An extensor muscle is any skeletal muscle that opens a joint increasing the angle between components of a limb, such as straightening the knee or elbow and bending the wrist or spine. With the exception of the knee joint the movement is directed backward. This action is known as extension.
The type of muscle that has the opposite effect is the flexor muscle, which closes a joint, decreasing the angle between limb components.
In humans, some muscles include 'extensor' in their name, some include;
- Extensor carpi radialis brevis
- Extensor hallucis longus muscle
- Extensor carpi ulnaris muscle
- Extensor carpi radialis longus muscle
For further reference see genetic origin.