How does running shoe design and construction affect injury risk, and how should this affect your shoe choices? Andrew Hamilton looks at what the recent research has to say MORE
Minimalist shoes: the road to reduced running injuries?
Minimalist running shoes claim to offer the benefits of barefoot running, without some of the drawbacks. But what does the science say about the benefits or otherwise of this type of footwear? Peak Performance looks at the evidence
It was back in the late 1970s that the running boom really got underway. Since then, tens of millions of people around the globe have enjoyed recreational and competitive running, both as a pastime and as way of getting and keeping fit. However, as any physiotherapist will tell you, the biomechanical demands of running are such that the potential for injury is comparatively high compared to other endurance sports such as cycling or swimming. Not surprisingly therefore, the growth in the popularity of running has been accompanied by an increasing number of running injuries.
Throughout the running boom, it has always been assumed that the most effective way of preventing running injuries has been the use of appropriate footwear. It’s hardly surprising therefore that the past four decades has seen an explosion in running shoe technology, with successive generations of shoes using increasingly sophisticated solutions to ensure the ideal running gait and the absorption of the impact forces, which are a particular problem when pounding the tarmac or pavements.
With this in mind, you might expect that the rate of injury suffered by runners per mile run has been steadily falling. However, studies on the incidence of running-related injuries conducted from 1989 to the present have found a remarkably consistent rate of injury(1). In other words, despite all the technological advances in shoe development over these past decades, the rate of injury has stayed pretty much the same.
The barefoot revolution
In more recent years, some runners have taken a different approach by embracing barefoot running. The proponents of barefoot running claim that this mode of running is far more ‘natural’ and allows the human foot to work in the way that Nature intended it to – something that can’t happen when the foot is shod in an artificial shoe. The claimed advantages of barefoot running revolve around the fact that an evaluation of experienced barefoot runners compared to shod runners reveals several striking (no pun intended!) differences.
Most of these differences stem from the fact that barefoot runners typically land with a mid-foot strike (where the foot lands rather flatly) or forefoot strike (where the ball of the foot contacts the ground first and then the heel is lowered). This contrasts with the fact that 75% of shod runners land with a rear-foot strike in a heel-to-toe fashion(2).
The significance of this fact is that the ground reaction impact forces generated from the collision of the foot with the surface are typically much higher when a runner heel strikes – the magnitude of the peak impact force during rear foot strike has been shown to be 1.5 – 3 times the body weight(3). The use of a cushioned running shoe typically decreases this impact force by around 10%, making it more tolerable. However, a much more effective way to reduce the magnitude of foot strike impact forces is to adopt a mid or forefoot strike (see figure 1). It follows that without any cushioning underfoot to reduce the impact of rear foot strike, barefoot runners naturally adopt a mid or forefoot strike, which actually turns out to be more effective at reducing impact forces, and so (according the proponents) reduces the risk of injury.
Figure 1: The ground reaction forces associated with barefoot vs. shod foot strike(3)
Note: a) = barefoot rear foot strike; b) = shod rear foot strike; c) = barefoot forefoot strike. The steepness of the curve (indicated by leftmost arrow) indicates the degree of deceleration and the associated impact force. Note how the forefoot strike barefoot trial (c) produces significantly lower impact force than in a rear foot shod strike (b).
Why is it that a mid or forefoot strike pattern can reduce the severity of ground impact forces compared to a rear foot strike? One explanation for this phenomenon comes from a study conducted by US scientists(4). In particular, the researchers looked at the joint kinematics and impact absorption characteristics of the ankle, knee and hip joints during rear foot strike running, forefoot strike running and barefoot running. One of the main findings was that in rear foot strike, there was a significantly greater reliance on the knee and hip joints to absorb impact forces compared to barefoot and forefoot strike running. Instead, these latter modes of running produced more impact absorption at the ankle joint (see figure 2). These results were confirmed in a later study on runners wearing minimalist shoes(5). It found that compared to normally cushioned shoes, minimalist shoes (see later) reduced loading through the patellofemoral (knee) joint).
Figure 2: Power (impact) absorption differences between rear foot, forefoot and barefoot running
RFS = rear foot strike; FFS = forefoot strike; Barefoot = barefoot running. Barefoot and forefoot runners demonstrated increased plantarflexion (toes pointing downwards) at initial ground contact, which increased peak ankle power absorption and decreased peak knee and hip power absorption.
Barefoot benefits or barefoot lies?
The data on foot strike differences between shod and barefoot runners are widely accepted across the sports science and running community. What this mean in terms of injury risk remains a topic of controversy however. Barefoot running proponents claim that (provided a gradual transition is made) the introduction of barefoot running to a training programme will lower the risk of injury. But this is disputed by a number of researchers. For example, Craig Payne, a senior lecturer at the Department of Podiatry at La Trobe University in Melbourne commented in a paper on this topic: “The barefoot running community has an appalling track record at how they misinterpret, misuse and misquote research. The simple facts are that not one risk factor study on running injuries has linked high impacts to running injuries, yet the barefoot running community claim that the evidence shows this and consider high impacts as the cause of all injuries.(6)”
Putting this argument to one side for the moment, there are some other, undeniable disadvantages to barefoot running. For example, running barefoot on extremely hot pavements/tarmac or in extremely cold conditions may damage the soles of the feet. In addition, there are hazards such as nails, glass, pebbles and other objects that can puncture the soles of feet or lead to stubbed toes. Moreover, even if heel strikes are eliminated by running barefoot, the use of shorter strides typically observed in barefoot runners means the feet hit the ground more often – what’s gained by reducing the force of impact might be offset by the increase in the frequency of impacts.
The evidence to date is that while barefoot running may offer some theoretical advantages in terms of reducing foot strike impact by promoting a more mid/forefoot strike pattern, there are disadvantages. In something of a halfway-house evolution therefore, running shoe manufacturers have recently begun to offer so-called ‘minimalist’ running shoes. Minimalist shoes (sometimes referred to as ‘barefoot shoes’) are designed to allow the foot to move through a similar range and pattern of motion during running as would an unshod foot, while at the same time offering some protection. Some minimalist shoes are completely minimal, providing just a thin slither of rubber under the sole, which leaves the foot to move as freely as possible. Others are slightly more substantial, being designed to help runners gradually transition from rear foot normally shod running to barefoot-style running.
The minimalist promise
Implicit in the marketing behind minimalist shoes is the promise that (providing they are introduced very gradually) their use will help runners to develop a more natural running style, leading to fewer injuries, a more balanced musculature and better running posture. But what does the science say about these claims?
One of the earliest studies into the use of minimalist shoes compared the biomechanics of barefoot running with that of running in minimalist footwear and conventional running shoes(7). In the study, the mechanical characteristics of the foot/shoe-ground interface were investigated in eight experienced barefoot runners in order to assess the ground pressure distribution, sagittal plane kinematics, and running economy. The researchers also sought to discover if a minimalist shoe (Vibram Fivefingers – see figure 3) was effective in mimicking the experience of barefoot running and both these conditions were compared to the use of conventional running shoes.
Figure 3: Vibram Five Fingers shoe
As hinted above, it turned out that compared to the standard shod condition, when running barefoot the athletes landed in more plantarflexion at the ankle, which reduced impact forces and changed stride patterns. In particular, significantly shorter stride length, foot contact times, and higher stride frequency were observed. The use of the Fivefingers shoe also resulted in peak impact forces that were significantly lower than shod running, and much closer to barefoot running. The lower limb kinematics with Fivefingers was similar to barefoot running, with a foot position that was significantly more plantarflexed than in conventional shoes. The researchers concluded: “The Fivefingers shoe seems to be effective in imitating the barefoot conditions while providing a small amount of protection.”
Another purported benefit of minimalist shoes is that they allow a more accurate estimate of static and dynamic foot position compared to wearing a standard cushioned running shoe. The theory is that this increased ‘foot awareness’ may help promote a more efficient running motion, especially over undulating terrain. In a study on this topic, researchers looked at the effect of a five-toed minimal protection shoe on static and dynamic ankle position sense (again, Vibram Fivefingers shoe) and compared it both with a conventional shoe and with barefoot running(8).
Static ankle joint position sense was assessed by asking fourteen experienced amateur runners to estimate the perceived direction and amplitude of a support slope surface board placed under their right foot while standing. The dynamic measures were performed with the subjects running on a treadmill at 12kmh and asking them to evaluate the treadmill surface slope.
The results showed that degree of movement away from flat (ie the degree of slope) experienced at the foot was underestimated, regardless of shoe or static/dynamic testing. However, in the static trials there was significantly more underestimation with the running shoe, while no significant differences were found between Fivefingers and barefoot condition. While running (dynamic test), the treadmill surface slope was significantly better estimated with Fivefingers than with conventional shoes, or barefoot running.
The early studies suggested that the use of minimalist shoes could have its place in a training programme; however, some of the research that followed proved far less positive. For example, a study by Australian researchers publishe looked at the changes in running mechanics that occur when highly trained runners run barefoot and in a minimalist shoe, and specifically if running in a minimalist shoe replicates barefoot running(9). To do this, ground reaction (impact) force data and movement patterns were collected from 22 highly-trained runners during overground running while barefoot and in three shod conditions (minimalist shoe, racing flat and the athlete’s regular shoe). The results showed that although there were significant differences between barefoot and shod conditions for movement patterns at the knee and ankle, there were no differences between shod conditions – ie that none of the shod conditions successfully replicated barefoot running.
Another study published the same year examined the claims that minimalist shoes can promote a more efficient running action, and therefore improve running economy(10). This study compared minimalist and standard running shoes (along with ‘rocker shoes’) for their effects on energy expenditure and running efficiency. Eighteen endurance female runners who were inexperienced at running barefoot or with minimalist shoes completed a 6-minute sub-maximal treadmill running test for each footwear condition, during which oxygen consumption, carbon dioxide production, heart rate and rate of perceived exertion were measured. The main finding was that compared to the standard shoes, the minimalist shoes did not decrease energy expenditure or improve running economy (despite being lighter). The rocker shoes actually increased energy expenditure, something that the researchers put down to the substantial extra mass of this shoe design.
An even more negative assessment of the minimalist shoe claims came from a US study(11). In the study, the researchers set out to determine if running in a minimalist shoe resulted in a reduction in ground reaction forces and alters movement patterns over standard shoe running. They also looked at whether any ‘within-session’ accommodation to a novel minimalist shoe occurs. Fourteen male, rear foot striking runners who had never run in a minimalist shoe were tested while running at 3.35 metres/sec for ten minutes on a treadmill in minimalist and standard shoes while a 3-dimensional movement analysis of the lower limbs and feet was carried out. Data were collected after one minute and then again after ten minutes of running in both shoe types.
The first finding was that minimalist shoe running resulted in no changes in step length or step rate. Secondly, while there was greater knee flexion (bending) and greater dorsiflexion angle at foot strike with the minimalist shoes, vertical impact peak and average vertical loading rates were actually higher then when conventional shoes were worn. To make matters worse, after ten minutes, the vertical impact peak and average vertical loading rate during foot strike increased. In other words, the minimalist shoes had not only increased impact and loading forces initially, as time went on, there was also no accommodation – ie things got steadily worse!
The researchers concluded: “Running in a minimalist shoe appears to (at least in the short-term) increase loading of the lower extremity over standard shoe running. Moreover, the accommodation period resulted in less favourable landing mechanics in both shoes. Our findings therefore bring into question whether minimal shoes will provide enough feedback to induce an alteration that is similar to barefoot running.”
Increased injury risk?
Given that running in minimalist footwear has been promoted as a means of reducing or eliminating running injuries by returning to a more natural gait, it might be surprising to learn that the literature contains a growing number of studies showing an increased risk of injury from minimalist shoe use. Moreover, this is not the injury risk arising as a result of switching abruptly to minimalist shoes without a gradual transition (see panel 1), but rather seems to be an inherent risk in the use of these shoes themselves.
For example, a US study reported on ten experienced runners, aged 21 to 57 years who were identified with injuries within one year of gradually transitioning from traditional to minimalist running footwear(12). Patients were interviewed to determine their running history, injury history, transition to minimalist footwear, and their new injury details. These runners had been running an average of 26 miles per week (ie not high mileage) for an average of 19 years. After switching to minimalist footwear, an injury occurred in these runners after an average of 2.8 months. These injuries included eight metatarsal stress fractures, a calcaneal stress fracture, and a plantar fascia rupture.
Another study looked at foot bone marrow oedema (a manifestation of added stress on the foot) after a 10-week gradual transition to minimalist running shoes (Vibram Fivefingers)(13). Thirty-six experienced recreational runners underwent magnetic resonance imaging (MRI) before and after a 10-week period. During the ten weeks, 17 subjects ran only in their traditional (cushioned) shoes while the other 19 gradually transitioned to the Vibram FiveFinger running shoes. The severity of the bone marrow oedema was scored on a range of 0-4 (0 = no bone marrow oedema; 4 = oedema in more than 50% of the length of the bone). A score of 4 represented a stress fracture. The pre-training MRI scores were not statistically different between the groups. However, the post-training MRI scores showed that in the Vibram group, 10 of the 19 runners showed increases in bone marrow oedema in at least one bone after 10 weeks of running. This suggests that even with a carefully structured transition period, minimalist-type shoes may increase the risk of stress fracture injuries.
Panel 1: The importance of a gradual transition
Despite any potential benefits of minimalist shoes demonstrated under experimental conditions in small study populations, it’s important to emphasise that any use of minimalist shoes should be introduced very gradually since athletes who have previously worn conventional running shoes may not possess adequate strength and flexibility in their feet to suddenly switch to barefoot running without consequences.
Indeed one case study reports two cases of ultra marathoners who, without changing their training routine or mileage, transitioned to wearing minimalist shoes(14). Both runners suffered metatarsal stress fractures within six weeks of wearing the new minimalist shoes. The fractures were unusual in that their location near the metatarsal shaft is not typical for runners, and that they presented in experienced, well-trained runners. Moreover, prior to these stress fractures, neither runner had had any similar kind of injury or had any condition that may have predisposed them to this kind of injury.
Another case study also looked at injuries occurring in three runners who suddenly switched to minimalist footwear, having previously worn conventional shoes(15). The authors concluded that any transition to minimalist shoe use should be performed very gradually. They also concluded that this type of injury is actually quite common and will continue to become more prevalent as more runners change to minimalist shoes. Further and more recent evidence also suggests that older, experienced runners may require a longer and more gradual transition (to a forefoot strike) than younger runners(16).
While barefoot running does seem to reduce impact loadings during foot strike, there’s still much controversy and little scientific agreement about any potential benefit in terms of injury reduction this may bring about. The evidence for the use of minimalist shoes however is rather more convincing – but unfortunately in the wrong direction! Whilst they may improve foot proprioception, there’s conflicting evidence about whether minimalist shoes can successfully mimic barefoot running. Indeed, some studies suggest that they may increase impact loadings during foot strike. The evidence for their ability to reduce injury rates is rather more damning as a number of studies appear to suggest that far from reducing injury risk, the use of minimalist shoes may actually increase this risk – even with an extended transition period into minimalist shoe use.
*On the balance of evidence to date, it’s difficult to recommend minimalist running shoes with confidence.
*Runners with a previous history of plantar fasciitis should be discouraged from using minimalist shoes, as should those with a previous history of stress fracture injuries.
*Where runners are determined to use minimalist footwear, an extended transition period of at least 3 months is absolutely essential and hilly running (with its greater associated impacts) should be avoided. A longer transition may be required for more mature runners.
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