Ground Reaction Force (GFR) chart

Sajer ingin berkongsi :)

Since the release of Dr. Lieberman's study I've had a few requests from folks wanting me to show my test results from a gait analysis I had done at the UVA human performance lab. They're curious to see how my Ground Reaction Force (GFR) chart compares with the one shown in the Nature video showing a barefoot runner (I'm assuming Lieberman himself) landing on a strike plate with a GFR chart beneath.

In the tests I did at UVA, I was wearing three different pairs of shoes (I was never tested barefoot): tai chi shoes, NB-MF800's and NB-790's. I've pasted in three graphs below. The first two are of a barefoot runner running with a heel strike, a barefoot runner running with a forefoot landing and the third graph is of me with the graphs of all three of my tests overlaid onto the same chart in three different colors.

GRF for a Heel Striker

GRF for a Forefoot Striker

GRF for Danny

Notice the differences in GFR between the three runners shown as a multiple of Body Weight measured during the initial weight-bearing phase. The Barefoot runner had a multiple of 1.85 x Body Weight. To make the comparisons fair, let's assume all three tests were done with a runner who weights 150 lbs., that would mean (with a GFR of 1.85 x Body weight) that his heels were absorbing a force of 277.5 lbs. upon impact. Let's say that his heel has an area of 5 sq. inches. that would mean that his heels were experiencing an impact of about 55.5 lbs./sq. inch. Ouch!!!

In the second figure, the runner is landing on his forefoot and his GFR is 2.64 x Body Weight during his support phase which figures out to 2.64 x 150 lbs. = 396 lbs. Now let's say that the area of his forefoot is approximately 16 sq. inches. That would mean that his foot would be absorbing about 24.75 lbs./sq. inch. … or less than half of what the heel striker feels.

In the third figure I'm landing in a midfoot (or fullfoot) strike which means that the entire bottom of my foot is landing as my foot hits the ground. My GFR was measured at 2.47 x Body Weight or 2.47 x 150 lbs. = 370.5 lbs. Now, if the surface area of the entire bottom of my foot is roughly 30 sq. inches. (I'm a size 9.5 shoe), that would mean that the impact felt by my feet would be approximately 12.35 lbs./sq. inch. or half as much as the forefoot striker. That's less than 25% of the impact per square inch the heel striker feels and half of what the forefoot striker feels. You can also see that my GFR was the same no matter which shoes I was wearing. I imagine that it would still have been the same had I been tested barefoot… possibly implying that there's something to be said for working on improving your running technique. It's about the runner, not the shoe.

I've used 150 lbs. as a standard weight for all three runners because I don't know how much Dr. Leiberman weights. I also used the size of my feet in my calculations because I also don't know his shoe size. BUT, the point I'm trying to make here is that whether or not you run with shoes on, your impact with the ground will be most if you land in a heel strike because there's a lot of force going into a relatively small area of your foot. Subsequently, if you land in a forefoot strike, you land with more area of your foot touching the ground and it spreads the force of impact over a larger area thereby reducing the amount of impact per square inch. And lastly, if you land in a fullfoot strike, you spread the impact out over a much larger area and lower your impact per square inch even more still.

Since greater impact with the ground can most likely be directly related to greater incidence of impact injuries, this all boils down to one big question. How do you lower your impact with the ground so that you don't hurt yourself when you're running. My premise is that although some types of shoes can reduce shock and impact, it's the runner who, in the end, is responsible for learning how to manage his or her impact with the ground in a way that consistently works. This is the need that is presented to every runner and it is one of the main reasons why ChiRunning was developed.

Dr. Liebermans study was a good beginning into seeing the difference in impact forces between shod and barefoot runners. Next, I would like to see a study comparing the difference in impact between heel strikers, midfoot strikers and forefoot strikers; with bare feet, with minimal shoes, racing flats, and with your basic high-heeled running shoes.