Tie a resistance band off to one side and stand with your leg to be exercised closest to the band.
Loop the band just below this knee so that the band travels away from you to the side.
Take a large step forwards on this leg, dropping your knees down towards the floor into a lunge.
The band will try to pull your knee outward.
Resist this movement by keeping the movement of your knee directly forwards over your toes.
Ensure your body remains upright with your core stability muscles engaged throughout.