筋肉 (kinniku) refers to muscle tissue in the body, which is responsible for movement and maintaining posture. Muscles are made up of fibers that contract and relax, allowing for various physical activities. There are three main types of muscle: skeletal, smooth, and cardiac, each serving different functions in the body. Skeletal muscles are attached to bones and enable voluntary movements, while smooth muscles control involuntary actions in organs like the stomach. Cardiac muscle is unique to the heart, facilitating its pumping action. Proper nutrition and exercise are essential for maintaining healthy筋肉 and overall physical fitness.