An "extensor" is a type of muscle that helps to straighten or extend a joint. When an extensor muscle 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 flexor muscles, which bend joints. Proper functioning of extensors is crucial for maintaining mobility and overall physical health.