The human body has more than 650 muscles, which make up half of a person's body weight. They are connected to bones by tough, cordlike tissues called tendons, which allow the muscles to pull on bones. The narrow bands on the top of your hands are tendons; you can see them lengthen and shorten as they pull on your fingers to make them move. Bones are fastened to other bones by long, fibrous straps called ligaments, which wrap around the joints. Cartilage is a flexible, rubbery substance that supports bones and protects them when they rub against each other. Together, bones, muscles, tendons, and ligaments form the musculoskeletal system.

Humans have three different kinds of muscle - skeletal, smooth, and cardiac. Skeletal muscles are attached to bone, mostly in the legs, arms, abdomen, chest, neck, and face. These muscles are called striated because they are made up of fibers that have horizontal stripes. They hold the skeleton together, give the body shape, and help it with everyday movements. Skeletal muscles can contract (shorten or tighten) quickly and powerfully, but they tire easily and have to rest between workouts. Their size varies greatly depending on the job they do, and they are known as voluntary muscles because you can control their movement.