"Over time, sitting for long stretches weakens your core-the muscles that should hold you upright-and leads to far too much pressure on your back muscles… I had one-on-one lessons with Madonna's Pilates instructor in London…I do Pilates sessions twice a week and ride my horse twice a week (horse riding is excellent for strengthening the core, legs, and arms.) I never sit at the computer for more than 30 minutes without standing up and stretching. I have hired an assistant who types while I dictate."