Running economy, often explained as the equivalent of gas mileage for runners, is a bit of a black art. There are plenty of theories about what influences it, and Does Running Burn Fat showed that Paula Radcliffe managed to improve her economy over a five-year period -- but no one is really sure how to reliably boost it.
A new study of "stride angle" produces unexpected results, just published in the Journal of Strength and Conditioning Research, takes a look at a parameter I hadn't seen discussed before, which they call "stride angle." It's the angle at which your foot leaves the ground, as shown in this diagram:
The researchers took a homogeneous group of 25 well-trained male distance runners and measured their running economy at three different sub-maximal speeds. They also analysed several biomechanical parameters, including stride angle.
What results should they have expected? Previous research (and common sense!) had suggested that runners with excessive up-and-down motion tend to be less efficient than runners who avoid bobbing up and down. That suggests that a smaller stride angle should predict better running economy, which is indeed what they predicted. But it's not what they found. Here's a typical graph from one of the running speeds, with running economy (in ml of O2 per kg per km) on the y-axis and stride angle on the x-axis:
It's pretty clear that the runners with the highest stride angle have the best economy (low numbers are good for economy, since it tells you how much fuel you're burning). The same results were found at every speed they tested. The data showed an equally clear inverse relationship between stride angle and ground contact time: the runners with higher stride angle stayed in contact with the ground for less time, and (consequently?) had better running economy.
Perhaps just as interestingly, none of the other biomechanical variables they assessed (e.g. stride length and cadence) had any significant relationship with running economy.
So what do we do with this? The authors of the article suggest that "specific exercises designed to achieve higher stride angles during running may promote efficient stride biomechanics." That seems a little premature to me, since we don't really understand what's happening. It would be interesting to repeat this study with a far more mixed group. These runners were ~32:00 10-K racers training about 60 miles per week on average. If we included more recreational runners, would we see a wider range of stride angles? Would the running economy line start to curve upward once the stride angle reached 8 degrees? Or 10 degrees? What if we increased the speed above lactate threshold (all three speeds they tested were sub-threshold)?
Anyway, lots of questions. But for now, I find it very interesting that the intuitively "obvious" prediction -- that a flatter, less bouncy stride would be more efficient -- didn't turn out to be true. Another reminder of something that Paula Radcliffe's success should have taught us long ago: you can't diagnose running economy just by watching someone run.
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