Muscles are a big part of your body. Skeletal muscles are also called voluntary muscles, meaning that they move when you tell them to. They are attached to your bones. Skeletal muscles are connected to a tough cord called tendons. Tendons connect bones to muscles. They also attach the periosteum to the bone. The muscles pull on bones to make motion. Muscles work in pairs to move the bone. When you pick something up your muscle contracts on one end and relaxes on the other.