Bending an elbow is similar to bending a knee. First, your brain sends impulses through your central nervous system and throughout your nerves. Then, the impulses cause flexible muscles to contract and relax, making them lengthen and shrink. Your muscles are connected to your bones by tendons, a connective tissue. As muscles contract and relax, they pull and push on the tendons to pull and push on the bones, allowing a joint to bend. Your biceps contract and triceps relax to bend your elbow. Your biceps relax and tripceps contract to straighten your elbow.