In search of a Big Theory of Injury Prevention

The sages are split on the fundamental philosophical topic of why bad things (i.e. injuries) happen to good people (i.e. runners who follow the ten percent rule). Some argue that this is because we haven’t worked out which form mistake, muscle imbalance, or training error causes a specific injury. Others argue that the issue is more complex, and that we need a full causal framework that connects training and life stresses, biomechanical loads delivered to various regions of the body, and the constantly changing capacity of each joint and tissue to absorb those loads.

Running injury experts Chris Napier of the University of British Columbia and Rich Willy of the University of Montana defend the latter viewpoint in a recent (and free to read) editorial post in the International Journal of Sports Physical Therapy. Simple guidelines concerning training load, for example, are guaranteed to fail since increasing the stress on a tissue like a tendon by 10 percent causes it to fail 50 percent sooner. That’s one reason why adding speedwork can cause problems even if you’re not going any faster. You can’t prevent an injury unless you know the chain of events that will most likely result in one.

Drawing that web of causal arrows, however, remains a difficult task, as evidenced by yet another new study, this one on Western States ultramarathoners. The results were published in the Clinical Journal of Sports Medicine by a team led by Emily Kraus, a sports medicine doctor and researcher at Stanford University, who investigated 123 runners (83 men, 40 women) who did the 100-mile race in 2018 and 2019. Their purpose was to investigate the components of the female and male athlete triads in ultramarathoners, as well as the risk of stress fractures and other bone stress ailments.

“The combination of low energy availability, menstruation abnormalities, and low bone mineral density (BMD) in women is known as the athlete triad.” Low levels of sex hormones like testosterone can be used to compensate for menstrual abnormalities in men. It’s a component of the larger problem of relative energy shortage in sports (RED-S).

The causal arrow here is, in theory, rather obvious. You’ll have lower amounts of sex hormones if you don’t eat enough, either overall or during the times of day when you need it most to support your training. This is what causes irregular or absent periods as a warning indicator. It can also lead to a decrease in bone mineral density over time, making you more susceptible to stress fractures, even at training loads you were previously able to bear. Every link in this chain has solid evidence.

Many Western States runners appeared to be at risk of disordered eating, according to a self-reported questionnaire (such question: “Are you trying to change your body weight or body composition to improve your performance?”). 62.5 percent of women and 44.5 percent of men appeared to be at risk. Low bone density, defined as a Z-score less than -1, was found in a large number of people (16.7 and 30.1 percent, respectively). Many (37.5 and 20.5 percent) have a history of stress fractures, which is consistent with data from other extreme runner studies. When you add in a few more factors like low body mass and irregular periods, you may get a cumulative triad risk score, which found that 61.1 percent of women and 29.2 percent of men are at moderate risk for bone stress injuries, while 5.6 percent are at high risk.

In some ways, this is a good example of Napier and Willy’s point. You won’t get a very useful indicator of injury risk if you focus on a single risk factor like bone mineral density. Few of the women had low BMD, but many suffered stress fractures; the opposite was true for the men. Hormone levels were also equivocal, as assessed by InsideTracker’s blood testing battery. There appeared to be a relationship between low testosterone and estradiol levels and decreased bone mineral density in women. Men, on the other hand, did not show the same pattern.

A more relevant evaluation of injury risk is obtained by broadening the causal diagram, as with the cumulative triad risk score. In one of Kraus’s previous studies, for every one-point increase in the cumulative risk score for male athletes (which doesn’t include any direct assessment of hormones because men don’t have a simple proxy like menstrual dysfunction), the risk of a subsequent stress fracture increased by 57 percent. It’s even more obvious in women: a moderate risk diagnostic doubles your risks of a stress fracture, while a high risk diagnosis quadruples your chances.

But that’s only one aspect of Napier and Willy’s grand scheme. Another recent paper (co-authored by Napier, Karrie Hamstra-Wright of the University of Illinois at Chicago, and Kellie Huxel Bliven of A.T. Still University) describes a “holistic approach” to bone stress injuries, claiming that “athletes have their own cumulative risk profile that influences their capacity to withstand specific training loads.” They’re no longer just talking about the triad risk factors; instead, they’re combining a vast web of ou pas (sex, race, age, genetics, alignment, prior injury…) and modifiable (strength, fatigue, flexibility, biomechanics, stress, recovery, nutrition…) intrinsic factors, as well as extrinsic factors like footwear, training surface, and training load).

This potential super-calculation of injury risk does not yet have an equation. In some ways, this is just an attempt to quantitatively express what happens in the mind of a good clinician while evaluating an athlete. The major question is whether the equation—or, more likely, a future machine-learning algorithm—can ever combine all of those inputs and give injury advise that is measurably better than, say, the 10% rule plus various warnings not to do something dumb. Only time and a great deal of meticulous research will tell.

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