Step forward with your left foot, lowering your hips until both knees are bent at about a 90-degree angle. Keep your front knee directly above your ankle, and lower your right knee to just above the floor.
Push away from the floor with your left foot — not coming completely back to standing but enough to straighten both legs and point your left foot forward — before lowering back down into your lunge.
Once more, push away from the floor, this time to your starting position with your legs together.
Repeat on the other side, and continue alternating for 30 seconds.