BONE HEALTH

Exercise Is Not the Path to Strong Bones—Or Is It? I asked Vanessa Yingling, a faculty member in my department who is an expert on bone to take a look at the New York Times articles and respond. –PMc Misconception: All you have to do is walk or do even the most modest strength training exercises to build strong bones. Actually: Exercise has little or no effect on bone strength. —“Exercise Is Not the Path to Strong Bones,” New York Times Thought to be an April Fool’s Day hoax by some, the article by Gina Kolata on April 1, 2016, titled “Exercise Is Not the Path to Strong Bones,” was real. Kolata’s article in the New York Times Misconception series stated that exercise was not beneficial to bone strength. The response to this article was so overwhelming that Kolata published a follow-up piece titled “A Second Look at ‘Misconception’ on Exercise and Bones,” clarifying her original points: that she only meant to focus on exercise and bone in adult populations.

Bone Health Although some studies indicate that exercise has minimal effect on bone strength in adults, only a portion of the large body of literature on this subject was discussed. Cochrane has for years produced systematic reviews of primary research in health care and is recognized as the highest standard in evidence-based resources. In fact, the Cochrane review in 2011 concluded that exercise will improve bone mineral density slightly and exercise X-ray absorptiometry (DXA). However, one limitation of the 2-D DXA measure is the lack of architectural information on our bones. The author in these pieces missed an opportunity to educate the public on key aspects of bone health: 1. Architecture is a key component of bone strength. 2. Increasing bone strength in adults is not the goal; the more realistic focus should be on mitigating the rate of bone loss. Kolata’s focus was on adults; however, adolescence is a key time period to optimize bone health through exercise, and adults can model that behavior for children. 3. Research has identified unique features for an exercise prescription focused on skeletal health. Bone Architecture Is a Key Component of Bone Strength A discussion of bone health, and specifically bone strength, cannot gloss over how these parameters are measured. Stronger bones could and should lead to healthier bones, but direct measurement of bone strength in humans is difficult since most of us would hesitate to have our bones By Vanessa Yingling, PhD Continue on Page 5 Exercise Is Not the Path to Strong Bones—Or Is It? I asked Vanessa Yingling, a faculty member in my department who is an expert on bone to take a look at the New York Times articles and respond. –PMc Misconception: All you have to do is walk or do even the most modest strength training exercises to build strong bones. Actually: Exercise has little or no effect on bone strength. —“Exercise Is Not the Path to Strong Bones,” New York Times Thought to be an April Fool’s Day hoax by some, the article by Gina Kolata on April 1, 2016, titled “Exercise Is Not the Path to Strong Bones,” was real. Kolata’s article in the New York Times Misconception series stated that exercise was not beneficial to bone strength. The response to this article was so overwhelming that Kolata published a follow-up piece titled “A Second Look at ‘Misconception’ on Exercise and Bones,” clarifying her original points: that she only meant to focus on exercise and bone in adult populations. will slightly reduce the chances of having a fracture in postmenopausal women. Exercise has the potential to be a safe and effective way to minimize bone loss. Kolata brought up, but did not fully discuss, a key point in understanding research on bone health in her follow-up piece: that bone density and bone strength differ. The diagnosis of osteoporosis is currently based on an estimation of bone mineral density (BMD), measured by dual-energy
gy.org broken, even for science. Therefore, new scanning technologies are continually being used to add to our understanding of bone strength. Best research practice requires the use of 3-D imaging techniques in addition to DXA measures. The strength of our bones cannot be described using just one parameter. A key study (Robling et al., 2002) using adult rats following an exercise protocol reported minimal changes (5 percent) in bone density (BMD) but large changes (64-94 percent) in bone strength. In addition, 50 percent of women and 70 percent of men who sustain a low trauma fracture don’t have a diagnosis of osteoporosis (Sanders et al., 2006). Take Home Point: Factors beyond BMD, such as bone size and architecture, are key components of bone strength. The strength of most structures is a combination of the material that makes up the structure, the amount of material, and the architecture of the structure. For example, bike frames made of steel are strong, but bike frames made of aluminum can be just as strong if the tubes are much larger. Hollow tubes are as strong or stronger than solid tubes. The strength of hollow structures such as bamboo, flower stems, bike frames, and our own long bones is due to the distribution of the material away from the center bending axis (second moment of inertia). Figure 1 is an example of two different architectures that yield similar bone strength. In this example, the 36-year-old male has a smaller, more slender bone and thus a smaller moment of inertia, which should render this tibia weaker than that of the 42-year-old male. However, the densities of the bones also differ. The 36-year-old male maintains his bone strength with a denser but smaller bone, and the 42-year-old male has a larger and less dense bone with the same strength. Take Home Point: One parameter, such as bone density, cannot fully describe bone strength. Conservation Is Not Acquisition The goal for much of the population is not to gain bone mass but to minimize bone loss. Our bones add and lose bone in a mechanically smart manner. As we age, we lose bone mass mainly from the endocortical surface (inner surface of the bone) but we also form a small amount of bone (even as we age) to the periosteal, or outer, surface of the bone—a mechanically smart process. Aging results in the overall decrease in the amount of bone, but the size (diameter) of our bone increases to take advantage of the moment of inertia concept. Take Home Point: Bone strength can be preserved as we lose bone mass (figure 2). In addition, data suggest that bone does not subscribe to the “use it or lose it” paradigm. Exercise results in increased bone mass added to the outer surface of the Figure 1. The bones have equal strength. Continue on Page
 bone (periosteal surface). A study using professional baseball players reported that the gains in bone size and architecture during youth persist later in life. They report that even though bone mass decreased after the players retired, the benefits of bone size and strength persisted (Warden et al, 2014). Take Home Point: Exercise bone and it may last a lifetime. Model Good Behavior: Exercise With Your Kids It may be true that less-than-spectacular results from exercise are reported in adult populations. But as noted in Kolata’s article, exercise has other benefits for fracture reduction, including increased muscle mass that can decrease the risk of falling. From my perspective, an important reason to exercise is to model behavior for our kids. Adolescence is a key time to optimize bone strength. As illustrated in figure 3, there are two major approaches to decreasing fracture as we age. 1. Decrease the loss of bone during aging 2. Increase the bone strength while we are young so we age gracefully and without fracture Take Home Point: Exercise during adolescence may strengthen bone for a lifetime. Exercise Prescription for Bone Health Although a specific exercise prescription for bone adaptation remains to be determined, Kolata missed an opportunity to highlight what we know at this stage regarding the positive effects of physical activity and sport on bone. Specifically, positive effects result from unique, variable, and dynamic loading. Static loading does not trigger an adaptive response in bone; dynamic exercise is necessary. Also, the applied load must exceed the usual loading conditions to stimulate bone formation. Currently, weightbearing activities are recommended to stimulate bone formation due to the greater magnitude of load that these activities apply to the skeleton. Activities that provide larger loads, such as weightlifting, volleyball, basketball, and gymnastics, produce a greater stimulus compared to endurance activities. such as distance running and triathlon. Variety in exercise modalities is a key to bone health. Your bones, although they need high-impact loading, also need rest. Emerging
evidence indicates that bone cells lose sensitivity to mechanical stimulation after a certain number of loading cycles and that recovery periods are needed to restore the mechanosensitivity. Animal studies have proposed that very few loading cycles are necessary to optimize bone forma - tion. Furthermore, for a given number of loading cycles, multiple daily sessions are more beneficial than a single daily session. Therefore, you should exercise your bones multiple times per day for short durations. Take Home Point: Short duration, highimpact exercise multiple times per day is optimal. The Wrap Up A misconception is defined as a view or opinion that is incorrect because it is based on faulty thinking or understand - ing. Exercise’s effect on bone strength, although complex and age dependent, is not based on faulty thinking. Exercise is effective for bone strength development and healthy aging. Cochrane Review. (2011, July 6).