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Physically active people have stronger bones and a lower risk of osteoporosis. But not every kind of exercise is equally useful for preventing fractures.

There is a broad scientific consensus that exercise prevents osteoporosis by maintaining healthy bone. But the type of exercise that does the most good for bone may surprise you.

What is the relationship between exercise and bone?

The way that exercise builds up bone is, counterintuitively, by breaking it down. The skeleton needs to be light and flexible for maximum mobility. When impact, say from hitting a ball, or jumping on the floor, or snatching a heavy weight, "bends" a bone, or when the muscles send signals to the bones that their connection to the bone is stressed, the bone activates a special class of cells called osteoclasts. These cells break down bone to allow it to assume a new, more efficient shape. Then another class of cells called osteoblasts lay down a scaffold of collagen to which new mineral crystals can attach to remodel the bone.

This process is activated by changes in physical activity. As long as you are doing the same routine the same way every day, there is no need to remodel the bone. Bone growth occurs in response to some strain or effort beyond the usual range. Just how much extra effort, impact, or stress is needed to build new bone depends on the location of the bone and the age of the person doing the exercise. If there is any good news for physically inactive people, it's this: The less active you are, the less extra effort it takes to help build new bones. As your bones get stronger and stronger and you get stronger and stronger, it takes more and more effort to build new bones. But physically active people generally have stronger bones.

What kind of exercise builds bone?

Aerobic exercise like swimming and bicycling are great for your heart and lungs, but they aren't particularly helpful for your bones. The kinds of exercise that stimulate bone growth come in three general categories:

  • Impact exercises provide a strong signal to bone that it is time to clear out old bone to remodel with new bone. Hitting a ball, jumping on the floor, snatching a weight, and getting hit in a contact sport all trigger the growth of new bone.
  • Eccentric exercises build the muscles that send the signals to bone to grow. An eccentric exercise isn't something like competitive ironing or clothes-free skydiving. An eccentric exercise "puts the brakes" on a concentric exercise. Lowering a weight is eccentric. Lifting a weight is concentric. Lowering your body during a pushup is eccentric. Raising your body during a pushup is concentric Eccentric exercises stretch muscles at the same time they are being contract. They send hormonal signals (growth hormone and sclerostin) to bone that it's time to stop building in one direction (they turn off obsteoblasts) and start recycling bone (they turn on osteoclasts). 
  • Resistance exercises exert force against another object. These kinds of exercises cause the muscles to send a signal to bone that it is time to reshape themselves to accommodate greater stress. These exercises are essentially any activity that forces the body to fight gravity. If you qualify as a star on "My 600 Pound Life," walking across the room is a resistance exercise for you. In fact, walking is the universal resistance exercise. Dancing, hiking, and stair climbing are great, too. Walking three to five miles (5000 to 8000 meters) a week makes a difference for bone health.
If you already have osteoporosis, all of these exercises are intimidating and some of them are dangerous. You can work with a professional trainer to do safe impact and eccentric exercises. But you almost certainly will get your doctor's advice to do resistance exercise.

Resistance exercise is the one safe form of exercise that is appropriate for just about anyone who has osteoporosis.

The special case of the hip bone

Many people with good reason dread the idea of breaking a hip. The femur, the thigh bone, which connects to the hip, is the strongest bone in the body, but it has a vulnerability. The proximal (upper) end of the hip resists up-and-down forces, but it is not as resistant to side-to-side forces. It is usually a fall to one side that breaks a hip. So what kind of exercise can we do to strengthen the femur against the most dangerous kind of fall?

The answer turns out to be hopping. A clinical trial with older men, aged 65 to 75, found that doing 50 hops a week for 12 months strengthens the bone in the superolateral femoral neck, the part of the femur most likely to be broken in a fall. Any one-legged jump increases the strength of the femur.

Of course, if you already have a fracture of the femur, it's not the greatest idea to do a bunny hop. Instead, ask your doctor about doing gentle exercises on parallel bars. Supporting your weight on the bars, you will tap, not hop, to build up bone mass. It is a much slower but much safer process for recovering bones.

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  • Photo courtesy of SteadyHealth

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