What is the function of extensor muscles?

Answer: Increase the angle at the joint
Extensors increase the angle at the joint. An extensor muscle is a muscle that, when contracted, extends or straightens that limb or part of the body.
Flexors reduce the angle at the joint. Flexor muscles bend the limb at the joint when contracted.
Abductors draw a limb away from the midline.
Adductors draw a limb back towards the body.