Muscles contract in different ways to move and stabilize the body. Prime movers or agonists are muscles that cause joints to bend or flex, while antagonists cause joints to straighten or extend and hold positions. Fixators work with agonists to maintain body position for movement, and synergists shorten to aid joint motion along with agonists. Isometric contractions occur when muscle length stays the same against resistance. During concentric contractions muscles shorten to move joints, like in bicep curls, while eccentric contractions lengthen under load, such as when lowering a weight.