Heel strike? Forefoot strike? Midfoot strike? There's been plenty of debate over the past few years about how your foot should hit the ground while running, but data about how people actually run in real life has been pretty sparse. You can use a force plate to measure a few strides in a laboratory, or a high-speed video camera to measure a few strides as someone runs past during an outdoor race. But what about measuring every single footstrike during a long race to find out how variable footstrike is, how it changes with varying terrain, and how it changes as you fatigue?

Last year, a group of researchers in France, led by Marlène Giandolini of the University of Lyon (now at University Savoie Mont Blanc) and the shoe company Salomon, published a paper in the Journal of Biomechanics describing a new way to measure footstrike in the field. They basically strapped two small wireless accelerometers to a shoe, one on the heel and one at the front of the foot, and synchronized the data from them. Each accelerometer produces a spike when that part of the foot hits the ground. If you heel-strike, then the heel accelerometer registers a spike before the toe accelerometer; if you forefoot strike, it's the other way around. If the two spikes happen at the same time (within a roughly 20-millisecond range, according to the calibration data presented in the paper), that's a midfoot strike.

Cool stuff. Now some of the same researchers have published a new report, in Footwear Science, taking the technique into the field and presenting some race data from the most famous trail/ultra runner in the world these days, Kilian Jornet. The data was taken during the 2013 edition of Kilian's Classik in Font-Romeau, a 45-K trail race with more than a mile of elevation gain. Jornet won the race in 4:23:18, wearing three accelerometers (there was an extra one on his shin) synced to a GPS unit (so they could match footstrike to the terrain he was running on). The only hitch: the battery on the GPS died after about two hours, so they only have data for the first half of the race.

Here's the main figure from the paper, showing the proportion of forefoot (top), midfoot (middle), and rearfoot (bottom) strikes during various segments of the race, superimposed over the altitude profile (the generally increasing line) and the speed (the line that goes up and down):

The first thing that jumps out is that you can't really say "Jornet is a _____ striker." He seems to use all three types of footstrike, which likely has a lot of do with the very rugged terrain he was running on.

There's a bit of an apparent trend to increasing rearfoot strikes as the race progresses. I asked Giandolini about that via e-mail, and she pointed out that Jornet was doing mostly forefoot striking, which uses your calf muscles, and running mostly uphill, which also puts stress on your calf muscles: "[W]e can suggest that the association of both these factors would lead Kilian to modify his running technique as a strategy to relax calf muscles." It would have been interesting to see if the trend continued in the second half of the race.

Of course, this was mainly a proof of principle. It will be interesting to see how Jornet's data compares with more "average" runners, and how trail data compares to road data. A couple of studies using video analysis at big road races have found that, on average, forefoot strikers tend to run a little faster than rearfoot strikers. One of the interesting points that Giandolini makes is that the best approach for trail running (with rough terrain and big elevation changes) might be quite different: rather than any one style of footstrike being "ideal," the ability to switch between different footstrikes might be more useful for a trail runner.

"Is the ability of the runner to switch from a technique to another a criteria of performance?" she wrote. "We think that it can. We think that the capacity of a trail runner to adapt to extrinsic (e.g. slope, surface) and intrinsic (e.g. fatigue, pains) would give him an advantage."

That suggests that road and trail running might have quite different footstrike demands. For trail runners, avoiding muscle damage and injury is more important than small changes in running economy, so switching footstrikes frequently makes sense. For road runners on relatively flat surfaces, running economy is more important, so it's ideal to have enough fatigue resistance to maintain your stride with as few changes as possible for the entire race.

This is quite speculative – but the nice thing is that this accelerometer set-up offers a way of gathering the real-world data needed to find out how top (and average) runners actually run in different conditions. It'll be fun to see more data.

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