Last time I discussed the common misperception that impact causes injuries.

Today, I’m going to show you the critical link in the chain describing the real instant of vulnerability. It’s actually pretty obvious when you think about it (kind of like you always find your car keys in the last place you look)…vulnerability occurs when your entire body weight is supported by a planted foot.

As you can see above, the knee joint torques and forces reach the top long after the impact arrows. Rather, at around 10-20% of the gait cycle, the foot becomes fully planted. Here is where the knee joint torques and forces reach their maximum. And indeed, it is these peak torques and forces, not the small ones at impact, that relate to knee osteoarthritis.

Although impact sounds dramatic and is certainly what causes injuries in falls (and motor vehicle accidents), impact is not what causes repetitive type injuries during normal walking and running. Any biomechanics research that combines force plate data with motion data demonstrates that most forces related to injury peak when the foot is fully planted. Importantly, this is the case for just about every injury prone area, not just the knee. For example, we get stress fractures in very specific anatomic sites. The peak forces through these sites occur when the foot is fully planted — not at impact.

I cannot say this enough, as it goes against the marketing spin of billions: peak stresses and strains associated with injury occur when the foot is fully planted.

So what does this mean with respect to footwear? This is where the rubber meets the road, and I will describe the science of it all in my next post.

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