Come into an all fours position on an exercise mat with your hands directly under your shoulders and your knees directly under your hips. Lift your knees so they hover a few inches off the floor. This is your start position. Keeping your core braced and a neutral spine, extend your knees so your body is in a pike position. As you do this, simultaneously lift one hand to tap the opposite knee. Quickly return to the start position, and repeat the same sequence using the opposite foot and hand. Keep alternating sides for the required number of reps (or time).