## 1 Introduction  
Antlers, used medicinally in Korea and China for over 2000 years, have been studied extensively. Pharmacologically active ingredients identified in antlers include ganglio-sides, pantorin (70% ethanol extract), amino acids, calcium phosphate, calcium carbonate, collagens, phospholipids, chondro, glucosamines, and hyaluronic acid [1; 2; 3].  
From the top to the bottom of an antler, the amount of collagen, mineral (including ash), and calcium (all indicators of chemical composition) increases, while the amount of functional material (proteins, fats, glycosaminoglycans, sialic acids, and uronic acid) decreases [4].  
For use as a herbal medicine, antlers are immersed in alcohol and then minced, which inactivates some of the key ingredients. To resolve this, fermented antler (FA) is produced by various fermentation processes, which increases the yield of effective ingredients, reduces or removes pesticide residues, increases the absorption rate, and increases the number of beneficial intestinal microorganisms [5; 6; 7].  
The human gut microbiota affects host environment by regulating several processes, including nutrient absorption, inflammation, immune control, and anabolic balance [8]. As interest in the gut microbiota increases, a connection with muscle strength improvement has emerged. Lactic acid bacteria (LAB) play a central role in the intestinal flora. Previous studies show that LAB have anti-obesity effects [9] and improve cholesterol levels [10], as well as showing anti-inflammatory [11], antioxidant [12], and anti-cancer activity [13]. Indeed, studies show that the anti-inflammatory, anti-reactive oxygen species (ROS), and mitochondrial regulatory properties of LAB are associated with prevention of sarcopenia [14; 15; 16]. LAB ferment food nutrients to produce short-chain fatty acids, which can be used as an energy source by muscle and liver cells; the resulting stabilization of blood sugar levels can improve endurance [17]. LAB also produces vitamins B12 and folic acid, which affect metabolic activity related to energy production [18], as well as promoting uptake of amino acids by muscle tissue, inhibiting fatigue caused by lactic acid production, and promoting muscle protein synthesis [19].  
Skeletal muscle, which makes up approximately 50% of the total body weight, is responsible mainly for generating force (which translates to movement) and heat [20]. The most important factor that determines muscle function is muscle mass, which is maintained by the balance between protein synthesis and decomposition. Currently, there are no treatments that effectively prevent onset of muscle weakness or fatigue.  
Antlers (which are regenerated annually) contain high contents of growth factors such as insulin-like growth factor (IGF), making them an ideal research model for skeletal growth and bone differentiation. Indeed, studies on FA and muscle strength have been conducted [21; 22; 23]; most of the previous studies have only seen behavioral experiments of exercise performance ability with antlers before fermentation. Even in studies proceeding fermentation, the analysis of the mechanism of action is inadequate. Furthermore, little is known about whether fermenting antlers with various LAB affects biological activity.  
Here, we show that products derived from LAB-fermented antlers increase mitochondrial biogenesis, muscle strength, and endurance in mice both in vitro and in vivo via the IGF-1/AKT/mTOR pathway and AMPK signal pathway.