p<0.001\).  
\end{table} 
Table 6: Relationship between values of selected anthropometric characteristics and age and rSO\({}_{2}\) values. 79 years, respectively. The average values of the rSO\({}_{2}\) from selected ROIs from the anterior and posterior sides of the upper and lower parts of the body have been measured. The 18 ROIs have been chosen because of their frequent involvement in overload injuries and following physiotherapy and their easy accessibility to NIRS techniques. The analysis began with assessing the symmetry of the distribution of the obtained saturation values for contralateral body areas, and then the comparative assessment of the rSO\({}_{2}\) values for individual areas was carried out. All analyses were performed independently for the entire group of subjects and then they were divided into groups, taking into account age and gender. Taking into account previous reports [34] on the relationship between changes in hemodynamic parameters (related to a sudden change in the position from lying down to standing) and the tissue saturation values recorded, especially in the distal parts of the body, we conducted our research on volunteers resting for 20 min in a lying position. Due to the above, reference to the values presented by us should be made not only considering the exact location of the sensor but also the lying position. Peripheral muscle rSO\({}_{2}\) is a hemodynamic variable that differs between healthy and severely ill patients [35] and is associated with clinically important outcomes [36]. Available studies were performed using different NIRS pulse oximeters and at different body sites, making standardization of measurements and establishing reference values much more difficult and this is a significant obstacle to widespread clinical use. A fundamental problem of NIRS tissue oximetry is the lack of a reference standard. Therefore, testing and comparing commercially available NIRS equipment at different locations and in different populations is important, as other researchers have also pointed out [37]. The median absolute resting rSO\({}_{2}\) values recorded in the area of the proximal part of the forearm flexor muscles by the three leading pulse oximeters (INVOS 5100C = 70.7%, FORE-SIGHT = 64.6%, NONIN EQUANOX 7600 = 68.4%) in the steady state were within 6 percentage points, while the values during deoxygenation varied significantly. Therefore, forearm rSO\({}_{2}\) values using INVOS, NONIN and FORE-SIGHT in occlusion testing cannot be used interchangeably. The available literature results do not determine which device is best for assessing rSO\({}_{2}\). Although the NONIN we used was characterized by the highest variability of measurements, at the same time, the results of comparisons of pulse oximeters suggest that good repeatability comes at the expense of low sensitivity to changes in oxidation [37]. Measurement of rSO\({}_{2}\) finds application in the detection of tissue hypoperfusion, which, being a pathophysiological process, leads to multiple organ dysfunction and death. In patients at risk of hypoperfusion, it is crucial to monitor and detect inadequate tissue perfusion and oxygenation early. In clinical practice, tissue oxygenation is assessed by global measurements such as blood pressure and cardiac output or with invasive methods measuring variables derived from oxygen and blood lactate levels. The use of NIRS appears to be an important and valuable alternative because it is non-invasive, allows real-time measurements and is clinically reliable [38].