It’s not the fall that hurts, but the sudden stop at the end. Scientists in the United States, Kenya and Scotland say this is also true for our feet when we run.
Recent research, published in the international science journal Nature by Leiberman and colleagues, compares foot strike patterns and vertical collision forces in habitually barefoot runners and runners who wear shoes.
There are three types of foot strike patterns in runners. Runners who strike the ground heel-first are called rear-foot strike (RFS) runners. Those who land on the ball of their foot are fore-foot strike (FFS) runners and those who strike simultaneously with their heel and the ball of their foot are mid-foot strike (MFS) runners.
It seems that foot strike pattern largely determines the vertical force experienced by a runner every time they hit the ground. It has to do with the biomechanics of the leg and the foot, and the conversion of translational force (the runner’s body moving up and down) into rotational force around the ankle.
In FFS runners, the calf acts like a spring during impact and some of the downwards force of the body is converted into rotational energy around the ankle joint. This means that much of the runner’s body mass can decelerate gradually rather than suddenly. In RFS runners, impact occurs just below the ankle and little downwards force can be converted to rotational energy, leaving a greater amount of body mass to decelerate quickly during foot strike. This results in a vertical force in RFS runners that is approximately three times higher than in FFS runners.
Furthermore, it appears that running shoes are influencing our foot strike patterns. Runners who normally run barefoot usually FFS or MFS. Runners who normally wear shoes tend to RFS.
Modern running shoes have an elevated cushioned heel to make RFS running more comfortable. However, running shoes may encourage RFS running. Foot strike patterns in a group of habitually-shod runners were recorded before and after six weeks of training in conditions to simulate barefoot running (no or minimal footwear). The proportion of RFS runners in the group decreased from 72% to 36% after barefoot running training.
This research has important implications for running-related injuries. Some members of the running community are staunch advocates of the benefits of barefoot running, and this research may support some of their claims. It is somewhat disconcerting, however, to find that this research was in part funded by Vibram USA, the designers of the new FiveFingers® footwear that aims to imitate barefoot running. We wait in anticipation of a controlled study, funded by bodies other than footwear companies, to compare the incidence of injuries among FFS, MFS and RFS runners, in shoes and barefoot.
Leiberman et al. (2010). Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature. 463:531-535.
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