I have always really struggled to understand why there is so much fuss about reducing impact forces to treat and prevent overuse injuries in runners. Everywhere you go in the crankosphere blogosphere heel impacts are the cause of all evil and have to be eliminated at all costs. Even in the scientific literature, for example, the very first sentence of the Lieberman et al (2010) article on barefoot running that made the cover of Nature was:
Running can be most injurious at the moment the foot collides with the ground.
You would have thought that such an emphatic statement, especially in a scientific publication would have a citation to support it. It doesn’t.
So, are impact forces, especially at heel contact a risk factor for injury or are those who claim it is, just engaging in the wishful thinking logical fallacy? In other words, they just state it as they wish it was true. Just what does the evidence say on impacts and injury in runners:
- Nigg (1997) summarized his and other research and reported no differences in injury between those with higher or lower impact peaks and found a correlation between higher impact loads and less injuries; concluding that: “Impact forces have been associated with the development of musculoskeletal injuries. However, results of epidemiologic studies that assess the association between impact loading and the development of acute or chronic injuries do not support this notion“. In the initial chapter of his recent book (2010), Biomechanics of Sports Shoes, Nigg concluded: “Currently, there is no conclusive evidence that impact forces during heel-toe running are responsible for the development of running related-injuries“
- van Mechelen (1992) reported no differences in injury rates between those that run on hard or soft surfaces (where presumably the impact loads would have been different)
- van Gent’s et al (2007) systematic review of published studies on the risk factors for running injuries did not identify impact loads as a risk factor.
- Bredeweg et al (2013) looked at the asymmetry in impact loads between the left and right side of runners and found no correlation between higher impacts and the side of the injury.
- The systematic review of the literature and meta-analysis by Zadpoor and Nikooyan (2011) on loading rates and stress fractures found that: “The currently available data does not support the hypothesis that there is a significant difference between the ground reaction force of subjects experiencing lower-limb stress fracture and control groups. Instead, the vertical loading rate was found to be significantly different between the two groups.” They reported that while impacts were not a factor, the loading rate was a factor. This was only for tibial stress fractures that make up ~4-5% of all running injuries¹.
- In contrast, there was an abstract presented at the ASB meeting in 2010 by Irene Davis in which they reported that impact factors were associated with an increased risk for injury. There is no more information available on this study apart from the abstract.
- Bredeweg et al (2012) in their prospective study did find that male injured runners had higher loading rates than noninjured male runners. They found no differences in the female cohort.
So I am unconvinced that impact loads and loading rates are even a problem or as big a problem as all the rhetoric and propaganda make them out to be. The evidence either way is not compelling. They are a problem for tibial stress fractures¹, that make up a small number of running injuries. It is possible to spin or cherry pick the research to try and make whatever point you are trying to make or story you are trying to tell. I am happy to be convinced otherwise, but the evidence is telling me that heel striking and high impacts are not the big evil that they often get painted to be. Even if they were and you change the running form or technique to reduce those impacts, to do so, you only increase the load in other tissues, exposing those tissues to increased risk for injury¹. Its six of one and half a dozen of the other.
As always, I go where the evidence takes me until convinced otherwise, and the evidence tells me that impact forces are either not really a big a problem as the rhetoric and propaganda says they are; or if they are, the evidence supporting that is far from compelling.
POSTSCRIPT: I was reminded of something via Twitter on this topic by@SethONeil: “Only time I worry about impacts is heel fat pad syndromes” (tweet). He is right. Despite all the rhetoric and propaganda that impacts are supposed to be so evil, clinicians when treating running injuries never take them into account! There is obviously a reason for that! Even if you look at all the risk factor studies for specific injuries, such as the most common running injuries of patellofemoral pain syndrome (runners knee) or medal tibial stress syndrome (shin splints), impacts are never mentioned or even found as being risk factors.
1. Even though the evidence shows that higher impact loading rates at heel impact are a risk factor for tibial stress fractures; forefoot striking probably increases the risk for metatarsal stress fractures. We keep being told that this is not a problem if the bone (metatarsals) are given time to progressively adapt to the loads. What I don’t understand is why the bone (tibia) can not be progressively adapted to the load of heel striking and adapt?