## 1 Introduction  
Muscle stem cell-derived extracellular vesicles (MuSC-EVs) play a central role in skeletal muscle repair and remodeling [1]. Upon injury to muscle, muscle stem cells (MuSCs) are activated, proliferate, and differentiate into myoblasts, which fuse into the preexisting myotubes to facilitate regeneration. Due to their role in regeneration and repair, MuSCs may provide therapeutic benefit to a range of muscle disorders and pathologies [2; 3]. Although current strategies in MuSC-based therapies have focused primarily on the isolation, culture, and transplantation of MuSCs in the treatment of muscular dystrophies, these cells may also hold therapeutic potential in numerous other muscle pathologies including cachexia, sarcopenia, mechanical ventilation, disease muscular atrophy, and muscle trauma [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Although initially promising, there are limitations to a transplantation-based strategy [14; 15]. For example, since skeletal muscle is the most abundant tissue in humans, a very high number of MuSCs are likely required to treat systemic muscle conditions, though the number of MuSCs required to achieve a lasting therapeutic benefit is equivocal [2]. Further, generating the large number of MuSCs needed for therapeutics requires expanding MuSCs ex vivo; however, the capacity to expand isolated MuSCs is limited before myogenic potential rapidly declines [16].  
Extracellular vesicles (EVs) are small, lipid-coated vesicles that contain molecular cargo and are generally classified into overlapping size ranges as exosomes (30-150 nm), microvesicles (100-800 nm), and large EVs (apoptotic bodies, 200 nm-5 \(\upmu\)m), and contain a variety of molecular cargo including protein, RNA (including microRNA), and DNA [17; 18; 19]. EVs participate in paracrine and endocrine-like signaling via the selective packaging of specific molecular cargo that are delivered into nearby and distant recipient cells [20]. Most, if not all, cell types including muscle cells, release EVs [18; 21]. Recent evidence indicates EVs play critical roles in allowing MuSCs to respond to stimuli as well as interact with other cell types within the muscle niche [22; 23; 24]. [PERSON] et al. (2020) demonstrated that satellite cells (Pax7+ MuSCs) release EVs during load-induced hypertrophy to communicate with muscle cells and facilitate extracellular matrix remodeling [25]. [PERSON] et al. (2020) showed that MuSCs instruct macrophages to acquire an anti-inflammatory phenotype via the release of insulin-like growth factor-2 and MuSC-conditioned media relieves inflammatory bowel disease in mice [26]. Additionally, EVs released by cardiosphere-derived cells transiently restore dystrophin in the myocardium and skeletal muscle of dystrophic mice, potentially via delivery of miRNA-148a [27].  
MuSC-EVs may provide therapeutic benefits for myopathies and may help overcome many of the limitations of traditional MuSC-mediated therapeutic approaches; however, limited information exists regarding MuSC-EVs. Therefore, the purpose of this investigation was to characterize MuSC-EVs and examine potential therapeutic benefits in vitro. Importantly, mitochondrial damage has been causatively linked to impaired MuSC function [28; 29], and impaired mitochondrial function is known to play a role in muscle dysfunction in models of muscular dystrophy [30; 31; 32; 33], cancer cachexia [34], disuse atrophy [9; 35; 36], mechanical ventilation-induced diaphragm weakness, [9; 37; 38; 39; 40; 41; 42], hyperthermia [33], sarcopenia, [43; 44], diabetes [12; 45], and chronic kidney disease [46; 47]. With the established links between MuSC function and mitochondrial function [48], we also investigated the extent to which MuSC-EVs could attenuate mitochondrial dysfunction. We hypothesized that MuSCs release an abundance of EVs that rapidly enter myotubes and attenuate oxidative stress-induced mitochondrial dysfunction.