With the increasing popularity of minimalist footwear and barefoot running, it is imperative that clinicians understand running mechanics and the potential implications of these footwear options.
Approximately 19 million Americans run at least 100 days per year.1 Although running is excellent cardiovascular exercise, a published systematic review reported that the incidence of lower-extremity injuries among runners ranged from 19% to 79%.2
In recent years, there has been a trend of runners switching from traditional running shoes to barefoot running or to minimalist shoes. This switch is likely driven by the possibility of increased performance3-5 and more important, the possibility of reducing injury.6-8 Up to 22% of runners have tried running barefoot, and 30% have reported using minimalist shoes.9
Understanding foot-strike patterns
Rather than being a function of what is worn (or not worn) on the foot, the potential benefits of adopting a barefoot or alternative running style (e.g. Chi running, pose running, minimalist footwear)10 may have more to do with what part of the foot impacts the ground first. Running is often seen as a rapid series of footfalls generating high forces and impacts several hundred times per mile. Impact-loading includes the vertical impact peak (indicated in Figure 1).
Within this curve, claims of decreased injury have focused on a reduction of impact-loading, which consists of decreasing the loads and rate of loading shortly after foot strike. Impact-loading includes the vertical impact peak—which occurs shortly after foot strike before the center of mass is over the foot—and the rate of loading from foot strike to that peak. Research suggests that runners with lower impact-loading have reduced injury risk.6,7,11,12
The way the foot strikes the ground can be categorized into three distinct foot-strike patterns:
- A rearfoot strike (RFS) pattern is a foot strike in which the heel, or rear third of the sole, is the first part of the foot to come in contact with the ground
- A midfoot strike (MFS) pattern is a foot strike in which the middle third of the sole makes contact with the ground first
- A forefoot strike (FFS) pattern is a foot strike in which the third of the sole makes contact with the ground first.13
Different foot-strike patterns affect the profile of the force curve generated when the foot hits the ground, and this in turn affects the transmission of energy through the musculoskeletal system.
As observed in Figure 1, the typical RFS is characterized by a pronounced vertical impact peak of the vertical ground reaction force, whereas the MFS and FFS patterns do not have a pronounced vertical impact peak.
The vertical impact peak, which occurs within the first 50 milliseconds of stance, results from the abrupt collision of the heel with the running surface. This collision causes part of the body’s mass to decelerate rapidly (i.e., the leg), while the rest of the body decelerates more gradually. Therefore, runners who land heel-first during a running stride (RFS) generate higher peak impact forces that are transmitted more rapidly throughout the musculoskeletal system compared with FFS runners.8,14,15
Because of this reduced impact-loading in MFS/FFS runners, it is hypothesized that runners who adopt a MFS or FFS pattern associated with an alternative running style will ultimately decrease impact loading.14,15
There are minimal data to describe the distributions of foot-strike patterns among recreational runners, as all of the data are from half-marathon or full-marathon races. Among marathoners, 75% to 89% ran with an RFS pattern, 3% to 24% ran with an MFS pattern, and only 1% to 2% ran with an FFS pattern.16,17
Certainly, a potential bias exists in the distribution numbers based on the study participants’ ability to train for and compete in a half-marathon or full marathon: Despite the increasing popularity of recreational road racing, only 2.2 million runners (including those running multiple races and international runners) completed half-marathons and/or full marathons in the United States during 2011.18,19 Since only a small percentage of American runners (<12%) run half- or full marathons, these reported distributions may not translate to all recreational runners.
Injury implications of foot-strike patterns
RFS with higher impact-loading appears to carry a higher risk of injury compared with RFS with lower impacts. Researchers have reported that RFS runners with retrospective tibial stress fractures had higher impact loads than did uninjured RFS runners.11 Additionally, a two-year prospective study found that RFS runners who sustained an injury of any type had higher impact-loading than did uninjured RFS runners.12
These studies suggest that even within RFS (and without changing foot-strike pattern), decreased impact-loading may reduce injury risk. As impact-loading is reduced in MFS and FFS runners, it has been theorized that running injuries can be mediated by changing one’s foot-strike pattern from RFS to MFS or FFS. To date, this conclusion has not been strongly supported by a large prospective study.
In contrast to RFS runners, experienced barefoot runners maintain an FFS pattern with a plantar-flexed ankle.15 Landing on the forefoot with a plantar-flexed ankle results in attenuation of peak impact forces when compared with running shod and utilizing an RFS pattern.15 A 2010 study found reduced impact-loading in barefoot runners compared with shod runners.14
Again, this reduction in peak impact force is not necessarily a function of running without shoes; it is more likely associated with which portion of the foot strikes the ground first. Although the authors of the 2010 study suggested that reduction in forces associated with an FFS pattern may reduce injury in runners, they acknowledged that there had only been anecdotal reports of injury reduction in barefoot runners.14
Only recently, a small retrospective analysis of 52 collegiate cross-country runners indicated that runners who utilized an RFS pattern experienced twice as many stress injuries as did those who utilized an FFS pattern.8
Altering foot-strike patterns
There is currently no justification for changing the foot-strike pattern in an asymptomatic runner. However, emerging evidence shows that changes to foot-strike patterns may be a useful tool for mitigating the effects of injuries related to running.
In a case series of 10 individuals who suffered from chronic exertional compartment syndrome, all were able to change their running style from an RFS to an FFS pattern over a six-week period of supervised instruction.6 The changed running style resulted in decreased intracompartmental pressures as measured by an orthopedic surgeon using an intracompartmental pressure monitor. The subjects continued to run and train with a reduction in pain one year after the intervention began.6
In another case series, three runners underwent eight supervised sessions of gait-retraining over a two-week period to adopt a non-RFS pattern, which reduced their patellofemoral pain. The new patterns were maintained for at least three months with a reduction in pain.7
Researchers have utilized real-time audio and/or visual feedback to decrease impact-loading in runners.20 Some individuals changed from an RFS pattern to an FFS pattern, and others decreased loads while maintaining an RFS pattern. Before recommending altering foot-strike patterns to mitigate injury, more research in larger cohorts of runners is necessary.
Other than the previously discussed studies, there is a paucity of literature supporting injury reduction in barefoot or alternative running styles. Other reports associating resolution of injury with alterations in foot-strike pattern have been anecdotal. Review articles have described the need for larger prospective studies to support more definitively the hypothesis that non-RFS runners are experiencing fewer injuries.21
No known large-scale studies systematically support either hypothesis. Larger-scale prospective studies are ultimately needed to determine the strength of relationship between foot-strike patterns and injury. Challengingly, these studies will need to control for running mileage, injury history, and other factors that may confound the results.
Minimalist footwear and barefoot running
Prior to the 1970s, endurance runners wore narrow, thin-soled shoes that would now be considered track flats. Since then, traditional running shoes have evolved to include a wide and thickly cushioned sole likely to reduce pain of repeated heel strikes in RFS.