1. Lunge and knee drive
Take a medium-sized step back into a lunge, bending back leg so knee almost touches the ground. Push into front foot to return to standing and drive knee back up toward chest. Do 10 to 12 reps. Switch sides. Repeat.
Perfect your form: Hold chest up (no slouching!) with shoulders facing forward.
Dial it down: Do the lunges without the knee drives.
Amp it up: Hold a pair of light hand weights by your sides.