How Exercise Can Improve Bone Density
For exercise to affect bone density, it needs to be high impact and weight bearing. That’s why activities like running, basketball, volleyball, gymnastics, soccer, tennis, weightlifting, and high-intensity interval training (HIIT) can have a profound effect on bone formation — they increase the load on your bones, forcing them to adapt so that they can better tolerate the strain imposed by those activities. It’s also why activities like walking, swimming, and stationary cycling (aka “spinning”), while effective for weight loss and improving cardiovascular fitness, have minimal impact on bone health. Being non-weight-bearing, they don’t increase the load on your bones, and thus don’t provide enough stress to cause an uptick in remodeling.
When you lift a weight, stretch a resistance band, jump (and land) repeatedly, or pound the pavement, you create a compressive force that causes fluid to flow within your bone tissue. Cells called osteocytes detect that flow, and trigger an increase in bone formation as a result. “The degree of flow is proportional to the strain imposed by the exercise,” says Stiles. The greater the strain, the greater the exercise’s potential impact on bone remodeling.
Weight-bearing exercise can also stimulate bone formation in a way that is similar to how it stimulates muscle growth: by damaging bone tissue on the cellular level. This “micro-trauma” initiates a healing response, but the body doesn’t just repair the damage — it reinforces the bone’s collagen and calcium matrix to make it stronger than it was before. “New bone isn’t just laid on top of old bone, like some people think,” says Stiles. “The bone changes its structure and increases its strength right down to its core.”
Just 12 months of resistance training is enough to increase bone mineral density more than one percent, according to a recent study by Hinton and her colleagues at the University of Missouri. That might not sound like much — until you consider that it roughly matches the rate of bone loss after the age of 40, and that participants in the study exercised as little as twice per week.
At this point, it’s important to note two things. First, “the effect is site specific, meaning only the loaded bone gets stronger,” says Hinton. If you’re a runner, that means you’ll need to add upper-body work (in the form of strength training) to your program if you want to optimize bone formation above your waist. Second, it is possible to get too much of a good thing.
“We don’t know the upper limit yet, but we do know that overtraining can be a problem, especially in women,” says Stiles, explaining that, much like menopause, it can cause a reduction in bone-regulating hormones, such as estrogen. “That’s one reason you need to build sufficient recovery time into your workout program.”
Here’s another: Like muscles, bones don’t grow stronger during workouts, they grow stronger between them. If you never allow your bones enough time to complete their repair process, you’ll never optimize your bone density. “You’ll also increase your risk of injury,” says Stiles.