The motivation for Dr. Lieberman’s thesis (1) on (barefoot) running is the fact that 30%–70% of all runners have a running-related stress injury each year and that, despite great efforts from biomechanics researchers over the past 40 yr, this injury rate has remained constant.
A secondary, but in the context equally important, observation is that barefoot runners, on average, run differently than shod runners (they tend to be more midfoot-forefoot strikers rather than heel strikers, and they tend to take shorter but quicker steps when running at sub-maximal speeds). Because barefoot running has evolved over thousands of years, whereas running in well-cushioned shoes has been practiced only for the past 40 yr, the basic hypothesis is that a “barefoot” running style evolved such that running injuries are limited, whereas running in well-cushioned shoes may cause adaptations that predispose runners to running injuries, or in Dr. Lieberman’s words: “How one runs probably is more important than what is on one’s feet, but what is on one’s feet may affect how one runs.”
Dr. Lieberman’s review is fascinating and covers many basic concepts, only a few of which can be addressed in a short commentary. Therefore, I will focus on the idea of “forefoot versus heel strike runners,” the notion of “stress-related running injuries,” and the question of “evolution in running.”
FOREFOOT VERSUS HEEL STRIKE RUNNERS
People have very different running styles when it comes to foot placement during the stance phase of running. Although most people are what Dr. Lieberman refers to as heel strike runners, some prefer to land with the midfoot or forefoot first. Heel strike runners often become midfoot-forefoot strike runners when they run barefoot or increase their speed, whereas “natural” midfoot and forefoot strikers (when running shod) rarely will change to a heel strike running gait independent of speed or shoe conditions. Therefore, it seems that midfoot-forefoot running is a more “ingrained” running style than heel strike running. The reasons for this are unclear, as are the reasons for the preference of foot placement among runners.
STRESS-RELATED RUNNING INJURIES
Many runners incur stress-related running injuries. Stress-related injuries typically are caused by cyclic mechanical loading applied in a stereotypical manner. Dr. Lieberman points out that barefoot running, compared with shod running, is more variable, and maybe increased variability could be a mechanism for decreasing stress-related running injuries. More generally, could we prevent stress-related injuries by running less stereotypically and more variably? For example, instead of running the same 8-km loop five times a week, maybe running five different loops, some hilly, some flat, some short and fast, others long and slow, with different surfaces ranging from flat grass to uneven forest trails, might be a recipe for limiting stress-related injuries, as internal structures would be loaded differently when we change speed, slope, and running surface. Another issue rarely considered is the idea that runners may get injured not because of how or how much they run but because of the strength of their musculoskeletal tissues. Bone, ligament, tendon, and cartilage tissues vary substantially across people, and these differences might be more important in predicting running injuries than running form or training mileage.
EVOLUTION IN RUNNING
Dr. Lieberman studied the evolution of running from a medical perspective. Avoiding stress-related injuries would have been paramount in peoples of the hunter-gatherer era, as injury could have affected severely their ability to provide for their dependents. However, maximizing average speed or minimizing the cost of transport equally may have been powerful determinants in the evolution of running. The world’s fastest distance runners mostly are natural midfoot and forefoot strikers. Could this point to an advantage of forefoot versus heel strike running, an improved economy, or the ability to run faster for longer periods? Laboratory testing does not support an economical advantage of midfoot-forefoot strikers, but these studies have been performed on flat surfaces and treadmills, and one wonders if the same results would be obtained on uneven, rugged surfaces as encountered by runners for most of the past 10,000 yr.
Dr. Lieberman raises a series of intriguing questions that have no immediate or easy answers. However, these questions make us think and might provoke studies identifying why so many people get injured while running. Maybe an alternative approach, not seriously considered yet, would be to analyze runners who have run for decades without ever becoming injured, and rather than studying why runners get injured, study why some of them never do.
Human Performance Laboratory
Faculty of Kinesiology
University of Calgary
Calgary, Alberta, Canada