 Therefore, PGC-1alpha gets expressed in muscles after acute and long-term exercise training.The function of this transcription factor is to increase the number and function of mitochondria, as well as to induce a switch of skeletal fibers to slow oxidative muscle fibers, as these are fatigue-resistant.Therefore, this UPR pathway mediates changes in muscles that have undergone endurance training by making them more resistant to fatigue and protecting them from future stress.