## 1 Introduction  
When slaughtering livestock, blood is treated as waste. Therefore, the 2.5-3.0 L of blood per pig, amounting to approximately 39,216 tons per year, has a major impact on water pollution [1, 2]. Blood, which accounts for approximately 7% of the total weight of pigs, generally contains 75-80% moisture and 15-17% high-quality protein; the protein is composed of albumin, globulin, and hemoglobin [3, 4]. Although this blood has the potential to become a valuable bioresource, it is disposed of due to a lack of research on possible industrial uses and a general aversion to blood. A recent study showed that the content of free amino acids, including branched-chain amino acids (BCAAs), such as valine (Val), isoleucine (Ile), and leucine (Leu), can be increased by enzymatic hydrolysis of pig blood [2]. In the human body, the 3 proteinogenic BCAAs Val, Ile, and Leu are included among the 9 essential amino acids (EAAs), accounting for 21% of the total protein content and 35% of the EAAs in muscle [5, 6]. Especially, the BCAA Leu plays important roles in the regulation of muscle protein synthesis [7, 8, 9].  
Increases in strength and endurance are related to muscle metabolism and mass. Pathological conditions, such as sarcopenia, cachexia, sepsis, burns, and trauma can weaken muscles and inhibit muscle damage repair [10]. The muscle regeneration process is necessary for muscle maintenance and damage repair systems and is precisely regulated by the expression of myogenic regulatory factors (MRFs), such as myoblast determination protein (MyoD), myogenin, and myogenic factor 5 (Myf5) [11; 12]. Myf5 is initially expressed after satellite cell activation, and newly formed muscle fibers sequentially express MyoD and myogenin [13]. Previous studies showed that Leu promoted the proliferation and differentiation of murine and porcine myoblasts [14; 15]. Leu deficiency was reported to inhibit the differentiation of both C2C12 myoblasts and primary mouse satellite cells via suppression of MyoD and Myf5 expression [16].  
The accumulation of muscle-specific proteins is associated with muscle growth, and protein deposition depends on the balance between protein synthesis and degradation [17; 18]. Previous studies have shown that nutrition can induce protein synthesis via the PI3K/AKT signaling pathway in rats and rainbow trouts [19; 20]. The mammalian target of rapamycin (mTOR) is the main mediator of the PI3K/AKT pathway, which plays an important role in protein synthesis [21]. mTOR regulates ribosomal S6 kinase (p70S6K) phosphorylation as one of the key downstream effectors and ultimately stimulates protein synthesis [22]. Leu promotes protein synthesis via the phosphorylation of translation initiation factors and ribosomal proteins of the mTOR signaling pathway [23; 24]. The anabolic action of BCAAs is also involved in the activation of insulin secretion and glucose uptake, which lead to beneficial effects on muscle maintenance and muscle loss [25].  
In this study, we examined the effects of porcine whole-blood protein hydrolysate (PWBPH) on exercise function in ICR mice and skeletal muscle differentiation in C2C12 mouse myoblasts and gastrocnemius (Gas) muscle of mice.