extension

For the lower limb, it is seen at its simplest at the hip joint:
here extension moves the limb backwards.

extension at other lower limb joints is more complex because (during embryological development) the limb undergoes torsion - otherwise you would stand on the backs of your feet rather than the soles! Thus, extension at the knee moves the limb forwards (e.g. as in kicking).
Similarly extension at the ankle is a movement in which the foot points upwards (NB The ankle presents additional complications in that extension is often called dorsiflexion).

The main extensors are:

1. for the hip - gluteus maximus
2. for the knee - the quadriceps
3. for the ankle - any muscle whose tendon passes into the foot anterior to the ankle
   (e.g. tibialis anterior, extensor digitorum longus, extensor hallucis longus).