Antiaging Dock: Muscle Age

There are over 600 muscles of various sizes in the human body, playing a crucial role in maintaining life activities. Based on their structure and function, they are classified into three types: skeletal muscles, which make up about 40% of the muscles and can be increased through exercise; cardiac muscles, which move the heart; and smooth muscles, which move the digestive tract and blood vessels, promoting digestion and blood flow. Muscles not only move the body, such as walking, running, and sitting, through the contraction and relaxation of skeletal muscles, but also protect organs and bones from external shocks, generate heat to increase basal metabolism and prevent lifestyle-related diseases, maintain body temperature, and promote the circulation of blood and lymph fluid, among other functions. The calf contains a large amount of muscle and has the function of pushing back the blood that has circulated to the leg, which is the farthest from the heart. For this reason, the calf is also referred to as the second heart. Muscles are also related to immunity, and immune cells, including lymphocytes, are activated by the amino acid glutamine, which is stored in large amounts in muscles. Therefore, it is said that if muscle mass decreases, immune function may decline. Consequently, a decrease in muscle mass may lead to physical discomfort. Skeletal muscles produce several types of hormones, collectively referred to as myokines. The first myokine was discovered in 2003, and more than 50 types of myokines have been identified to date, all of which are secreted from skeletal muscles through exercise and exert various systemic effects [1]. Their typical functions include (1) formation and regeneration of muscles and bones, (2) antiinflammatory effects, (3) involvement in carbohydrate and lipid metabolism, and (4) protection of cardiac muscle cells and vascular endothelial cells.