Archive for the ‘Biomechanics’ Category

The Biomechanical Basis for Barefoot Running

June 2, 2011

As discussed in the last post, there is significant evidence for running in humanity’s evolutionary past. Habitual shoe use only came onto the picture somewhere between 40,000 and 26,000 YA (BBC). Even then footwear was comparatively minimalist and  would not have modified the biomechanics of the foot in the way that modern running shoes are made to do. So the question now would be: how  did humans manage to make running a significant part of their survival strategy while being limited to running in bare feet. More simply put, how do humans run effectively with bare feet?

The mechanism of the human foot is a very complex and intricate one. There are 26 bones in each foot and even more muscles, tendons, and other connecting tissues. At first look, the muscles in the foot of the average habitually shod person would look atrophic as if they could serve no real function and where perhaps vestigial  remnants from tree climbing ancestors. This appearance, however, is the result of continuous use of restrictive shoes rather than the natural state of the human foot. Examination of the feet of habitually barefoot people  reveals significant morphological differences from those of habitually shod feet (The effects of habitual footwear use). These differences can be both in the skeletal formation of the foot and the muscular strength of the foot and surrounding structures. With the proper perspective of a healthy well formed foot in mind rather than the atrophied foot commonly depicted, we can shift our focus from protecting the foot toward using its innate mechanical properties.

The major factor that any runner has to overcome, whether they wear shoes or not, is the sudden vertical impact of the foot hitting the ground. Running is in essences a series of small jumps. Immediately after the foot meets the ground the weight of the entire body in motion is loaded onto that foot. On average the foot experiences over 2 times the body weight during and immediately after impact. This period is referred to as the stance phase of the running gait. The weight alone is not what makes this part of running significant, rather it is the rapidness with which that weight is applied that causes so much stress on the body. It is commonly accepted that most running injuries are the result of the large forces at play during the stance phase. Thus any sustainable approach to running must involve a way of adequately dealing with these forces in a way that does not lead to injury.

In the standard heel striking gait adopted by most shod runners, the foot is dorsiflexed (pointed upwards) making the Calcaneus (heel bone) the primary structure contacting the ground at touchdown. Because of the the positioning of the Calcaneus bone and the nature of the heel striking gait, all of the joints of the leg are lined up straight to receive the impact of the weight of the body, which is moving downward along the same vector as the leg. In this set up the energy from the initial impact cannot be dispersed at all and travel through the leg causing every joint involved to receive equal stress.

In barefoot running there are three major factors that allow the human body to endure the stress of running: the spring like structures of the foot and leg, the medial longitudinal arch,  and the sensory feed back systems known as proprioception.

The foot and leg are positioned in a significantly different manner during barefoot running then the way just described for heel strike. This manner of landing is referred to as a forefoot strike. While there are variations on this technique, the basic principles remain the same. In the forefoot strike the bodyweight is largely positioned over the foot in stance phase. The knee is bent and the foot is plantarflexed (point down). In this way the leg forms a number of levers that can absorb the initial shock of impact. The primary lever is that of the foot, holding the ankle as the fulcrum. The forefoot, or ball of the foot, is the area comprised of the ends of the long metatarsal bones, or where the toes connect to the foot. Allowing this portion of the foot to fall first and then slow the decent of the rest of the foot and subsequently the rest of the body greatly cushions the loading of the rest of the bodyweight.  In mechanical terms this is accomplished by converting  linear momentum into angular momentum and then applying resistance. The same is mechanic is mirrored in the bent knee and to a lesser extent the hip.

Once again I refer to another article co-written by Lieberman titled Foot strike patterns and collision forces in habitually barefoot versus shod runners. In this article the forces of running are analyzed and Lieberman come to this conclusion:

 At similar speeds, magnitudes of peak vertical force during the impact period … are approximately three times lower in habitual barefoot runners who FFS (forefoot strike) than in habitually shod runners who RFS (heel strike) either barefoot or in shoes

Both RFS gaits generate an impact transient, but shoes slow the transient’s rate of loading and lower its magnitude. FFS generates no impact transient even in the barefoot condition.

The next structure to aid in running is the medial longitudinal arch of the foot, or as it is commonly known  just the arch of the the foot. Presence of this structure in early Proto-Humans is used as an indication of bipedalism (BBC). The arch is dealt with as a rigid structure in the case of most footwear. Most shoes have some form of arch support that is intended to stabilize the arch and keep it from moving.  In the case of barefoot locomotion, however, the exact opposite is done for the arch. Studies of habitually barefoot people show that the arches of their feet are significantly higher on average then those of their habitually shod counterparts. When allowed to behave normally the arch of the foot deflects or spreads under weight. This topic is covered throughly in running related injury prevention through barefoot adaptations.

 …medial arch rising and shortening due to activation of intrinsic musculature allows the foot to act as a dynamic impact dampening structure rather than merely as a lever for propulsion…

Because of this deflection it acts as yet another spring like structure absorbing shock and then returning energy on push off.

The deflection of the arch is facilitated by intrinsic muscles within the foot. As the muscles strengthen the arch becomes more prominent. Additionally as these muscles take over much of the load bearing function in the foot, stress on the plantar fascia is reduced. This can help prevent cases of Plantar fasciitis, a very common overuse injury.

Activation of this intrinsic musculature is in part or in whole initiated by sensory feedback the feet receive from the ground. The sensory feedback system is referred to as Proprioception.

The irregular character of the contact surface seemed to be the element that was present in the subject with the greatest adaptation. this is consistent with the hypothesis that plantar sensory feedback may induce intrinsic foot shock absorption.

Reduction of sensory feedback by means of a shoe or other device that keeps the foot from accurately feeling the impact of  touchdown disallows the engagement of these adaptive features in the foot.

The modern running shoes and footwear in general have successfully diminished sensory feedback with out diminishing the injury inducing impact, a dangerous situation.The mode of injury follows the medical model of a neuropathic injury.

Given the function of the few structures and systems touched on here, as well as many others not discussed, it becomes clear that not only can humans run with bare feet effectively but that they may do so with better proficiency and safety then with shoes. From the perspective of evolution and from human physiology, barefoot running is solid. Never the less there remains a large number of practitioners of podiatry and sports medicine that recommend standard running shoes. These recommendations at best come from a fear that the case for barefoot running is only theoretical and may not be born out in practice. As with any procedure or new medicine, a fully controlled test of the effects in the real world is a must before it can be accepted. Unfortunately the reality of testing  in the world of running is complicated and less then stellar.

In the next post I will be discussing the Statistical Analysis of Barefoot and Conventional Running.