Stand with your feet flat on the floor at hip width apart.
Tie a resistance band in a loop around a secure object.
Place your affected leg in the loop with the band behind your knee.
Step back so you have some tension in the band.
Lift your other leg up so you are balancing on your affected leg only.
Keeping your body straight, bend your knee a little and then straighten back up slowly.
As you straighten your knee back up, you will be pulling against the resistance of the band, working your thigh muscles.
Relax and repeat.