A Literature Review done for my Masters in Sports Medicine (MsSpMed), University of Queensland, Australia 2017

Introduction

Even though barefoot running is not new, the barefoot running movement only started in the last decade, with Christopher McDougall, a prominent advocate of barefoot running, paving the way. In 2009, he published  a book, Born to Run: A hidden tribe, Superathletes and the Greatest Race the World Has ever seen, in which he discussed how the Indian Tribes in Mexico ran for long distances without shoes and did not suffer from running injuries that runners in modern society suffered from30. He attributed this to the modern shoes that despite technological advances were unable to prevent these injuries12.

Since then, there has been a surge of runners attempting this form of running in a bid to prevent injuries. In a large survey (n=785), 75.7% of runners were interested in running barefoot or in minimalist footwear and that 30.4% had already tried running in them24. This trend also drove footwear companies to develop minimalist shoes which hoped to not only allow the feet to mimic barefoot running but to also protect them from the harsh, hard surfaces that many urban runners are running on. These shoes most commonly have a 0mm heel-to-toe drop to mimic a flat surface unlike traditional shoes which can have as much as 14mm heel-to-toe difference in height. In addition, because of a very thin midsole and upper, they are very lightweight and can be as much as 400g lighter than a pair traditional shoes3. The thin midsole also causes the shoe to be highly flexible, recruiting the intrinsic muscles of the foot when running.

Running barefoot or in minimalist shoes naturally influence the runner’s gait.  This is due to the tendency for runners to run with a mid foot strike rather than a rear-foot strike in a bid to reduce the ground reaction force and its resulting pain1.  This leads to an increase in cadence and accompanying it, a reduced ground contact and flight time4. There has also been found to be a reduced vertical oscillation of the center of mass9. With these observations, barefoot advocates claim that running barefoot or in minimalist footwear can not only prevent injuries but also improve running performance.

With this, numerous studies have sought to determine if these claims are true. In terms of running performance,  there are 2 main quantifiable determinant of distance running success which are VO2max and Running Economy. However, RE has been shown to be a more accurate predictor of distance running success than the former15. RE is determined from the oxygen requirements of the runner at a fixed velocity of submaximal running. When running at given speed, a runner with better RE will cover the same distance in the same amount of time while utilising less oxygen.

Sports Physicians need to be able to give sound and evidence-based advice on the benefits of barefoot running such as improving RE, in view of the increasing popularity of running as a sport of choice. This review article examines the impact of barefoot running and the use of minimalist footwear on running economy.

Results and Methodology

A literature search was done on Pub Med to elicit recent studies on the effects of barefoot running or the use of minimalist shoes on running economy. The results are presented in Table 1.

Most studies1,2,6,7,8,10,11 show a positive influence of barefoot or running with minimalist shoes on running economy, improving it by up to 6.9%10. Some studies3,4,5,14 however did not find any significant differences among the two. Only one study6 recorded a negative impact on RE when running barefooted after accounting for mass difference.

However, it is important to note certain methodologies undertaken by these studies may impact the validity and generalisability of these results.

Mass of Shoes

Shoe mass is an important determinant of running economy and it is well accepted that every extra 100g of shoe mass per foot yielded a 1% increase on oxygen cost17. This is due to the need to accelerate and decelerate an increased shoe mass with and against gravity for every cycle of the running gait, which requires more muscular effort by the runner, inadvertently increasing RE9. Thus, in order to make a fair comparison and to ascertain if the improvement in RE is not just due to the mass difference, it is important it into account when comparing them. Only 3 studies in this literature review did this, including Divert et al. 4 , who used a ultra-thin diving socks with hand-sewn masses to load the sock while maintaining the same coordinates of the center of mass comparable to that when running in the shoes as shown in Figure 1. This innovative idea allowed the runner to run as if barefoot but still took into account the weight of the shoe that it is being compared to.

Familiarisation Period

A familiarisation period is key for participants to get used to running in minimalist shoes if they had no prior experience in barefoot running and it has been recommended that at least 1 month of familiarisation be given for runners so that any adaptations to the new running form can be internalised and possibly translated to better RE28. Thus it is possible that testing runners on their first setting of wearing the shoes without a period of familiarisation will lead to an underestimation of their true RE.

Only 2 of the studies3,10 reviewed had a substantial familiarisation period of at least 4 weeks and they showed significant improvement in RE post-familiarisation. However, in the study by Warne et al.2, which had the longest 8 week familiarisation period, did not see any significant change in RE, similar to other previous research18.

Nevertheless, having a familiarisation period also simulates a real-world situation where a runner trains in a new shoe for a substantial amount of time before taking part in a competition, providing us a better insight to the true effects when newly adopting this form of footwear.

Experience of runners running barefoot

Experience in  barefoot running can again influence the results of the study. This is because if a barefoot running form can indeed lead to better RE, more experienced runners will naturally demonstrate better RE  during the test than non-experienced runners especially in the absence of a familiarisation period. To remove this confounding exposure, all participants should be controlled for experience levels which was not done in 2 studies 1,4.

As such, perhaps the best approach2, 3, 10 would be to recruit runners with no experience and combine this with a familiarisation period to ascertain the true influence of running barefooted or in minimalist shoes on RE.

Speed of test

All of the studies reviewed used a fixed speed for their testing across all participants. This wrongly assumes that the participants are at the same fitness levels and that the pace chosen is a pace within the runners’ submaximal range which may not be the case as the participation selection was not specific in terms of recent race performances or personal bests. Thus, to get around this, runners should be tested at a speed relative to their velocity at VO2Max (vVO2Max). This was only done in one study by Hanson et al. 1 in which they tested the runners’ RE at 70%vVO2Max, to ensure that all runners are running at a similar intensity.

Discussion and Recommendations

Multiple factors affecting RE

There is no doubt that running barefoot or in minimalist shoes affects running mechanics and kinematics8,11. However, the differences in RE described in this literature review has not shown convincingly that it is due to these changes and not due to mass difference. Despite many studies showing a positive influence of barefoot running on RE, most of these did not take into account mass difference. Franz et al.6 found that there was a decrease in RE of 3-4% (significant)when weights are placed on the runner’s ankles when running barefoot as compared to shod running. On the other hand Perl et al. found an improvement of 2.42 – 3.32% (not significant) in RE when using minimalist shoes with weights as compared to traditional shoes.

Interestingly, a meta-analysis found that when using 440g as the demarcation between light and heavy shoes, it was found that light shoes and barefoot was significantly more economical than heavy shoes, but found no difference between light shoes and barefoot9.

This observation is crucial in practice. Cushioning27, stiffness26 and comfort25 of the shoe has all been shown individually to improve running economy and are possible confounding factors in these studies. This may also mean that running barefoot or in minimalist shoes that do not come with as much cushioning, stiffness and possibly comfort for the runner, is disadvantageous as compared to running in a traditional racing shoe which easily come under 440g.

This again highlights the many factors affecting RE and it is crucial that investigators break them down into its components to investigate them individually. For example, it is still debatable if the benefits of an altered running mechanics when running barefoot and minimalist footwear outweighs the benefits of cushioning provided by traditional shoes. To test this, one could have participants run barefoot on a treadmill surface that has the cushioning characteristics similar to those of cushioned running shoes6,21.

Elite Population

Currently, all of the studies were carried out on recreational runners making it difficult to generalise the results for elite distance runners. Unlike recreational runners who are more likely to utilise a rear foot strike when running in traditional shoes and only adopting a mid food strike when running barefoot or in minimalist footwear, elite runners are found to naturally run with a mid-foot strike despite running in traditional shoes22,23. As a 3% improvement in RE is estimated to allow the average runner to increase his speed per unit cost by approximately 0.147ms-1, which for the average runner, can  save him about 9.5 minutes over the course of a marathon1, 7. Even though it will be much less for the elite runner, it can still determine the breaking of a world record.

It is also important to note that  the speeds of the tests also affect the ability to generalise the results to elite runners who typically complete marathons at a speed of 18-20km/h. None of the studies had testing speeds close to this.

Gender

Peculiarly, most of these studies2,3,5,6,8,10 have been carried out on male-only subjects while only 3 studies1,3,7 were done on a mixed-gender population and one11 specifically investigating female runners. Previous research19,20 suggest that females adopt different movements at the hips and knees during running and thus may respond differently to barefoot or minimalist footwear running. This may imply that males and female may respond differently when adapting into barefoot or minimalist footwear running and thus results in all male studies cannot be generalised to the entire running population.

Impact of Barefoot and Minimalist Footwear Running on Shod running

Even before recreational running became popular, coaches of various levels already incorporated some degree of barefoot training into their athletes’ regimes12. This was likely due to the belief that regular running in barefoot provides stimulation of the intrinsic foot muscles which are otherwise more dormant when running in plush cushioning shoes.

An interesting observation in 2 studies showed that training regularly in minimalist footwear seemed to also increase RE when running in traditional shoes, suggesting that coaches of the past knew something that we could not prove until now. In the study by Bellar et al, after 5 week of training with minimalist shoes, there was an increase in RE when running in both the minimalist and traditional shoes. This trend was also observed in the study by Warne et al. where after 4 weeks of familiarisation, not only the RE observed in minimalist footwear running improve by 8.09%, RE when running in traditional shoes also improved by 2.32%. This is suggested to be because these runners ran with a barefoot running style even when in shoes8.

More conclusive research in this area may impact coaching practices in athletics in the future.

Use of RE as an assessment of running performance

Despite how time and distance measures are the reference standard in the assessment of running performance9, none of the studies used it while choosing to measure running economy at submaximal speeds instead. Even though the use of Running Economy to measure running performance is widely accepted, a strong association between RE and running performance has not always been observed29. Perhaps the use of time and distance measures such as time-to-exhaustion and time-trial tests could be considered in future studies.

Conclusion

There appears to be a positive association of barefoot or minimalist footwear running with an improvement of Running Economy. However, wether it is due to a mass difference or truly due to an improvement in running form and mechanics is still debatable. Even so, the results may be confounded by other factors such as the presence of cushioning, stiffness and comfort of the runner. As such, further research is required to confirm this finding. Thus at present, advice on barefoot running for patients need to be based on the individual’s running ability and comfort.

References

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30. Born to Run: A Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen (- Wikipedia, the free encyclopedia) https://en.m.wikipedia.org/wiki/Born_to_Run:_A_Hidden_Tribe,_Superathletes,_and_the_Greatest_Race_the_World_Has_Never_Seen