Muscles are used to express power and strength within the body. From walking up a flight of steps to pushing heavy objects, muscles make it possible for us to do many different types of movements.
Muscles have to have adequate energy supplies, oxygen, and carbon dioxide levels in order for them to work. Muscles pull on bones at joints to generate movement; they also act as pumps to move the blood around the body.
Muscle fibers are multinucleated cells. A single muscle may be composed of many myofibrils. A myofibril is made up mainly of actin and myosin filaments in a repeating pattern (known as the sarcomere).