The Anatomy of Bone
Bone is a living tissue with a rigid, honeycomb-like structure comprised of collagen and calcium. Like every other tissue in the body, it has nerves, blood vessels, and cells, and is in a constant state of “remodeling,” breaking down and rebuilding itself like an endless construction project. Indeed, about 10 percent of the average adult’s bone mass is remodeled each year.
“People tend to think that bone is a solid, static thing,” says Stiles. “But it is always responding, always regenerating.”
Bone formation outpaces breakdown until sometime between the ages of 25 and 30, at which point the body achieves peak bone mass and remodeling plateaus for about a decade. “Bone-strengthening exercise is important throughout the lifecycle,” says Pamela Hinton, Ph.D., an associate professor in the department of nutrition and exercise physiology at the University of Missouri. “But during periods of skeletal growth, exercise is especially effective at increasing bone mass and strength.”
That’s important because the more bone you build before the age of 30, the more you’ll have in the bank when bone density begins its slow, steady decline at around the age of 40. The process is hastened in women thanks to a decline in estrogen (a key bone mass regulator) that occurs during menopause, but it can have serious consequences for men as well: After the age of 50, approximately one in two women and one in four men will break a bone — usually in the hips, spine, or wrists — due to osteoporosis, according to the National Osteoporosis Foundation. If such a fracture happens after the age of 80, the potential consequences are grim: Thirty percent of octogenarians who break a hip die within a year of doing so, according to researchers at the Mayo Clinic.
“But even after the cessation of skeletal growth, exercise can still increase bone mass and slow the rate of age-related bone loss,” says Hinton.
In short, it’s never too late to start exercising to preserve bone density, and once you do, it’s not something you ever want to stop.
How Exercise Strengthens Bones
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.
The Importance of Nutrition
Proper nutrition plays a key role in the recovery process as well. But it’s not just about consuming the recommended 1,000mg of calcium per day or getting enough of the vitamins and nutrients that help you store it (such as vitamin D, potassium, and manganese) — you also have to make sure you’re consuming enough total daily calories, according to a review in the journal Current Sports Medicine Reports. The researchers found that some runners’ bone density is no greater than that of people who don’t exercise at all, and one of the primary reasons is that many runners don’t take in enough calories to meet their energy needs.
Another reason is the repetitive nature of distance running, according to the researchers. “When you do a lot of steady-state exercise it becomes a bit like white noise to bone cells — they become desensitized to the constant stimulation and basically switch off,” says Stiles. “Brief bursts of intense exercise seem to work best for improving bone density.”
That doesn’t mean you should stop running if that’s your thing — just make sure that “long, slow distance” isn’t the only way you enjoy it. Incorporate intervals, tempo runs, and other intermittent, high-intensity efforts into your training plan, and don’t forget to strength train once or twice a week. If you’d like to cover all of your fitness bases (muscular, cardiovascular, and bone) at the same time, consider HIIT) You’ll find it in many of the programs available on Beachbody On Demand, including CORE DE FORCE, TurboFire, and INSANITY.
Of course, all of this advice assumes you’re healthy enough for high-impact, weight-bearing exercise. Not everyone is. “I’m getting there,” says Daikeler. “I’m slowly figuring out how to perform weight-bearing exercise that doesn’t aggravate my hip, and it involves a lot of patience and upper-body work, which isn’t something I’m used to doing.”
But like every other fitness goal, building stronger bones is worth doing. Many people consider increased bone density a fringe benefit of exercise, but the reality is that it’s right up there with losing weight and becoming stronger — especially when it comes to aging.