Muscles are pieces of soft tissue throughout your body. They help you do everything from holding your body still to running a marathon. Muscles also move and support your organs. Your heart is a hard-working muscle that beats thousands of times a day to keep you alive.

Muscles control all movement in the body. There are more than 650 muscles in the human body. Muscles work together with bones to help you move. Muscles and bones (your skeleton) are part of the musculoskeletal (muh-skyuh-low-SKEH-luh-tl) system.

Muscles are attached to bones by tendons and help them to move. When a muscle contracts (bunches up), it gets shorter and so pulls on the bone it is attached to. When a muscle relaxes, it goes back to its normal size. Muscles can only pull and cannot push. Therefore muscles have to work in pairs to move a joint.

Muscle powers the movements of multicellular animals and maintains posture. Its gross appearance is familiar as meat or as the flesh of fish. Muscle is the most plentiful tissue in many animals; for example, it makes up 50 to 60 percent of the body mass in many fishes and 40 to 50 percent in antelopes.

The muscles that move the human skeleton vary greatly in shape and size and extend to every part of our bodies. The muscular system contains over 600 skeletal muscles alone, which make up about 40% of our mass. Blood vessels and nerves run to every muscle, helping control and regulate each muscle’s function.