## 4 Discussion  
Natural products are being re-evaluated as the demand for traditional medicines increases. Antler is a popular traditional medicine in Korea, China and South East Asia. However, concerns about potential side effects are discussed/reported rarely. Previous studies suggest that fermented and non-fermented antler are safe for in vitro and in vivo use [24; 25; 26]. Dear antlers contain the highest concentration of medicinal agents in the tip. Previous studies show that _Bacillus_ spp. or _Cordyceps militaris_ increase the amount of uronic acid and sialic acid (ganglioside) [27], which increase the capacity of the musculoskeletal system and improve exercise performance [28].  
Based on the results of previous studies, we fermented the middle/lower antlers with various LAB candidates and then studied the effects of one LAB strain on exercise performance.  
There are many ways to improve exercise performance. One is to activate the IGF-1 pathway. IGF-1 is contained within antlers. When the IGF-1 pathway is activated, exercise performance is improved [29]. Muscle hypertrophy inducers trigger protein synthesis by phosphorylating proteins downstream of the PI3K/AKT pathway in muscle cells. Among them, _mTOR_ is recognized as a major growth signaling factor that integrates various growth signals in cells [30]. Activation of _mTOR_ can increase muscle mass by inducing muscle protein synthesis via _p70s6k_, confirming that activation of the IGF-1 pathway increases muscle mass and exercise performance [31].  
Here, we showed that HY7602-FA increases IGF-1 levels in mouse cells. IGF-1 signaling was also identified in EDL muscle tissue from mice. The IGF-1 signaling pathway in the FA-treated group was activated to a greater extent than that in the exercise control, positive control (creatine), and NFA groups. This suggests that FA activates pathways associated with protein synthesis, thereby improving muscle strength and inducing hypertrophy.  
Another way to improve exercise performance is to increase muscle mass by increasing the number of mitochondria, a process called mitochondrial biogenesis. When the number of mitochondria increases, fatty acid oxidation and ATP production increase [32]. Therefore, mitochondrial biogenesis increases energy production and improves exercise performance. Mitochondrial biogenesis is regulated by a coactivator called _PGC-1a_ (per-oxisome proliferatorator-activated receptor-gamma coactivator 1 alpha), and the activity of _PGC-1a_ is regulated by the AMPK pathway and _SIRT1_ (sirtuin 1) [33].  
We confirmed in mouse cells that FA increased muscle protein synthesis and expression of genes associated with mitochondrial energy metabolism. Therefore, we conducted animal experiments to verify these in vitro observations.  
Expression of _AMPK/SIRT1/PGC-1a_ (genes related with mitochondrial energy metabolism) increased significantly in the HY7602-FA group compared with the exercise control, positive control (creatine), and NFA groups. The forced swimming and treadmill exercise tests showed that HY7602-FA significantly improved muscle endurance, even at relatively low concentrations. Furthermore, mice fed HY7602-FA were able to exercise for longer and showed decreased levels of serum markers of fatigue. Serum ALT, LDH, CK, lactate, and creatinine are indicators of accumulated fatigue stress resulting from exercise [34]. Under anaerobic conditions, the level of lactic acid in the blood is a product of carbohydrate conversion, which is the main source of energy for short-term intensive exercise. Accumulation of lactic acid in the blood causes fatigue; rapid removal aids faster recovery [35]. FA mitigated increases in fatigue and stress indices caused by intense exercise.