Step-Up to Reverse Lunge
Face the stairs with your left foot on the second step and your right leg behind you on the floor.
(A) Lift your right knee to your chest, then quickly step back to the starting position.
(B) Step down to the floor with your left leg, positioning it behind your right, then do a lunge (with your right knee directly over your ankle). Quickly step back up, left knee to chest, and return to the starting position. Do 12 reps, then do 12 more on the other side.