Resistance training, also known as strength training, not only helps to build your bone density, but also may help build your muscle mass and improve your flexibility and balance. It may also reduce your risk of developing arthritis or back pain or reduce existing symptoms.
A Tufts University study of hip and spine bone density in postmenopausal women found that women who participated in resistance training two days every week for one year gained one percent of bone mass, saw a 75 percent gain in strength and a 13 percent increase in balance [source: Centers for Disease Control and Prevention]. Aim for 30 minutes of resistance exercise two to three times per week. Remember, start with five minutes at a time if that's all you can manage and gradually build your length of exercise as you become stronger.