## 4 Discussion
It is important to note the lack of significant differences between rSO\({}_{2}\) values recorded from symmetric ROIs on the right and left sides of the body, demonstrated both by us (Table 2) and in the results of other authors in healthy volunteers [34] and patients with respiratory sepsis [12]. The above has an important clinical implication that rSO\({}_{2}\) measurements do not need to be performed bilaterally or on the dominant side. This is particularly important in the case of local factors contraindicating or interfering with rSO\({}_{2}\) measurements, such as local injections, tattoos, bandage applications or kinesiology taping. Nowadays, the use of NIRS has expanded significantly beyond the assessment of cerebral oxygenation and is increasingly enabling the monitoring of local tissue and muscle oxygenation and perfusion [13; 39]. This paper focuses on the use and utility of NIRS in monitoring tissue/muscle oxygenation, leaving aside the already well-developed aspect of assessing cerebral perfusion in the literature. However, the assessment of rSO\({}_{2}\) in the forehead area, as one of the most frequently analyzed areas in the first research on the use of NIRS, was not abandoned. Muscular activation and muscle oxygenation are considered to be the main approaches in the monitoring of working muscle. From this perspective, NIRS was used in our study as a measurement to establish reference values for regional oxygen saturation. The research results published so far mainly concern lower extremity muscles (i.e., biceps femoris, gastrocnemius, rectus femoris, tibialis anterior, vastus lateralis, vastus medialis) [40, 41] and to a lesser extent the muscles of the upper limbs (i.e., biceps brachii, brachioradialis muscle, shoulder muscle, forearm flexors, triceps brachii) [41, 42]. Only a few studies refer to the saturation of the trunk muscles (i.e., dorsal extensor, intercostal, multifidus, condylar and anterior dentate) [43]. Although many of the superficial skeletal muscles were assessed by independent authors, no regional oxygen saturation reference values have been proposed. Moreover, almost all of the evaluations have been carried out regarding the influence of various types of exercise (weight training, running, cycling, rowing, Wingate test) or physical stimulation (electrical stimulation, whole-body vibration) on muscle saturation [13, 44, 45, 46]. We would like to point out that it is difficult to assess the exact impact of therapeutic treatment or physical activity without knowledge about the resting values for a given area, as well as the individual factors that influence the regional saturation values. Knowledge of normative reference values for individual ROIs seems to be particularly useful in assessing the condition of a patient staying in an intensive care unit. Patients in a critical condition due to cardiovascular disorders and respiratory failure are also characterized by worse parameters of microcirculation and perfusion of tissues sensitive to hypoxia, e.g., muscles [38]. The correct rSO\({}_{2}\) value is an indicator of maintaining the balance between the supply and demand for tissue oxygen and may also be an important clinical criterion for the patient's prognosis [12, 34]. [PERSON] et al. determined that an rSO\({}_{2}\) value \(<60\%\) recorded from the brachioradialis muscle at admission to the ICU is associated with a significant increase in mortality in patients with sepsis. The authors concluded that the rSO\({}_{2}\) value may be a useful tool for quantitative assessment of microcirculatory dysfunction but also for predicting the development of critically ill patients [12], which has also been shown in previous studies [8, 11, 47]. Normative reference values for individual ROIs may also be useful in assessing the condition of a patient in the intensive care unit and the effectiveness of resuscitation in situations of sudden cardiac arrest. Studies have confirmed the possibility of monitoring skeletal muscle rSO\({}_{2}\) in hospital cardiac arrest.