It’s easy for us to picture our muscles as being alive, responding to the demands we put on them, like lifting a toddler, pushing a lawn mower, or carrying a suitcase. Envisioning our bones as living is a lot more difficult. But our bones are indeed alive. Made up of living bone cells, fat cells, and blood vessels, along with water, collagen, calcium, and other nonliving materials, our bones build up, break down, and build up again in a dynamic cycle that continues throughout our entire life.

This cycle of building up (bone formation) and breaking down (bone resorption) is appropriately called ‘remodeling.’ As we approach, and especially pass, middle age, the balance between building up and breaking down shifts in favor of the latter, and we gradually lose bone mass. But because bone is living tissue, it continues to be influenced by the same things that affect other parts of the body. Exercise is one of those things, and almost regardless of our age, it’s something we can use to build better bones.

Bones respond to extra demands
Two types of exercise are especially good for building bone: weight bearing and strength-training. Weight-bearing exercise is simply exercise that makes your bones and muscles bear your weight. Jamie Tellner, Fitness Director of the Health and Wellness Center, explains it in a somewhat more descriptive way: “It’s any activity during which at least one foot impacts the ground.” From either perspective, weight-bearing exercise includes activities such as walking, climbing stairs, or aerobics (see box on page 13). Swimming and biking, although great for building muscle and for general fitness, aren’t considered weight-bearing because your body doesn’t have to work to support your weight.

How does weight-bearing exercise build bone? Just as a muscle gets bigger and stronger the more you use it, bone gets denser and stronger when you place extra demands on it. In practical terms, the body makes bones stronger to handle the added stress. Astronauts who spend extended periods of time in the weightlessness of outer space suffer significant bone loss because their zero-gravity environment makes no demands at all on their bones. When no demands are made, there is less new bone formation and a net bone loss.

Gaining strength through resistance
Strength training involves lifting, pulling, or pushing against resistance. Examples are lifting free weights (like light hand and ankle weights) or using weight machines. “It needs to be do-able, but still an effort,” explains Jamie. “The amount of weight providing the resistance should be greater than what you’d be handling in a normal day.”

According to Jamie, you shouldn’t do strength training two days in a row, because the muscles stressed by the additional weight need about 48 hours to “rebuild” and become stronger. The extra pull of muscles on your bones sends a message to your body to strengthen the bones, too, so that they can withstand the added strain. To a lesser extent, a weight-bearing activity such as hiking will also strengthen muscles that, in turn, tug on bones and make them denser and stronger.

Dealing with bone loss at midlife
Many people who exercise to build bone do so with the hope of preventing or reducing the bone loss that frequently accompanies middle age … bone loss that can lead to osteoporosis, a condition characterized by low bone density. Especially for women, this can be a major concern. “Once estrogen is gone following menopause, there is bone loss,” says Eileen Engle, MD, Doylestown Hospital’s Director of Women’s Services. “However, there is a great deal a woman can do, including getting 1200-1500 mg of calcium and 800 IU of vitamin D each day, taking bone-building medications if appropriate, and engaging in regular weight-bearing exercise.” Dr. Engle offers a word of caution, though, for women who have already been diagnosed with osteoporosis. “While certain types of exercise can be very beneficial for many women with low bone density, inappropriate exercise or activities could actually precipitate a fracture. That’s why it’s important for a woman with osteoporosis to speak with her physician before starting an exercise program.”

Taking control… and loving it
Reaching midlife can be the impetus for starting an exercise program for many people, and Libby Wheeler of Doylestown was no exception. “Last May, I joined the Fitness Center so that I could get started with some real exercise,” says Libby. “My mother had osteoporosis, so I want to do whatever I can to avoid what she went through. I know that walking is a good weight-bearing exercise and I used to take long walks around town,” she recalls. “But if it was too hot, too cold, too rainy, or too snowy, I wouldn’t go. I decided I needed to take control of the situation and get into an environment where I can time my walks on the treadmill, work through my strength-building routine, and just generally know what I’m doing.”

Libby feels that her time has been well spent, for both fitness and personal reasons. “I feel much more physically fit than when I started,” she says. “And I’m doing this for me – by myself and for myself. No interruptions. No phone calls. This is my time … and I’m loving it!”

Dr. Eileen Engle is Medical Director of Women’s Services at Doylestown Hospital. She is also with the Women’s Midlife Health Center at the Health and Wellness Center by Doylestown Hospital, Warrington.

If you have osteoporosis, osteopenia (milder bone loss), arthritis, or another chronic medical condition, speak with your physician about the best type of exercise for you before you start an exercise program.