Stress fractures: is it weak bones or muscles?
A new study from researchers at the University of Calgary, published in the November issue of Medicine & Science in Sports & Exercise, looks at bone quality and leg muscle strength in a group of 19 women who have suffered stress fractures in their legs, and compares them to a group of matched controls. The basic results:
- the women who got stress fractures had thinner bones;
- at certain key locations, the quality of the bone was lower in the stress fracture group;
- the stress fracture group also had weaker leg muscles, particularly for knee extension (lower by 18.3%, statistically significant) and plantarflexion (lower by 17.3%, though not statistically significant).
Now, this sounds very similar to the results of a University of Minnesota study published a couple of years ago. Here‘s how I summed up the conclusions reached by those researchers:
What’s interesting, though, it that the bone differences were exactly in proportion to the size of the muscles in the same area, and there was no difference in bone mineral density. What this suggests is that the best way to avoid stress fractures is to make sure you have enough muscle on your legs — presumably by doing weights and (it goes without saying) eating enough.
What I don’t understand is that, in the new Calgary study, even though they mention the Minnesota study repeatedly in their discussion, they don’t discuss at all this idea that it’s the lower muscle strength that dictates the reduced bone size and thus the stress fracture risk — even though that was the primary conclusion of the Minnesota study. Instead, they say “the role of muscle weakness in [stress fractures] is unclear from previous studies,” and suggest that weaker knee extension might change running form to produce a “stiffer” running stride or somehow alter the direction of forces on the bone during running — both of which seem like unnecessarily complex and speculative ideas compared to the straightforward link between muscle strength and bone strength.
It’s entirely possible that I’m missing something here, because the paper is quite complex. But what I take away from it is, once again, that strengthening your legs is likely (though not yet proven in a prospective trial) to reduce your stress fracture risk.