Doing standard sit-ups? If so, that could be the sole cause of your low-back pain, especially if you have a desk-bound job. According to Stuart McGill, a leading low-lumber pain expert at Canada's University of Waterloo, sitting at a desk all day causes low-back flexion – the same thing that happens when you do traditional crunches and sit-ups. And all that spinal flexion can add up to low-back aches and pain. "The primary reason men ages 25 to 55 get low-back injury is because of repeated or prolonged spinal flexion," says McGill. But that doesn't mean you should ditch ab work: Strengthening your core without flexing your spine can help prevent low-back pain. McGill recommends doing planks and "stir-the-pots." Never done a stir-the-pot? Kneel in front of a Swiss ball with your back neutral or flat. Place your forearms on the ball, and without rotating your upper back, use your abs and glute muscles to draw circles with the ball on the floor, "stirring" in one direction, and then changing to the other side.