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Biomechanics

Barefoot running strikes back

Subjects

Detailed analyses of foot kinematics and kinetics in barefoot and shod runners offer a refined understanding of bipedalism in human evolution. This research will also prompt fresh studies of running injuries.

A commitment to walking and running on two legs distinguishes humans from apes, and has long been the defining adaptation of the hominins — the lineages that include both humans and our extinct relatives. This form of locomotion (bipedalism) has been around for millions of years, and we have been unshod for more than 99% of that time1. The uniquely specialized anatomy of the human foot2 is thus a product of barefoot bipedalism, which is still the norm in parts of the world. Lieberman and colleagues' biomechanical research on the subject (page 531 of this issue)3, therefore has implications for interpreting human evolution. It also has some potentially useful and thought-provoking implications for sports medicine and running-shoe design. (Full disclosure: I don't run any more, either barefoot or in shoes.)

Most shod runners today make initial contact with the ground heel-first (rear-foot striking, or RFS). Experienced barefoot runners, like the ones observed for this study3, land on the ground in many ways depending on the conditions — sometimes RFS, but more often avoiding landing heel-first because it hurts owing to repetitive, high-impact forces (or transients). A more anterior landing on a flat foot (mid-foot striking, or MFS), or on the lateral ball of the foot (fore-foot striking, or FFS), has predictable, and some would say desirable, consequences for pedal biomechanics. In FFS and some MFS, the foot's centre of pressure necessarily starts more anterior at contact and then moves backward briefly before moving forward again for toeing-off. (Sprinters, whether shod or not, also run on their forefeet, but for different mechanical reasons.) Among other differences, FFS barefoot runners tend to take shorter strides and to run with greater vertical leg and ankle compliance (the lowering of the body's centre of mass relative to the force of the impact). This serves to blunt the transient force and results in a less jarring, 'smoother ride'. The elevated and cushioned heel of most modern running shoes is designed for comfort, stability and to attenuate the transient forces of heel-strike in RFS running that may be linked to some orthopaedic injuries.

Lieberman and colleagues' multifaceted study3 corroborates and extends what is known about the basic mechanics of barefoot running, as they develop a collisional model of the foot and leg as an 'L-shaped double pendulum' with the same dimensions as a typical shank and foot. They then calculate how much energy such a pendulum exchanges with the ground when it collides at different points and with a stiff or compliant ankle. They also broaden the comparative human database by studying the phenomenon not only in long-term, habitual barefoot runners in a laboratory setting but also on the runners' home turf in Africa (Fig. 1). They find that FFS (and some MFS) reduces the effective mass of the foot and converts some translational energy into rotational energy; the calf muscles control heel drop, and the FFS runner can take fuller advantage of elastic energy storage in both the Achilles tendon and the longitudinal arch of the foot. FFS and MFS runners therefore require more calf- and foot-muscle strength, but avoid uncomfortable and potentially injurious impact transients even when barefoot on very hard surfaces.

Figure 1: Different footfalls.
figure1

These photos are of two Kalenjin runners from Kenya, a barefoot 12-year-old girl (left) and a boy of the same age in running shoes. Note the differences in foot angulation as the girl prepares for a forefoot touchdown and the boy prepares to land heel first. (Photos courtesy of D. E. Lieberman.)

The findings of this study complement and strengthen Bramble and Lieberman's influential endurance-running hypothesis (ERH) for the transformation of the human body plan with the emergence of the genus Homo4. The much earlier australopithecine version of bipedalism (as seen in 'Lucy', Australopithecus afarensis) was long-lived and a great success by any standard. However, this skeletal design received a major make-over near the Pliocene/Pleistocene boundary about 2 million years ago. Longer hindlimbs and shorter toes are part of this new package, and if the ERH is correct, the evolution of these features along with a fully arched foot is probably linked directly to barefoot running as an integral part of an adaptive strategy for pursuit hunting5,6. The australopithecine bare foot was well-suited for heel-to-toe walking and perhaps for short, rapid bursts of sprinting7. But Lucy was not a marathoner.

The blogosphere and popular magazines are full of debate about barefoot running, with testimonials to it as a more 'natural' and less injury-prone style, and often with a nod to the ERH and an appeal to the evolutionary primacy of the unshod foot. Sometimes barefoot advocacy can take on evangelical overtones8. But it is also clear that running-shoe companies and other footwear designers are paying attention, and they should take note of this new study3. Barefoot-like designs for footwear are currently the rage, even if they still constitute only a small slice of the enormous running-shoe industry. Many shod runners never develop injuries, but the available data indicate that at least some (19–79%) do9. Although there is no hard proof that running in shoes, especially hi-tech or PCECH (pronation control, elevated cushioned heel) versions, causes injuries, in my view there is no compelling evidence that it prevents them either10,11. However, there are data that implicate shoes more generally as a plausible source of some types of chronic foot problems12,13.

More studies like that of Lieberman et al.3 are required to provide data instead of opinion, and testable models and scientific explanation instead of anecdotes. It is also apparent that a carefully designed biomedical study with an evidence-based approach is badly needed to assess the competing claims as to what, if anything, is the best cover for a runner's foot. It will be interesting to see where the next foot falls, and how it is wrapped.

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Jungers, W. Barefoot running strikes back. Nature 463, 433–434 (2010). https://doi.org/10.1038/463433a

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