Pretty much all types of aerobic exercise help maintain muscle strength, coordination, and balance, all of which help keep you from stumbling and breaking a bone. That's a huge boon for bone health. But weight-bearing workouts such as running, hiking, racquetball, soccer – anything where your legs or arms must work against gravity to bear your bodyweight – are also vital for "loading" your bones, a.k.a. keeping them strong. Unfortunately, cycling and swimming don't count as weight-bearing – both are awesome aerobically, but they don't require bones and muscles to hold up your body. "Although it's really tough for adults to increase bone mass with exercise, doing weight-bearing workouts most days of the week helps maintain bone mass and prevents the drop in density that occurs when you don't exercise," Recker says. "Stay as active as possible. Your bones almost can't get too much physical activity."