Thus, flexion at the ankle is a movement in which the foot points downwards
[N.B. the ankle presents additional complications in that flexion is often
called plantar flexion and extension called dorsiflexion]. 
flexion is a movement at a limb joint which brings ventral (anterior)
surfaces closer together.
For the lower limb, it is seen at its simplest at the hip joint:
here flexion moves the limb forwards and upwards until the front of the
thigh meets the anterior abdominal wall.
flexion at the 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, flexion at the ankle is a movement in which the foot points downwards
[N.B. the ankle presents additional complications in that flexion is often
called plantar flexion and extension called dorsiflexion].
The main flexor muscles are: