For America's baby boomers, a generation weaned on Jack LaLanne, shaped by Jane Fonda videos and sculpted in the modern-day gym, too much of a good thing has consequences. Encouraged by doctors to continue to exercise three to five times a week for their health, a legion of running, swimming and biking boomers are flouting the conventional limits of the middle-aged body's abilities, and filling the nation's operating rooms and orthopedists' offices in the process. They need knee and hip replacements, surgery for cartilage and ligament damage, and treatment for tendinitis, arthritis, bursitis and stress fractures. The phenomenon even has...