Start in a plank position with your hands directly under your shoulders and your feet in line with your hips. Lift your left leg off the ground so your heel is even with your pelvis.
Keeping your torso stable, bring your right elbow to your left knee under your body. Do not round your back or twist your spine, and keep your pelvis down. Then reach the arms and leg out and away from each other.
Repeat on the other side, keeping the movement steady and controlled.
Continue alternating sides for 40 seconds, and feel free to take a Child's Pose during your 20 seconds rest before moving on to jumping lunges.