Bone of Contention

“Vulnerability is not a weakness or something we can arrange to do without...it is the ever present and abiding undercurrent of our natural state.” - David Whyte

We think of our skeleton as being solid and inert, but bone is a dynamic tissue that is always in flux, sensing and responding to the stress encountered as we move through our day. Although bone physiology is complex, bone formation and loss can be understood as a balance between two different types of bone cells--osteoblasts and osteoclasts--that respond to how much load a particular bone is required to bear over an extended period of time. Mechanical loading of the bone is transmitted through the tendons, which are cords of strong fibrous collagen tissue that arise from the muscle to attach to bone. When the tendon pulls on the bone repeatedly and with sufficient force—which happens during exercise--it stimulates the osteoblasts, which build bone mass and strength. When there is less loading, as when we have long periods of inactivity, osteoclasts predominate, breaking down bone so that it can be reabsorbed.

If you are a postmenopausal woman, you may very well be thinking quite a bit about your bones, especially if you’ve been diagnosed with osteoporosis or osteopenia. Low bone mass, the main characteristic of osteoporosis, has been part of the human condition for thousands of years—well preserved skeletal remains from 6000 years ago show evidence of thinning bones in women who were as young as 45. Scientists have understood the physiology of bone remodeling and bone loss since the mid-19th century, but it was always considered a normal part of aging; it is only recently that it has been categorized as a disease.

In the search for nonpharmaceutical interventions to build bone, exercise is increasingly prescribed as a treatment for osteoporosis. Just how good is it at building bone? In the past two decades, there has been a good deal of research studying the effects of exercise on building bone; overall, the results have yet to offer any definitive answers, yielding a somewhat mixed bag of results. What is clear is that exercise can be good medicine for our bones, but it can be a hard treatment to prescribe and an even harder pill for many to swallow.

What do we know so far? First and foremost, bones get stronger only when they are exposed to loads that exceed what is encountered in everyday activities. The most common way of exposing bones to load is with strength training or high intensity movements such as jumping and stair climbing. To be effective, these exercises require a certain degree of discomfort, and we must be willing to push beyond our comfort zone to get stronger. Lots of repetitions with light weights may work at the onset for those who are very weak or deconditioned, but only to a point. Bones have a threshold level of adaptation; loads above this set point make our bones stronger. Once we stop challenging ourselves, the bone becomes desensitized and, if the loads stay the same, will eventually trigger osteoclasts to start the process of bone reabsorption. Decades of research makes it clear that exercising to strengthen bone is not a once and done activity but a commitment to continually push our limits.

It has been said that what is good for the bones is good for the muscle. Science is showing that the opposite is also true--bone loss seems to mirror decreases in muscle strength, especially when it comes to our ability to create power. As we age, due to either inactivity or injury, we tend to engage in more endurance activities-- walking, cycling, or swimming--that maintain a steady pace and rhythm over an extended period of time. Power moves, common in bootcamp classes, include rapid changes in pace, unusual patterns, or use fast, dynamic, and explosive movements. These are high intensity moves that require a solid base of fitness, have an increased risk of falls and injury, and are much harder to do with back issues, poor posture, or arthritis. The ability to produce power has been shown to decrease rapidly with disuse. Endurance activities and power moves use different types of muscle fibers; it is the fibers associated with high intensity moves that have been found to stimulate bone growth. People with osteoporosis show significant atrophy in the muscle fibers that produce power. The good news is that anyone can build their capacity to engage in high intensity exercise—even 90-year-olds have learned to safely jump onto a low box—but it takes effort, commitment, and practice.

We must keep in mind that skeletal adaptations are site specific, not systemic. That is, shoulder raises with a heavy weight will not build the bones in the spine. For exercise to be effective in addressing osteoporosis, it must load common fracture sites. Walking at a very brisk pace (faster than 4 mph) wearing a weighted vest while maintaining good posture has been shown to improve bone density, with some studies showing an improvement as high as 40%. Before you rush to buy a weighted vest, be advised that it is hard to walk at a 15-mph pace—very hard--and even harder to do so wearing a heavy vest. Importantly, exercise interventions take time, so one needs to be ready and willing to work intensely at least twice a week. The response time for bone is slow, with remodeling taking as long as three to eight months, and only when the program is followed consistently and with progressions in load.

Although we may look at medical practices from previous centuries as quaint or backwards, we must remember that science can only provide us with treatments that reflect evidence from what they were able to see and quantify at the time. In the mid-19th century, fevers were considered the cause of many diseases. It wasn’t until scientists developed the germ theory of disease that physicians understood fevers as symptoms, not causes. Studies related to exercise and bone health are also limited by the methods currently available to study them. The gold standard for testing any intervention is the randomized controlled trial (RCT), which for osteoporosis requires at least 7,000 participants who are all at a high risk of fractures followed over five years. In an RCT, some receive the intervention, some do not. Only then can we definitively identify the exercises that build bone. Since osteoporosis is a condition that is typically diagnosed in the later decades of life, a time when arthritis, muscle atrophy, and general deconditioning is common for many, finding 7,000 participants who have similar health profiles is a daunting task. It is challenging enough to get frail people to exercise; it is a herculean task to find 7,000 who are willing and, importantly, able to jump, squat, hop, walk/run fast, and lift heavy weights, twice a week for at least eight months.

Given the limitations of RCT, our best resources are systematic reviews of studies from which we draw general conclusions. We have learned a good deal about exercise from decades of research. Due to the evolving nature of research, we know quite a bit about what does not stimulate bone growth and are gathering data about which exercises are effective. Walking at a pace slower than 4 mph, cycling, and swimming, although excellent for cardiovascular health, offer only low levels of bone strain and have little or no effect on density. Strength training can place a diverse range of load on bone but reverses osteoporosis only when the program includes movements that target fracture sites, uses heavy weights with fewer repetitions, and includes dynamic, explosive movements.

What we have learned from systematic reviews is that, even though an exercise may not create significant changes in bone density, it may still have a significant impact in reducing our risk of falling. Osteoporosis presents two health risks—thinning bones that can become brittle and weak, leading to fractures, and an increased risk of falling due to postural changes, decreased hip range of motion, and poor leg strength. Exercises that include changes of pace and direction, stair climbing, and novel movement patterns have been found to reduce falls by as much as 50% in frail women with osteoporosis, even though they did not create any significant increase in bone density.

The current medical focus on screening and treatments for osteoporosis may be clouding our understanding of the larger, more contentious issue related to bone loss: Is osteoporosis a disease that needs to be treated or, with the exception of a certain subgroup of people who have other risk factors, is it just one of many conditions that are more likely to appear with aging—much like cataracts, dry skin, and atrial fibrillation? This may be a radical thought today at a time when osteoporosis is considered pathological, but not so long ago it was seen as a natural part of the aging process. Up until the mid-19th century, if you were fortunate enough to reach your sixties, declines in physical and mental health were considered as simply part of the aging process. Even when Julius Wolff in the 19th century identified the way bone is remodeled, porous bone was not considered to be pathological. It wasn’t until the 1940’s that bone porosity became associated with disease when the physician Fulton Albright distinguished between what he termed “postmenopausal osteoporosis”—an advanced, debilitating condition—and “senile” osteoporosis, which was normal loss due to aging. The debate between osteoporosis’ classification as a disease has raged for years, colored by political, social, and cultural norms related to femininity, aging, and the role of women in society. All of this changed in the 1970’s when our culture shifted to one that highly valued youth, found faith in the redemptive properties of medicine, and the influence of the pharmaceutical companies in medicine and medical research grew exponentially.

Which brings us to 2023, where we still do not have a clear, evidence-based answer as to whether osteoporosis poses a significant threat to our overall well-being. We do know that certain factors increase a person’s risk for spontaneous fractures—genetics, endocrine disorders, steroid use, cancers, neuropathies, postural deformities, poor nutrition, alcoholism, and smoking history, to name a few. Clearly there are people who benefit from targeted exercise interventions and pharmaceutical treatments. For those who lack these risk factors, exercise interventions are still important, but for different reasons, which are mostly to keep our muscles strong and help us prevent falls.

Unless you have factors that increase your risk of fractures, perhaps the best approach to osteoporosis is to weigh the pros and cons of both pharmaceuticals and exercise interventions. What is important to you in life? How do you want to spend your time? If you have the time, motivation, and interest to commit to engaging in high intensity exercise, then you may enjoy the challenge of building bones along with strong muscles. If injuries, family obligations, or your general interest in exercise is low, then you may want to consider taking bone building drugs and staying physically active. Just know that the jury is still out, that the medical profession is not completely on board with bone loss being a disease, and that our current treatments for osteoporosis—both pharmaceutical and exercise—are still controversial.

Perhaps our current obsession with building bone reflects a deeper fear of becoming frail, of succumbing to the disabilities we associate with old age, or of being vulnerable. We all know that 85-year-old marathon runner and the 90-year-old bodybuilder who inspire us to stay strong and fit, free of disease, so we can be healthy right up to the moment we die in our sleep. Rarely does the future unfold in the way we hope it will, especially when it comes to aging. Searching for exercises that keep our bones strong gives us a temporary feeling of power over the inevitability of aging. The only real choice we have is how we accept our inherent vulnerability. Do we live in fear of disability, or can we accept the aging process, letting it unfold while allowing it to make us wiser, more courageous, and compassionate?