by Alex Hutchison
It was Daniel Lieberman, the barefoot-running evolutionary biologist from Harvard, who first got me thinking about impact forces. Back in 2010, he and his colleagues published a hugely influential study in Nature that compared the “collision forces” of feet hitting ground for runners with and without shoes while landing on their heel or forefoot. The stark differences in the paper’s force curves provided crucial scientific ballast for the minimalist running movement—but a new study suggests we may have been interpreting those curves all wrong.
The new study, which appears in the Journal of Applied Physiology, comes from Peter Weyand’s biomechanics group at Southern Methodist University, led by his former doctoral student Andrew Udofa, along with Ken Clark (who is now at West Chester University) and physicist Laurence Ryan. By rethinking what running force curves tell us, they’re able to resolve a longstanding mystery about the role of shoe cushioning and potentially offer an accessible and individualized way of testing which shoes are best for you. To understand why, we first need to look back at Lieberman’s curves.
If you ask someone to run on a fancy force-measuring treadmill (which costs several hundred thousand dollars) or force plate installed in the ground, you can get a characteristic curve that shows you how much force the runner is applying vertically to the ground with each step (and conversely, as Isaac Newton figured out, how much force the ground is sending into your body). Here, from Lieberman’s 2010 paper, is a typical example for someone running barefoot and landing on their hee
The key feature to notice is the little spike on the left side of the curve. That’s your heel slamming into the ground, a split second before the full force of the rest of your body presses into the ground. The dominant view among biomechanists is that it’s not the overall size of the force (which tops out at about 2.4 body weights here) that determines how likely you are to get injured; instead, it’s how quickly the force is applied, otherwise known as the loading rate. Thanks to the mini-spike of the heel strike, the force in this picture climbs very steeply, which is presumably bad.
Now compare how the force looks when you land instead on your forefoot:
The little spike is gone! That means the loading rate is lower, and you’re less likely to get injured. At least, that’s the theory.
In truth, according to the new study, the little spike is still there; it’s just obscured by the bigger one. Back in 2014, Weyand, Clark, and Ryan proposed what they called a “two-mass model” to explain how these sorts of force curves vary under different conditions. In this model, the force curve always consists of two distinct components: a small spike that corresponds to your foot and lower leg smacking into the ground and almost immediately jarring to a halt; and a bigger, slower spike that corresponds to the rest of your body reaching the lowest point of its up-and-down motion. The overall force is simply the sum of those two spikes.
Here’s what looks like:
Read more at: https://www.outsideonline.com/2390686/barefoot-running-biomechanics-study