An extensor muscle
is any muscle
that opens a joint increasing the angle between components of a limb, such as straightening the knee
and bending the wrist
. With the exception of the knee joint the movement is directed backward. This action is known as extension
Flexor muscles have the opposite effect, closing a joint and decreasing the angle between limb components.
In humans, some muscles include 'extensor' in their name, some include;
For further reference see genetic origin.