If you've got knee pain, you might need to strengthen your hips. That's an increasingly common prescription these days, thanks to a bunch of research over the past decade or so linking conditions like runner's knee (also known as patellofemoral pain, or PFP) and more recently iliotibial band syndrome to sub-par hip strength. But there's a key question lurking behind these studies: do you develop knee pain because your hips are weak, or do your hips get weak because your knees hurt and you're forced to alter your movement patterns?
A systematic review by Danish and Australian researchers, published recently in the British Journal of Sports Medicine, tries to answer this question by pooling the data from 24 different studies of hip strength and PFP. The headline result is that cross-sectional studies showed an association between hip strength and knee pain, but prospective studies didn't. In other words, if you take a group of runners at a single point in time, the ones who have weak hips will be more likely to have knee pain (and vice versa). But if you take a group of healthy runners, measure their hip strength, and then follow them over subsequent months and years, you won't be able to predict who will develop knee pain and who won't. That's precisely the pattern you'd expect if knee pain causes hip weakness, rather than the other way around.
But let's not get too carried away with this finding. As the authors are careful to point out, only three of the 24 studies are prospective! So the failure to find a statistically significant predictive relationship isn't that surprising, especially given the mixed study populations (one included adolescents, who have different characteristics). This ends up looking like one of those cases where there's insufficient evidence to say that weak hips cause knee pain, but also insufficient evidence to say that they don't. Here how the authors put it:
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"Considering favourable clinical outcomes with hip strengthening protocols in individuals with PFP, the findings of this review do not bring into question the potential benefit of such programmes for symptom reduction. However, they do indicate that hip strengthening protocols may not be sufficient to prevent the occurrence or reoccurrence of PFP."
So the evidence isn't there to suggest that every runner should be doing hip-strengthening exercises. But if you start having knee problems, that's probably a good place to start.
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