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A few years ago, researchers at the University of North Carolina asked runners which part of their foot struck the ground first when they ran. Of the 57 subjects, 37 identified themselves as forefoot strikers, while the rest said they were heel strikers.

The catch? When the runners were analyzed with high-speed video on a force-sensing treadmill, it turned out that more than one-third of the self-identified forefoot strikers were actually crashing down heel-first.

Figuring out how your foot lands while running is clearly trickier than expected; figuring out how it should land is even harder. Though runners and researchers have been arguing for years over the merits of heel, midfoot or forefoot landings for efficiency and injury prevention, real-world data about how people actually run have been surprisingly sparse.

But a new technique developed by French researchers offers mobile monitoring of landing patterns, and its initial results reveal that foot strike can change dramatically over the course of a single run – and, at least for trail runners, that's a good thing.

"We think the capacity of a trail runner to adapt [to different foot strikes] would give him an advantage," says Marlène Giandolini, a researcher at the Université Savoie Mont Blanc in France and the shoe company Salomon.

The usual approaches to measuring foot strike rely on either a force-sensing treadmill, a high-speed video camera or both. The problem with these methods, explains Giandolini, is that they provide only a brief snapshot of a few strides under controlled conditions.

But factors such as fatigue and varying terrain are likely to affect how your foot hits the ground, as a few previous studies have hinted. For example, a 2011 study placed high-speed video cameras at the 10-kilometre and 32-kilometre marks of the Manchester City Marathon in New Hampshire. Of the 288 runners filmed at both points, the proportion of heel strikers increased from 88 per cent to 93 per cent between the first and second checkpoint, presumably as a result of muscle fatigue among midfoot and forefoot strikers.

Giandolini and her colleagues are particularly interested in foot strike among trail runners – Salomon's target market – so they devised a system involving two tiny wireless accelerometers, one mounted at the front of the shoe and the other at the back, that can be taken out of the lab into the wild.

Their initial report, published in the Journal of Biomechanics last year, showed that the sensors could distinguish between a heel and forefoot strike depending on which accelerometer registered a spike of deceleration first. If the two spikes are within about 20 milliseconds of each other, the stride is considered a midfoot strike, with the front and rear of the foot landing essentially simultaneously.

For their first real-world case study, Giandolini and her colleagues recruited Kilian Jornet Burgada, a 27-year-old Spaniard widely regarded as the greatest trail, mountain and ultra runner on the planet at the moment. They wired him up during a 45-kilometre trail race in Font-Romeu, France, that included more than 1,600 metres of elevation gain. The only hitch: The GPS unit synched to the accelerometers ran out of batteries halfway through the race, so they were able to analyze only the first 20 kilometres of data.

The results, published in the journal Footwear Science, show that Burgada's footstrike is … complicated. Of the 5,530 steps analyzed, 18.5 per cent were heel strikes, 32.6 per cent were midfoot and 48.9 per cent were forefoot, a remarkable display of diversity. As the race progressed, the proportion of heel strikes increased steadily, perhaps because the predominantly uphill course put extra stress on his calf muscles – the same muscles required to maintain a forefoot landing.

This adaptability is likely one of the traits that make Burgada so good, Giandolini says. Mixing up different foot landings and adjusting to take stress off tired muscles should reduce the risk of injury, especially while running on demanding trails.

That view marks a stark contrast to the idea, advanced in books such as Christopher McDougall's 2009 bestseller Born to Run, that every runner should aspire to land on their forefoot. Instead, the context matters, says Ellie Greenwood, a two-time world-champion ultramarathoner from Scotland who now lives in Vancouver.

"When trail running, I am more thinking about where to place my feet rather than how to place my feet," she says. When running on the roads, on the other hand, she consciously adjusts her foot strike from heel to toe depending on how fast she's running.

The upshot, unfortunately, is that there's no simple prescription for how your foot should hit the ground when running. Instead, focus on other cues, such as trying to run as quietly as possible and making sure your feet land underneath your centre of mass rather than far in front of your body – and let your feet strike how they may.