Bring yourself to the ground and enter a high plank position with your hands directly under your shoulders and your legs extended directly behind you. This is your start position. Keeping your core braced, a neutral spine and without dropping your hips, slowly extend your right leg up off the floor, keeping it fully extended. As you do this, simultaneously extend your left arm out and up in front of you. Hold this top position for a count of one before gently reversing the movement, returning to the start position. Repeat the same sequence using the opposite leg and arm. Keep alternating sides for the required number of reps (or time).