Have you seen those weird and stunningly trendy glove-like shoes that have five toe compartments? Or people running on the side of the road in a shoe that seemingly has less support than a sandal? Or, better yet, people running without wearing any shoes at all? Well, as you are surely aware, we have dialed back the clocks and are now entrenched in the barefoot running era.
Current evidence suggests that relative to traditional shoe running, conditioned barefoot runners -- such as those in Kenya -- demonstrate a tendency to run with a forefoot strike pattern (i.e., landing on the balls of the feet). This allows for a slow lowering of the body through eccentric (lengthening muscle contraction) activity of the calf muscles. In contrast, habitual shoe runners strike the ground with the heel, and then use the foot as a rocker for the body to fall forward.
Reconstruction of foot contact patterns and lower extremity kinematics during typical heel-strike (left) and forefoot strike (right) running. The red arrow represents the vertical ground reaction force.
Relative to a heel-strike, the forefoot strike of barefoot running is associated with a reduction in the rate at which the body collides with ground, and, presumably, the muscle and ligamentous demand to the knee, both of which are associated with risk of knee injury. According to our preliminary findings at the University of Southern California, however, the catch-22 is that in barefoot running there is a shift in demand from the knee to the ankle and its supporting structures. Unsurprisingly, recent anecdotal and clinical reports have linked this running style with an increased potential for plantar fasciitis, tibial stress fractures, and calf pain.
Similar to their conditioned barefoot running counterparts, when habitual shoe runners run barefoot, or in barefoot shoes, there is an immediate change from a heel-strike to a forefoot strike. This inherent change occurs in order to prevent collision between the heel and the ground. However, per our initial reports, unlike habitual barefoot runners, the calf of a novice barefooter is typically not conditioned to maintain this increase in demand, leading to calf muscle fatigue. As a result, there is a reversion to the traditional heel-strike pattern following a short bout of barefoot running, but now without the shock-absorbing layer of a shoe. Consequently, the rate of collision with the ground can rise upwards of 150-percent higher than initial barefoot running, likely increasing the knee-injury risk. It appears that the only way to prevent this is to train the foot and calf to this new running pattern, highlighting the importance of a long-term transition to barefoot running prior to ditching your shoes.
The purpose of a transition to barefoot running would be to develop the motor pattern, as well as muscular strength and endurance to optimize barefoot running performance, while dually minimizing injury risk. Thus emphasis would be placed upon eccentric training of the calf in order to develop the ability to consistently land on the balls of the feet. In contrast to a concentric exercise (muscle shortening), eccentric exercise aims to strengthen the muscle when it's being lengthened. During eccentric activity, the load being resisted exceeds that of the muscle force output; thus the muscle is effectively absorbing energy. This is akin to what occurs when you forefoot strike and rely on the calf -- in its now-lengthened position -- to slowly lower the body and prevent collision with the ground. As a result of the altered loading mechanism, eccentric exercise is associated not only with strength gains but increases in the size of fast-twitch muscle fibers.
Running magazines would lead you to believe that barefoot running reduces injury risk and potentially maximizes performance. However, it appears that, as this is a new trend, there is simply insufficient prospective data to say otherwise. But, as in all changes in life, there may be a few trade-offs. From an injury perspective, does the forefoot strike and reduction in loading rate now synonymous with (conditioned) barefoot running counteract the increase in demand to the calf, Achilles, and ankle-joint complex? Will this increase in energy absorption induce muscle tears or tendonitis? And does the reduction in knee-joint loading warrant a lengthy and strict adherence to a barefoot running transition protocol? As all good scientists say when posed with difficult questions and trying to buy more time to answer them: It depends. But only time will tell.
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