Bring yourself to the ground and enter a high plank position with your hands directly under your shoulders. This is your start position. Keeping you core braced, reach your left arm back underneath you to touch your right foot. Feel the contraction of your core as you move. Reverse the movement to return to the start position. Then reach your right arm back underneath you to touch your left foot. Return to the high plank position. Lower your chest towards the ground by bending your elbows until your chest is just above the ground or you feel a stretching of your chest and shoulders. Hold for a count of one then press upwards from your chest and shoulders, straightening your arms as you return to the high plank position. Repeat this full sequence for the required number of reps (or time).