An extensor is a type of muscle that helps to straighten or extend a joint. When an extensor contracts, it pulls on the bones, allowing for movements such as straightening the arm at the elbow or the leg at the knee. These muscles are essential for various activities, including walking, running, and lifting. In the human body, common extensors include the triceps brachii, which extends the elbow, and the quadriceps, which extends the knee. Extensors work in opposition to flexors, which are muscles that bend joints. Together, these muscle groups enable a wide range of motion and functional movements.