A flexor is a muscle that makes a limb bend. A flexor usually has two points of attachment on either side of a joint. For example, the biceps muscle attaches to the forearm and the upper arm. Flexing the biceps causes the arm to bend. Like all flexors, the biceps has an antagonistic ("opposite") muscle, called an extensor, that causes it to straighten – in this case the triceps (see antagonist).