CKMT2 and rheumatoid arthritis: The gastrocnemius muscles of RA rats were predicted to be under the energy/ATP-insufficient status because in addition to insufficient energy/ATP production from glycolysis and mitochondria, there was a significant decrease of CKM (cytosolic) and CKMT2 (mitochondrial) proteins, which are involved in the phosphocreatine/creatine (PCr/Cr) shuttle associated with ATP reservation and regeneration [46].