## 4 Discussion
Patients whose spontaneous circulation was restored had higher skeletal muscle rSO\({}_{2}\) at the beginning of advanced life support (basal rSO\({}_{2}\)) and during cardiopulmonary resuscitation (maximal rSO\({}_{2}\)) [48]. In the presented study, we accurately determined ROIs for the most commonly assessed skeletal muscles of the upper and lower extremities and back, as well as additional ROIs for tendons (Achilles, plantar fascia) and joint areas (cubital fossa, popliteal fossa and carpal tunnel) (Figure 2). We have provided age- and gender-specific reference values) (Tables 3 and 4). The mean rSO\({}_{2}\) value for the analyzed ROIs ranged from the lowest values for the Achilles tendon (40.34%) to the highest values for tibialis anterior (69.94%). This distribution of values was independent of age and gender. Significant differences in rSO\({}_{2}\) values were found between all of the selected ROIs (Table 5). Such differentiation is confirmed by literature data, which show higher rSO\({}_{2}\) values in deltoid muscle than brachioradialis muscle. This clearly indicates area differences in rSO\({}_{2}\) values related to the blood supply and metabolic activity of tissues. Our findings support the conclusion that NIRS-derived values are not universal for all areas and should be carefully interpreted according to the underlying physiology of the monitored tissue. Such results are consistent with the assumption that human skeletal muscle is composed of a heterogeneous collection of muscle fiber types. Most skeletal muscles in a human contain all three types of fibers: type I (slow-twitch oxidative), IIa (fast oxidative glycolytic (FOG)) and IIx (fast glycolytic (FG)), although in varying proportions. Moreover, even amongst fibers of the same type, there are structural and functional characteristic differences [44, 49, 50]. In addition, given the importance of the capillary bed for the delivery of oxygen to the muscle, it is expected that highly oxidative muscles have a denser capillary network than highly glycolytic muscles [51]. We also noticed that higher rSO\({}_{2}\) values were recorded for muscle ROIs (from 63.39% of the C-Th to 69.89% of the mid-neck) than for tendon and joint areas (from 40.34% of the Achilles tendon to 66.32% of the ulnar fossa) (Table 3). This seems to confirm once again the need to standardize the measurement of the signal site, as other researchers have also stressed [12]. However, only a few studies have tried to quantify the oxygen consumption of human tendon in vivo. The tendon's blood circulation is poor, mostly peripheral (peri-tendinous sheet), and rises from the muscular belly and periosteum, although tendons are metabolically active tissues [52]. In [PERSON] et al. 's research, the usefulness of NIRS to study oxygen extraction and blood flow in the patellar tendon during ramped isometric knee extensions in healthy young subjects has been confirmed [53]. Similarly, [PERSON] et al., using NIRS and red laser light, found that the oxygen consumption of the Achilles tendon as well as that of the medial gastrocnemius muscle increased significantly after repeated muscle contractions [54]. This result implied that tendons as well as muscles consumed oxygen during contraction and changes in oxygen consumption within the tendon need to be elucidated to understand human tendon physiology in vivo. The regional oxygen saturation index can also be used to evaluate the effect of therapeutic, rehabilitative interventions and new therapies on the status and possible changes in tissue perfusion. For example, rSO\({}_{2}\) was used to assess the effect of a lower limb elevation procedure after injury or surgery on the perfusion status of the tissues. The analysis of rSO\({}_{2}\) values at various degrees of elevation of the control and experimental limbs showed that as the elevation of the human limb increases, muscle oxygen saturation deteriorates, which may have important clinical implications [55].