## 2 Topics and Results
### Blood Flow
#### 2.3.1 Vasodilation  
The results from numerous studies prove that under normal environmental conditions (atmospheric pressure and ambient temperature), skeletal muscle blood flow increases linearly with increasing exercise intensity [8]. The increase in blood flow is a consequence of the increased tissue demand for oxygen [39; 40]. This conclusion also holds for exercise performed by small muscle groups, where skeletal muscle blood flow is regulated primarily by dilation of the vessels feeding the active muscle tissue, combined with increases in cardiac output and perfusion pressure [41]. Signals of neural and/or mechanical origin are suggested to be the likely mediators of the rapid increase in blood flow (within 1 s) observed at the onset of exercise [42; 43]. The processes involved in mechanically induced vasodilation are not fully understood. They can be induced and regulated by both endothelium-dependent and endothelium-independent factors [44]. Once the vasodilation process begins, the muscle's blood supply and oxygen demand require maintenance to ensure that the corresponding workload is met. Blood flow follows the energy and oxygen demands of working muscle tissue [45]. This means that metabolic signals produced by contracting muscles and/or blood flow may be involved in the dynamic control of vascular tone either directly or through the blood endothelium they stimulate [8; 46]. At a constant intensity of a given exercise, skeletal muscle blood flow may eventually stabilize or smoothly increase in response to increasing oxygen (O\({}_{2}\)) demand [47]. If the load involves large muscle groups (cycling or running), the relationship between blood flow and increasing load intensity is not linear. The degree of vasodilation seen, for example, during intense knee joint extensions involving the entire muscle mass would require an enormous cardiac output (about 100 L/min, far exceeding the pumping capacity of the human heart). In this way, the maintenance of adequate blood pressure is disturbed [48]. Therefore, muscle vasodilation during whole-body exercise is largely limited at high exercise loads, when skeletal muscle blood flow typically increases linearly with increasing exercise intensity up to about 80% peak power. Flow subsequently stabilizes and even decreases as muscle groups approach or exceed their maximal \"aerobic threshold\" [45; 49]. During intense physical exercise, the change in blood flow in the skeletal muscles can be strongly influenced by environmental conditions, for example--different temperature and oxygen concentration. Decreased levels of oxygen (O\({}_{2}\)) in inhaled air lead to a decrease in its content in arterial blood [50]. In conditions of reduced oxygen content in the arterial blood (at unloaded muscle groups or at submaximal loads through knee extension), compensatory vasodilation begins, increasing skeletal muscle blood flow, which maintains the balance between the amount of O\({}_{2}\) needed and delivered to the muscle tissue [51]. Aerobic exercise training is known to induce structural and functional changes in the cardiovascular system. They lead to an improvement in maximal muscle blood flow: e.g., during maximal knee extension exercise or cycling exercise [52; 53]. At the same time, aerobic exercise maximizes muscle oxygen uptake [54; 55]. Conversely, peak (maximal) muscle blood flow is markedly reduced in various pathological conditions [56; 57], and its normalization and/or improvement can be achieved by appropriate exercises in a regular training program [58]. The improvement in muscle blood flow resulting from these exercises and training is thought to be a primary result of increased functional vessel vasodilation and/or increased vessel cross-sectional area [39; 59].