Put weight in the left foot, lift your right knee up, and step into a forward lunge. The front knee should be at a 90-degree angle with the left knee just barely hovering above the ground.
Push into your right heel, come to stand, and immediately step the right foot behind you into a reverse lunge with the left knee at a 90-degree angle.
This counts as one rep.
Do as many reps as you can for 15 seconds on this side, then do another 15 seconds on the other side.
