Stand with both feet together. Take a step forward with your right foot like you're going into a lunge, allowing your left heel to come off the ground. Place your hands on your hips for stability.
Slowly bend your knees until your right thigh is parallel to the floor and your left knee nearly touches the ground. (At the bottom of the move, both knees should be bent at about 90 degrees.) Keep your torso straight and core pulled in toward your spine.
Drive through your feet to push back up to standing.
Immediately go back into a split squat, and repeat for the full 40 seconds.
Once the 40 seconds is up, rest and complete the move with your left leg forward.