http://www.runbare.com/389/new-study-by ... e-journal/
In the barefoot runner who heel strikes, the typical impact force that you’ll get landing on your heel is two times your body weight, essentially the same as hitting your heel with a hammer with two times your body weight.
So if you land (with a heel strike) with a shoe on, you have basically the same force (perhaps 10% less as a little force is dampened by the shoe) so the magnitude of the force is the same, wearing a shoe or not when heel striking.
But when you land with a barefoot running style (much more of a forefoot strike), then the actual magnitude, or the height of that peak, is half the body weight, much less collision force is occurring.
If you look at the rate of loading of that force, how fast that force ramps up, which many people think is the primary cause of injury (nobody knows for sure), the rate of loading, when you run barefoot and heel strike is 450 bodyweights per second—that’s fast.
The reason a shoe is comfortable is that it has a lot of elastic elements in the heel and it slows the rate of loading when heel-striking to about 70 bodyweights per second.
However barefoot runners (most of them) had about 30 bodyweights per second.
The point is that when you run barefoot you can have magnitudes of loading that are much lower than running in a shoe, and rates of loading that are equal to or lower than wearing a shoe…which is why it’s comfortable and potentially less injurious.
Specific Points:
First Point – Higher Impact and Lower Efficiency When Heel Striking
Based on the study, a heel strike converts little impact into forward or ‘rotational energy’ instead transmitting the energy up through the body. In contrast, a forefoot strike decreased impact by storing energy in the muscles of the calf and the achilles tendon, and rebounding it in rotational kinetic energy. In short, running on the heel increases impact (up to three times the impact force of forefoot landing, and up to 7 times the impact loading of running barefoot) and decreases efficiency (causing a braking effect with each stride) while running on the forefoot decreases impact and increases efficiency, by translating stored kinetic energy in the muscles into rotational or forward propulsion.
In essence, if you picture your stride as a wheel, when running with your heel down, you’re running with a cogged or geared wheel, hitting hard and stopping with each stride. When you’re running with a forefoot stride, your impact is less, and is translated into rotational force, spinning the wheel and moving you forward.
Second Point – Barefoot Runners Lower Center of Gravity to Reduce Impact
The study found that barefoot runners suffered no greater impact when landing on hard surfaces than soft surfaces. This is because forefoot runners lowered their centers of gravity more (bent their knees and ankles more) or were more ‘compliant’ to the ground than their rear foot striking compatriots. This allows barefoot runners to ‘adjust leg stiffness depending on surface hardnesses.
This point dispels the myth that we could once run barefoot on the Savanna, but cannot do so on harder ‘modern’ surfaces such as asphalt and pavement as the study found ‘no significant differences in rates or magnitudes of impact loading in barefoot runners on hard surfaces relative to cushioned surfaces’.
Third Point – We Evolved to Run on Our Forefoot
Natural selection suggests that if endurance running was important to our survival, then forefoot running come about to protect the foot and reduce the chance of injury.
While barefoot runners or those who wear minimalist shoes avoid rear foot landings and the associated impacts, in contrast, most shod runners today land almost exclusively on their heels.
From an evolutionary perspective, the study theorizes that ancient man, such as the early Australopithecus afarensis had an enlarged heel and probably walked with a rear foot strike, but was missing many features of the modern foot, such as a strong longitudinal arch.
They found that a strong longitudinal arch which acts as a spring, storing and releasing energy, would have increased performance in a forefoot strike or perhaps mid-foot strike, but not for a rear-foot strike. This not only would have better protected the runner, but likely accounts for the ‘lower (economical) cost of barefoot running relative to shod running’. In short, the longitudinal arch (if not locked in a shoe with arch support) helps us rebound with each stride, reducing the amount of energy necessary to run.
Daniel Lieberman is a professor of human evolutionary biology at Harvard University.
