Knees in Need of Help

Pity the '80s aerobics teacher. She was once a taskmaster who urged students to "feel the burn" of seemingly endless reps—and one! and two! Come on, just 5,000 more! Now she's the one who's been burned. Those gung-ho aerobics gurus didn't have adequate cushioning in their shoes (or their floors), and today, many of them are struggling with stress injuries—from constant hip pain to plantar fasciitis, an inflammation of connective tissue around the foot. "It's very common for boomers who engaged in aerobics to have pain in the lower extremities," says Cedric Bryant, chief physiologist for the American Council on Exercise. "All that repetitive pounding seems to have worn them down."

Longtime runners, too, are hurting more as their bodies approach the finish line. They have "pristine cardiovascular systems," says Bryant, and in general they're healthier than their sedentary peers. But as they age, their ligaments and tendons shorten and tighten, and they develop muscle imbalances—tight lower backs, taut hamstrings, overdeveloped calf muscles and comparatively weak shin muscles. The result, says Bryant, is that "if you've run, run, run every day, and then you join the company softball team, you can easily strain your hamstring sprinting to first base." After which, of course, you'll want to walk (don't run) to the doctor's office.