3.3.3. Myostatin

All these pathways are additional potential exercise under-BFR mechanisms and are mostly accompanied by lower myostatin levels [64]. Previous research has shown that the expression of myostatin is reduced in response to BFR training and is associated with increased muscle mass and strength after eight weeks of resistance training with the BFR application [67]. As myostatin harms protein synthesis, lower levels thereof can lead to larger training e ffects. Furthermore, ribosomal protein S6 kinase beta-1 (S6K1) is stimulated under BFR after a single low-intensity strength training of the lower extremities (20% of 1RM, duration approximately four to five minutes) [62]. The S6K1 is involved in the regulation of mRNA translation and, again, may be an important contributor to muscular protein biosynthesis [62].

The mechanisms of how BFR leads to positive training e ffects are, conclusively, found in metabolic stress, ischemic hypoxia, and an increased expression of vascular endothelial growth factors [68], elicited by the training, BFR, or a combination of both. The increased fluid shear stress caused by ischemia and reperfusion between the repetitions and sets during the training intervention with BFR could be a stimulator for arteriogenesis [69,70]. The hemodynamic stimuli amplified by BFR lead to an increased release of endothelial NO synthase, among other responses [71]. Additionally, recent studies have shown that a single bout of strength training under BFR leads to changes in the miRNA expression profile [72]. So far, the parameters in animal and human studies to determine the mechanisms have only been identified by invasive measures such as muscle biopsies and whole blood samples. The exact mechanism of low-load training under BFR has not ye<sup>t</sup> been finally clarified.

#### 3.3.4. BFR and LEAD

Summarizing the findings, both exercising with LEAD and BFR share, inter alia, reduced blood flow, altered miRNA expression, and changes in the hemodynamic stimuli (e.g., fluid shear stress) as the main mechanisms of the adaptions to training. One of the main di fferences is the return of blood through the veins. When the blood flow is reduced via an external application (in BFR), the pressure on the veins increases, as well. That leads to a reduced backflow of the (venous) blood. At the same time, the transport of metabolic products is elicited by the applied reduced blood flow. This mechanism does not occur in LEAD. Furthermore, in LEAD, mitochondrial activity is increased. Under BFR (in training), myostatin levels decrease, whereas S6K1, heat shock proteins, surrounding tissue pressure, fast-twitch muscle fibers involvement, and venous blood flow increase. Both mechanisms (exercising with LEAD or under BFR application) have in common that blood and mucous cell lactate, nitric oxide synthase, miRNA, fluid shear stress, and VEGF concentrations are increased; hypoxia/ischemia is induced and the arterial blood flow is decreased. These micro and macro level adaptations lead to neovascularization (LEAD), a decrease of inflammatory processes, and expression of proinflammatory immune cells. Furthermore, the endothelial function is increased, the involved skeletal muscles are remodeled, and alterations in capillary density and in the ratio of type I to type II muscle fibers occur.

Despite considerable di fferences, there are, thus, many mechanisms that the two conditions have in common. Especially, the ischemic situation, the changes in miRNA expression, and the increased production of NOS, with their associated arteriogenesis after training with blood flow reduction, attract attention when comparing the underlying adaptation mechanisms to reduced blood flow applicated via BFR or pathophysiologically via LEAD. An overview of the similarities and di fferences in the mechanisms of exercise in LEAD and under BFR is provided in Figure 1. At the bottom level, the di fferences and commonalities are displayed as Venn diagrams. These exercise (plus BFR or LEAD)-induced mechanisms lead to (upper level of the figure) several e ffects on di fferent biophysiological levels. These are, again, displayed as Venn diagrams, to show commonalities and di fferences between the exercise e ffects of the training with LEAD or under BFR. Despite broad knowledge on several factors, many of the mechanisms are only suggested by anecdotical evidence and not ye<sup>t</sup> proven by high quality studies.

**Figure 1.** Mechanisms and pathways of how exercise leads to training success in peripheral arterial disease (lower extremity arterial disease (LEAD), right side) and under blood flow restriction (blood flow restriction (BFR) left side).
