Your bones are meant to break – and why that’s a good thing

Bone is a resilient tissue. That’s why we dig up skeletons, not muscles or skin. 

But bone is not invincible, which is why many endurance athletes suffer bone stress injuries (BSI). Understanding your skeleton’s behavior and tendencies is crucial if you are trying to keep yourself or your athletes off crutches and smashing PRs. 

At a microscopic level, your bones are constantly changing. Pounding away on the road or treadmill causes your bones to slowly and steadily crack under pressure. Sounds problematic. But that damage signals your brain to act, sending resources to clear out the damaged area and build denser bone tissue. Like ocean tides coming in and out, your skeleton ebbs and flows. 

David Burr, maybe the world expert on bone biology, states,” From an evolutionary perspective, humans have adapted to environments and diets in which it was necessary to travel long distances on two legs, sometimes at great speed.”

His take is that over the last several thousand years, human skeletons have evolved to move. The built-in breaking down of your bones has allowed us to survive and thrive. This microdamage actually prolongs your bone’s integrity and delays a complete fracture. It’s a strategy for “strategically failing” somewhat unique to humans. When we compare human bones to animals like birds, we see they don’t fail in this same manner. Their bones have evolved to help with flight, while ours is for weight-bearing movement. If you asked a bird to run a marathon, its bones would snap! 

Microdamage accumulation is good for survival but tricky for distance running. If you accrue damage too quickly, you can develop a bone stress injury (BSI). BSIs happen because there is a disagreement between the breaking and rebuilding of your bones. Let’s go through how to keep that microdamage from collecting and put your bones in a good place to recover for your next run.

You’ve likely heard most running injuries happen because a runner did “too much too fast”. Often that’s true, but that broad idea lacks practicality. What’s too much? How fast is too fast? 

The uniqueness of you

The rate you can progress your training has just as much to do with your genetic makeup and your life during your developing years. You can significantly increase or decrease your stress fracture risk by your parents and activity choices as a kid. Any advice on training needs to start with the uniqueness of you, your health history, and your current situation. I bet you know a few runners with massive training spikes yet are healthy. While sensible advice like “don’t progress more than 10% each week” may be helpful in some instances, we can zoom out on big ideas and themes to consider as you go through the slog of training. 

Your bones deal with more than just running. Outside of helping you move, they’re a reserve for calcium, help produce blood cells, and provide metabolic support to your body. They’ve got a lot to do! Your bone growth, development, and preservation primarily rely upon hormonal regulation somewhat independently of mechanical loads (i.e., training) throughout life to facilitate these other functions. 

Hart and colleagues state, “Hormone function majorly influences bone health and metabolism, ascending into domination in adulthood and later life.” 

So your training habits affect your likelihood of accruing microdamage to develop a stress fracture. But your overall health and well-being are likely more significant than your weekly mileage. And runners often struggle with their overall health. 

Your relationship with food

The nature of distance running and the negative culture around fueling, body weight, and body image place runners at a high risk of low energy availability (LEA), a state when your body doesn’t have enough resources. LEA shifts your body’s priority from being able to reproduce (hormone function) to simply staying alive. Your body doesn’t care about your upcoming race or your potential to have kids if it’s chronically under-fueled.

Here are a few stats. 

  • Five days of a 240-400 calorie deficit alters your bone metabolism
  • Menstrual dysfunction has been shown in 50-65% of female distance runners
  • Menstrual dysfunction leads to a 6x increase in stress fracture risk
  • Male and female runners with LEA have less dense bones than athletes in other sports

These stats highlight the importance of developing your nutrition knowledge alongside a skilled dietitian. 

Chill on the speedwork

You have to train a lot to get better at running.
You can progress through three different variables in training. How often, how far, and how fast you run. Warden et al. make the case that the most “bone-friendly” way to progress your training is to progress your running frequency, duration, and lastly intensity. I see training errors around intensity more frequently than any other variable. That’s a big reason I’ve seen many runners struggle to PR their marathon, half, or 5k but succeed as they get into ultramarathon training. They’re training at a much higher volume with a lot less intensity. Besides limiting intensity, Implementing simple changes like keeping 24 hours between training sessions, taking an entire day off every week, and your varying terrain can help. Your ability to minimize microdamage accumulation and facilitate the remodeling process is essential. You want to make sensible training decisions that slowly allow your muscles and other tissues to adapt to training demands. As much as you want to “grind out” every session, you should save those for 1-2x a month.

Take your foot off the gas after a breakthrough performance

Some runners can handle massive training intensity and volume spikes that lead them to a few successful performances — and then quickly fall off a cliff on the other side of the breakthrough. The temptation to continue to press is there after a PR. Runners often say, “I wanted more,” right before they end up in a physician’s office with a walking boot. It’s human nature to crave the next thing, but significant performance improvements often need time to back off.

Get out of shape

Body image is challenging for all athletes, runners being no different. Your body, mind, and lactate threshold should change throughout the year. That means there should be seasons if you weigh more or less, are faster and not so fast, and spend more or less time in training. Easier said than done, I know. This doesn’t mean simply taking off two weeks after your big race. It also doesn’t mean you have to shelf running for months at a time. The intensity and volume of training should come down as you explore different types of running. Try your hand at trail running in the winter. Go for a long hike instead of a long run. There should be times in the year when you lose your fitness and allow your bones, brain, and body a break. 

Build big muscles

Your muscles and bones are intimately connected. A muscle turns into a tendon that attaches to a bone. And a bigger muscle often means a stronger bone. Runners tend to break their femurs, their shins, and their feet. Your muscles help dissipate the forces on your bones by contracting together. When your muscles function correctly, they keep some of the stress of running off your skeleton. You should build big, bulky muscles in the weight room. Muscles also control the formation and growth of bone microdamage by contracting in coordinated patterns that regulate high levels of bone strain. Muscle is less capable of dissipating energy when it’s tired. It’s been shown that skeletal microdamage increases by 10-20% following muscular fatigue. 

If you want to keep your bones from breaking, don’t constantly push yourself to exhaustion in training. You need to be sensible with your training progressions while building big, strong, powerful muscles in the weight room. 

Here are a few of my favorite lifts to target common stress fracture locations. 

Be realistic with your situation

It’s crucial to be sensible with your unique situation because everyone’s life is different. Work, finances, training, kids – it ain’t always easy! If you are in a stressful season of life, it’s ok to change your relationship with training. I deal with this often with runners transitioning to a different phase of life. Post college running with a full-time job, training at a high level with a newborn, or taking care of a sick family member often requires adjusting the training expectations and pressure you put on yourself. 

While being persistent in the face of adversity can be an excellent quality, it can also lead to trouble if the healthier route is to say now isn’t the time, understanding that stressful seasons come and go. If you’re struggling to determine if it’s a time to push or pause, it’s helpful to ask someone you trust what they think of your situation. Your friends may be kinder to you than you are to yourself.

Find joy in things outside of running

Distance running is a huge time commitment, but it shouldn’t consume all your waking hours with no time for things outside of work and training. That extra pressure of running being “the only thing” makes it much more challenging to adjust your training when necessary. Every runner I work with gets asked this question at some point. What do you enjoy doing that isn’t exercise or work/school?” Putting less pressure on running as your identity helps keep a long-term perspective easier. 

I hope this blog gives you some actionable information for yourself, your patients, and your clients. To learn more about stress fracture management, I’d love it if you joined my newsletter below.

Thanks for reading!

Nathan Carlson PT, DPT, USATF

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References – 

Burr D. Why Bones Bend but Don’t Break. J Musculoskelet Neuronal Interact 2011; 11 (4): 270-285.

Burr D. Basic and Applied Bone Biology. 2014.

Higher Incidence of Bone Stress Injuries with Increasing Female Athlete Triad-Related Risk Factors: a Prospective Multisite Study of Exercising Girls and Women. AM j Sports Med. 2014. 42 (4): 949-958.

Rauh M, Nichols J, Barrack M. Relationships among injury and disordered eating, menstrual dysfunction, and low bone mineral density in high school athletes: a prospective study. Journal of Athletic Training. 2010. 45 (3), 243-252.

Warden S, Davis I, Fredericson M. Management and Prevention of Bone Stress Injuries. Journal of Orthopaedic and Sports Physical Therapy. 2014. 44 (10). 749-765.

Laddu D, Farr J, Lee V et al. Muscle Density Predicts Changes in Bone Density and Strength: a Prospective Study in Girls. J Musculoskelet Neuronal Interact. 2014 June ; 14(2): 195–204. 
Daly R, Saxon L, Turner C et al. The Relationship between Muscle Size and Bone Geometry and in Response to Exercise. Bone. 32 (2). 2004. 281-287.

Warden S, Edwards B, and Willy R. Optimal Load for Managing Low-Risk Bone Stress Injuries: The Science Behind the Clinical Reasoning. Journal of Orthopedic and Sport Physical Therapy. 2021. 51 (7). 322-330.

Barrack M, Rauh M, Barkai H et al. Dietary restraint and low bone mass in female adolescent endurance runners. The American journal of clinical nutrition. 2008. 87 (1) 36-43.

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