Extensor muscles are a group of muscles responsible for extending or straightening a joint. They are primarily found in the arms and legs, allowing movements such as straightening the elbow or knee. When these muscles contract, they pull on the bones, increasing the angle between them. In the human body, extensor muscles work in opposition to flexor muscles, which bend joints. For example, the triceps brachii is an extensor muscle that straightens the arm at the elbow, while the quadriceps group extends the knee. Proper functioning of extensor muscles is essential for activities like walking, running, and lifting.