A Trend Without Sole: Barefoot Running Explained
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.