**1. Introduction**

Arteriogenesis is defined as the growth of functional collateral arteries from pre-existing arterioarteriolar anastomoses [1,2]. An initial trigger is the occlusion of a main artery, which occurs during peripheral artery disease (PAD). Such an occlusion redirects the blood flow to the pre-formed collateral arteries and thereby alters the fluid shear stress (FSS) [3]. The increased blood flow initiates vascular remodeling and diameter growth [4]. Several mechano-sensors and transducers that convey the FSS message during collateral remodeling have been proposed, including ion channels [5], the glycocalyx layer of endothelial cells (ECs) [6], nitric oxide (NO) [7], and microRNAs (miRNAs) [8].

These small, non-coding ribonucleic acids have been shown to play a decisive role in processes such as heart development, vascular regeneration, and tissue repair [9–12]. miRNAs are involved in post-transcriptional gene regulation by binding to mRNAs, causing the repression of translation and mRNA degradation, thus fine-tuning protein expression. Several miRNAs have been shown to control the response of vascular cells to hemodynamic stress [8]. In addition, miRNAs can be secreted and can thereby contribute to intercellular communication [13] or serve as circulating biomarkers [14].

Arteriogenesis can be amplified by exercise, as documented in human trials [15–17] and animal studies [18]. Therefore, according to international guidelines, PAD patients in Fontaine stage I or IIA/B (Rutherford 1–3) should be recommended for exercise training [19,20]. Mechanisms involved in the exercise-mediated benefits of treating PAD are thought to be the suppression of inflammation [1], expression of pro-inflammatory immune cells [21,22], and the improvement of endothelial function [23]. Beyond that, physical training has the potential to promote additional vascularization [24,25].

Comparable mechanisms have been discussed for the training effects of blood flow restriction exercises: Blood flow restriction training (BFR) is a resistance training method in which blood flow is reduced artificially. The decreased blood flow is usually caused by applying a blood pressure cuff at the origin of the extremity (arms or legs) to be trained. The mechanisms of BFR are thought to involve ischemic hypoxia and the increased expression of vascular endothelial growth factors [26]. The hemodynamic stimuli amplified by BFR (e.g., shear stress at the endothelium) lead to an increased release of the endothelial NO synthase, among other responses [27].

To achieve systematic effects during BFR, a lower resistance load is used than in classic resistance training without BFR: An intensity of 20% of the single repetition maximum (1RM) and a reduced training time of about 4–8 weeks have been demonstrated to have effects on muscle hypertrophy and muscular strength [28–30]. BFR training with a lower load in a shorter time can lead to the same results as resistance training with significantly higher loads (at 65% 1RM). In particular, increases in muscle thickness and strength are comparable between these strategies [31,32]. Due to the comparable effects and lower loads, BFR is of grea<sup>t</sup> relevance for training persons with physical limitations (e.g., patients with injuries, patients with cardiovascular diseases, or elderly persons) [33,34].

Despite the promising results derived from BFR as a method to mimic exercise effects under different occlusion conditions like PAD and the potential role of miRNAs as effectors after hemodynamic stress or FSS, nothing is known about the acute effects of strength training during BFR on miRNA levels. Therefore, this study was designed to determine whether the profile of circulating miRNAs is altered after resistance training during BFR, as compared with low- and high-volume training protocols with no BFR. Our hypotheses were: (1) Blood flow restriction leads to a reduced blood flow velocity; (2) low-intensity blood flow restriction training leads to metabolic responses that are similar to those of high-intensity strength training without blood flow restriction; (3) low-intensity blood flow restriction training and training without blood flow restriction lead to different expression characteristics of miRNAs.
