About two million years ago, proto-humans began to evolve features necessary for long-distance running: long legs for a longer stride, large buttocks for balance, shorter toes, and long, flexible arches in the foot. Though humans and their ancestors have been running for millions of years, shoes have only been in existence for the past 10,000 years. In the early 1900s the first athletic shoes were invented. Modern running shoes, as they exist today, did not come onto the market until the 1970s. The vast majority of runners wear shoes, but barefoot running has recently developed a small but devoted following. Today's athletic shoes are a multibillion-dollar business. Some people wear them for their style, while others wear them mainly for their intended purpose - support and protection while running and exercising. Manufacturers typically claim their shoes offer more extensive benefits, and have been carefully and scientifically designed to maximize running ability and protect against injury. But is this true? Barefoot runners claim that they can run faster and with fewer injuries without shoes. Could modern shoes hinder a running style that was millions of years in the making?
Humans exhibit many musculoskeletal specializations for running. Running employs compliant limbs in which muscles and tendons in the legs sequentially store and then release strain energy during the stance phase of the stride cycle. These springs can have comparatively little effect on energy savings during an inverted pendulumlike walk, particularly at heel strike when the limb is not compliant, but are estimated to save approximately 50% of the metabolic cost of running. The most important of these springs is the Achilles tendon, which connects the heel with the major plantar flexors of the foot.
Another well-developed set of springs important to human running is the longitudinal arch of the foot. During walking, the plantar arch helps to maintain mid-tarsal rigidity for powered plantar flexion during toe-off, and absorbs some impact force, but only after heel strike. During running, the elastic structures of the plantar arch function as a spring, returning around 17% of the energy generated during each stance. In addition, for the plantar arch to be an effective spring during running, the transverse tarsal joint must restrict rotation between the hind foot and the anterior tarsal, allowing passive stretching of the plantar ligaments during a mid-foot strike.
Stability is also important in human running as gaits are inherently unsteady. Most obviously, the trunk and neck of human runners are more forwardly inclined during running than walking, resulting in a greater tendency to pitch forward, especially at heel strike. Researchers found that most runners who normally wear shoes strike the ground with their heels, whether they are wearing shoes or not. Runners who normally run barefoot strike the ground with the ball of their foot while shoeless, but some of those runners strike with their heels when wearing shoes. Force measurements of rear-foot striking, where the runner lands on his heel, found that this results in a quicker and stronger impact. Thus, rear-foot striking does not take advantage of the arch.
In forefoot landings, where the runner lands on the ball of the foot, the springy arch spreads out the impact of the collision over more time as it compresses and releases. The large cushioned heels on running shoes absorb some of the impact caused by rear-foot striking, but the impact is still greater than in forefoot striking. Rear-foot striking also changes the angles of the ankle and the knee, making the leg stiffer as compared to forefoot striking. Since the average runner strikes the ground about 1,000 times per mile, it is believed that these rear-foot collisions may play a role in the repetitive stress injuries that are common in distance runners. Heel-striking is painful when barefoot or in minimal shoes because it causes a large collision force each time a foot lands on the ground. Barefoot runners point their toes more at landing, avoiding this collision by decreasing the effective mass of the foot that comes to a sudden stop when you land, and by having a more compliant, or springy, leg.
Another factor to consider when evaluating running styles is the human skeletal strength. Running exposes the skeletal system to much higher stress, especially when the foot collides with the ground, producing a shock wave that passes up the body from the heel through the spine to the head. Peak vertical ground reaction forces (GRFs) at heel strike are approximately two to four times higher during running than during walking and may approach four times body weight at higher speeds. Human runners reduce these stresses to some extent through limb compliance and mid-foot striking, thereby also storing elastic strain energy in the leg and foot, but must otherwise dissipate impact forces within their bones and joints.
So should runners lose their shoes? Not exactly. Shoes are comfortable and they protect the foot from the perils of urban running, like rocks and glass. Shoes work because they cushion much of that force by slowing it down. There is also now a paradigm shift in technology for shoe insoles from traditional cushioning towards shock-suppression technology. Soon to be implemented by major athletic shoe manufacturers, this insole evolution will result in less vibrations traveling up the lower extremities, eliminating fatigue occurring within muscles that lead to injury. Unequal Technologies, the innovator behind this technology is ahead of the industry curve as they combine the shock suppression of Kevlar with stateof- the-art cushioning technology to support the foot. Kevlar acts as the middle reinforcement layer and lateral structure that prevents the insole from flattening or bottoming out, much like the foam and rubber materials found in other insoles.
However, if runners are curious about trying the barefoot running style, they should start slowly and do lots of stretching to avoid injury. Most people have been heelstrikers all their life and will have to transition slowly to build strength in their calf and foot muscles. Remember, humans were born to run - putting on shoes is your choice.