Running Science

by Owen Anderson

  In the platform research, the athletes perceived that the impact forces were greatest on the hard surface even though they were actually of the highest magnitude on the soft surface. The yielding surface was creating a feeling of comfort, an illusion of lower GRF, and as a result, a higher actual impact force was transmitted through the leg.

  Soft and Thick Midsoles May Increase Impact Forces

  Such findings have suggested to some sports medicine experts that modern running shoes are being created on the basis of an incorrect paradigm: the notion that relatively thick, softer, more deformable midsoles provide better cushioning and therefore less stiff midsoles are optimal for a large number of runners. An extension of this thinking is the conception that runners’ feet and legs are fragile objects that must be protected by extensive cushioning from the hard impacts of running. However, this cushioning might produce a feeling of comfort that knocks out a runner’s intrinsic neuromuscular defense mechanisms against impact forces, providing an explanation for why athletes landing on soft surfaces permitted higher forces to shock their legs. Expanded midsole cushioning might spike force transmission and perhaps augment the risk of running injury.9

  Research in this area has yielded interesting results. One study found that when runners used both hard and soft shoes and maximal vertical forces were similar, the runners had slower rise times to peak vertical force during foot strike when they wore the softer, more cushiony shoes.10 The phrase “rise time to peak vertical force” refers to the period after the foot hits the ground during running and acknowledges the fact that the GRF actually increases for the first 20 to 50 milliseconds of stance. One hypothesis is that the rate at which the force increases is linked with a higher risk of injury. In other words, slower rise times to peak vertical force would be connected with a reduced likelihood of injury, presumably because the slower rise times give a runner’s nervous system more time to react and change leg stiffness.

  A separate study found that harder shoes were linked with a slower rise time to peak vertical force and reduced peak vertical impact forces, supporting the idea that less stiff midsoles thwart neuromuscular responsiveness.11 However, a third inquiry found no relationship at all between shoe midsole hardness and force-loading magnitudes.12

  Taken together, these studies suggest that running-shoe midsole hardness appears to have an unpredictable effect on the impact forces experienced by the legs during running. The true take-home lesson is that endurance runners should not believe that soft, cushiony shoes—or running shoes advertised as having greater cushioning—will actually reduce the impact forces passing through their legs. Runners’ legs are not inert objects of fixed stiffness but rather are complicated, responsive structures that can change configuration and overall stiffness very quickly—even immediately before they encounter a particular running surface.

  Running Shoe Design and Injury Prevention

  A popular belief among endurance runners is that certain running shoes, usually the more expensive models, provide greater protection against injury. When surveying the running shoe market, runners find models with air cushioning, honeycombed midsoles, foam springs, microchips, microspheres, and specialized gels—and accept the notion that such advances must be linked with greater injury protection. Scientific research indicates that this is not the case. As running shoes have incorporated more so-called injury-preventing features since the 1970s, injury rates have not decreased at all. Injury investigations indicate that currently from 50 to 91 (!) percent of endurance runners are injured over the course of a training year when injury is defined as a physical problem severe enough to limit normal training.13

  Misconceptions About Shoe Price

  The available scientific evidence indicates that higher-cost, seemingly more protective running shoes are actually linked with a greater risk of injury. In one study, a Swiss physician analyzed the training habits and shoe preferences of 5,038 runners as they prepared to compete in a 16K road race in Bern, Switzerland.14 The physician found that during the 16-month training period, 14 percent of the runners who spent less than $40 for their shoes were injured as were 17 percent of the runners investing $40 to $60 and 21 percent of the athletes who shelled out $60 to $95. In contrast, 32 percent of the runners who paid more than $95 for their shoes were injured—that’s more than double the injury rate of the runners with the cheapest shoes.

  Training mileage and history of injury were equivalent in these four groups, which meant that the reduced risk of injury for those who bought the lower-cost shoes was not the result of a more modest training regime or a higher frequency of prior injuries among the those who purchased the more expensive shoes. The physician concluded that the amount of money spent per pair of running shoes was a predictor of the income of the purchaser and an inverse indicator of the quality and injury prevention characteristics of the shoes.

  Recent research has shown that more expensive shoes do not provide better cushioning than cheaper models. Researchers from the University of Dundee in the United Kingdom provided runners with low- (US$80-$90), medium- (US$120-$130), and high-cost (US$140-$150) running shoes and then measured plantar pressure as the individuals walked and ran on a treadmill. The assumption was that better cushioning should decrease the pressure placed on the plantar surface of the foot during running.15 Plantar-pressure measurements were recorded under the heel, across the forefoot, and under the great toe. Results indicated that the low- and medium-cost running shoes provided the same—if not better—protection from plantar pressure compared with the most expensive shoes. There was also no difference in comfort between the three types of shoes.

  The mechanism underlying the Swiss physician’s discovery of the link between injury and more expensive running shoes described previously has not been fully explained by scientific research. It is clear that more costly running shoes often have more features than their cheaper peers, including thicker midsoles and more developed motion-control features such as heel counters that are supposed to prevent overpronation of the ankle during the stance phase of gait. Unfortunately, thicker midsoles actually tend to increase medial and lateral rocking of the ankle during the stance phase of gait, effects that could put more stress on the ankle and knee during each impact with the ground. In addition, antipronation devices in the heels of running shoes have actually been linked with faster and more extensive amounts of pronation, which might increase the risks of ankle and knee injury.16

  The explanation for this latter, surprising finding may be that the hard heel structures, or counters, promoted as pronation preventers may actually serve as stiff levers that accelerate medially, that is, toward an imaginary midline running down the middle of the body, during stance more quickly than do softer-sided heels. If this seems paradoxical, bear in mind that lever speed is a direct function of lever length: As a lever increases in length, the speed at which the end of the lever moves must also increase. Having a hard structure against the side of the heel of a running shoe creates a lever on the inside of the heel, and the top of this lever must accelerate rapidly once natural pronation of the ankle is initiated during stance. Without the counter, there would be no such effect.

  As mentioned, it is also possible that wearing more expensive shoes fools runners into thinking that their lower limbs are better protected during running. Scientific research indicates that this idea is not as implausible as it might appear to be at first glance. In one study, researchers asked subjects to step down, barefoot, on separate force-measuring platforms.17 Each platform was covered with exactly the same shoe sole material, but the materials were made to look different because of cloth coverings of varying colors. The subjects were told that the material on one platform provided superior impact absorption and protection (a deceptive message) and that the material on a second platform offered poor impact absorption and was linked with a high risk of injury (a warning message).

  The results revealed that the subjects landed with the highest impact forces on the surface a
ssociated with the deceptive message; apparently believing it to be safer, they made fewer adjustments in their lower limbs to absorb shock. The subjects landed with the lowest impacts on the surface associated with the warning. The authors concluded that injury rates are highest in wearers of the most expensive running shoes because advertising has seduced them into believing that the more costly shoes provide a higher level of safety. Such misperceptions may attenuate impact-moderating changes in lower-limb action during landing and thus heighten impact forces and increase the risk of injury.

  Rethinking Duration of Shoe Life

  To prevent injury, endurance runners are frequently advised to throw away running shoes that have been used for more than 200 to 300 miles (322-483 km), or at least to use them for gardening rather than running. Research does show that the midsoles of such shoes have lost a significant amount of compression-set resistance, or the ability to deform and then spring back into original configuration with each footfall. Such a loss would apparently require the foot, ankle, and perhaps higher regions of the leg to soak up more force with each ground contact since less impact force would be used to deform the midsole.

  However, research has shown that as running shoe midsoles lose their compression-set resistance over time, individuals wearing the shoes actually improve the control of their feet during running, an effect that should decrease injury risk.18 The presumed mechanism is that cushiony midsoles that have not lost their compression-set resistance prevent a runner’s feet from truly feeling the surface upon which he or she is running and therefore prevent the nervous system from reacting effectively and with enhanced coordination. As shoe researchers S.E. Robbins and G.J. Gouw have stated, cushiony midsoles may create a kind of “pseudo-neuropathy” in runners.19 Research does reveal that modern running shoes tend to create a perceptual illusion causing runners to consistently underestimate impact forces.19 A surprising conclusion that can be drawn from such research is that running shoes may actually get better—not worse—with age from the standpoints of motion control and injury prevention.

  Potential Improvements for Midsoles

  The solution to the problem of loss of impact-force detection might be straightforward. Research has indicated that the mere addition of surface irregularities on running shoe insoles on the sides of the insoles in contact with the soles of the feet significantly improves runners’ estimates of impact forces,19 presumably because such irregularities provide runners with a heightened sense of the pressures exerted during ground contact on various parts of the bottom of the foot, and also because the bumps, or irregularities on the midsole, provide runners with a better estimate of horizontal shear forces during ground contact. Unfortunately, no such commercial insoles exist.

  Making running-shoe midsoles thinner should also improve coordination and balance during running. In one study, male subjects walked along a beam while wearing shoes with soles of different hardness and thickness. The thinner, harder soles were linked with significantly better balance.20 Many endurance runners and coaches might worry that thinner soles would increase impact forces experienced by the legs and thus increase injury risk, but research does not support the idea that thicker midsoles provide truly better cushioning. As noted previously, running shoe midsole thickness has burgeoned since the 1970s, and there has been no decrease in running injury rates. Runners’ legs appear to be quite capable of learning to absorb increased forces—assuming they are passed up the legs as a result of the use of thinner midsoles—in a safe and effective way provided that the transition from thick to slim midsoles is made carefully over time.

  Brand Loyalty’s Perils

  Swiss physician B. Marti’s research on training habits and shoe preferences described in this chapter also showed that runners with no preference for running shoe brand had a 25 percent lower risk of injury over a 16-month period of training as compared with runners who favored one particular shoe model. One hypothesis emerging from this finding is that each model of running shoes produces unique stresses on the musculoskeletal system; if the shoes are worn long enough, these stresses can lead to injury. If the shoes are changed, the stresses change, and the risk of injury is reduced because the previously stressed tissues get a break while other structures begin to take a pounding.

  Marti also found that runners who chose their shoes based on style or color had a 20 percent reduction in injury rate compared with competitors who attempted to choose running shoes with orthopedically correct design and construction(!). This suggests that either runners are unaware of the true nature of orthopedically correct running shoes or else that orthopedic optimality in running shoe design does not currently exist.

  Facts About Motion Control

  Running shoe companies market motion-control running shoes that are alleged to reduce excessive pronation and thus limit the risk of knee and other injuries. This view is proclaimed to be a mainstream development in running shoe technology.21 Such shoes usually have an assortment of unique features, including greater stiffness on the medial side of the midsole, greater support in the vertical wall of the medial side of the heel, and valgus (laterally inclined) heel wedges. It is certainly true that such shoes can change the kinematics of the ankle joint during running. There is no scientific evidence, however, to support the claim that the use of motion-control shoes reduces the risk of knee problems—or any other kind of injury—in endurance runners.22

  Why is this so? It is very possible that motion-control shoes have an impact on the movement of the feet and ankles during running, but the changes in movement that are produced are not optimal. It is also possible that motion-control shoes produce changes in kinetics that are often outweighed from an injury-prevention perspective by other factors, including training volume, nutrition, and recovery. It seems highly probable that strengthening the foot and ankle joint with running-specific exercises would have a greater impact on foot, ankle, and knee injuries than wearing a motion-control shoe. Research supports the idea that strength training limits the risk of running injury but has never reinforced the notion that motion-control shoes reduce injury rates.

  Scientific evidence suggests that the primary function of running shoes may be to protect the bottom of the foot from harsh, potentially damaging surfaces. All other proposed functions (e.g., stability, motion control, increased cushioning, advanced energy return) may lie within the domain of marketing rather than exercise science and foot and ankle biomechanics. Science also suggests that the purchase of expensive running shoes may represent a symbolic act rather than the actual acquisition of higher-quality shoes.

  Barefoot Running

  Since modern running shoes get failing marks from the standpoints of injury prevention, motion control, energy return, stability, and cushioning, it is not surprising that barefoot running is becoming increasingly popular among endurance runners. The popular press and a large number of runners and coaches have proposed that unshod running strengthens the feet more than shod running, helps relieve current injuries, and also lowers the risk of future injury. Proponents of barefoot running also suggest that the technique diminishes impact forces with the ground and the rate at which such forces are transmitted up the legs during running and that barefoot running can even improve performances. From the performance standpoint, research reveals that donning a pair of running shoes instantly harms running economy by about 1 percent compared with barefoot running, an effect that should slow distance-running performances to a similar extent.13

  Barefoot running is linked with lower impact forces and enhanced running economy.

  Courtesy of Owen Anderson

  The strong interest in barefoot running is partly the result of the publication of Christopher McDougall’s bestselling book Born to Run, in which the author claims that barefoot running dramatically enhances endurance and is the panacea for nearly all running-related injuries. McDougall also proposes that modern running shoes are a principal cause of injury in endurance runners.23

  Research on Bare
foot Running

  Research carried out by Daniel Lieberman and colleagues at Harvard University has bolstered the movement toward barefoot running.24 These researchers looked at the kinematics and kinetics of running in five different groups: (1) habitually shod athletes from the United States, (2) currently shod runners from the Rift Valley in Kenya who had grown up running barefoot, (3) U.S. runners who began running using shoes but have now adopted a barefoot-running style, (4) adolescent Kenyan runners who have never worn shoes, and (5) adolescent Kenyan runners who have run shod for most of their lives. The study led to the following findings:

  Habitually shod runners (groups 1 and 5) who grew up wearing shoes are usually rear-foot strikers (RFS), meaning that their heels make the first impacts with the ground during running at the beginning of the stance phase of gait. The strong link between running in shoes and heel striking has been detected in other research.25