Lower-body strength translates into good balance, flexibility, and endurance. As you get older, those attributes are key to reducing your risk of falls and injuries—particularly hip fractures, which often quickly lead to declining health. Up to 20% of hip-fracture patients die within 1 year because of complications from the trauma. "Having weak thigh muscles is the number one predictor of frailty in old age," says Robert Butler, MD, president of the International Longevity Center–USA in New York City.

To strengthen them, target your quads with the "phantom chair" move, says Joan Price, author of The Anytime, Anywhere Exercise Book. Here's how: Stand with back against wall. Slowly walk feet out and slide back down until you're in a seated position, ensuring knees are behind toes and lower back is pressed against wall. Hold until your thighs tell you, Enough! Do this daily, increasing your hold by a few seconds each time.