Get down on all fours with your hands at shoulder width apart. Then, stretch your legs out in back of you with your toes tucked (push-up position). Keeping your back straight, extend your right arm in front of you while simultaneously lifting up your left leg (keeping it straight). Slowly come back to the starting position and repeat with the left arm and right leg.
Do two sets of eight reps (one rep = each time you lift one arm and leg).