Things like lunges could help but check your posture: knees never forward of your ankles and knees inline and not to the side. It shows your thighs are inbalanced or weak if this happens and need work.
I was told about inner leg/ankle stregnth if thats any help, its something better youtubed as its hard to explain but basicly is about having your foot on the edge of a step with the instep off the edge. You raise your instep WITHOUT raising the heel, bending the knee to pick it up or rolling your foot. It took me ages to get the hang of it and a physio explained it was because the muscle in my inner calf was very weak and that my ankle had also become weak as a result.
Other lower leg straightening could be cycling, kicking a ball, calf raises for the posterior lower leg (standing on a step, heels off the edge and rasing yourself up and lowering yourself down with control. Holding on to the walls around for balance.
Another for the anterior calves would be, sitting down, knees bent, picking up a weight from the ground with it balanced between your feet. Lowering down with control.
As with most, reps of 8-15 depending on what you feel, rest for 15-30 seconds and then begin again. If you can do more then 3-5 sets in one go then up the weight or resistance. If you really can't manage because its too painful or you feel too weak doing the exercises in a swimming pool could be an option.
