Running Asymmetry, Loading Rate and Injury Risk

Asymmetry in the gait is common and is assumed to not necessarily be a good thing for a unidirectional activity such as running, especially if the asymmetry is in impact loads. Various intervention strategies are often used to make the gait more symmetrical, if the asymmetry is assumed to contribute to any injury or gait inefficiency that is present. There is always a few exceptions to these assumptions by appealing to the occasional elite runner who does run with an asymmetrical gait. The actual evidence that underpins this is limited. Now in pre-publication is a paper that looks at kinetic asymmetry in injured and non-injured runners that adds to that knowledge base: Here is the abstract:

Differences in kinetic asymmetry between injured and noninjured novice runners: A prospective cohort study
S.W. Bredeweg, I. Buist, B. Kluitenberg
Gait & Posture; Article in Press
The purpose of this prospective study was to describe natural levels of asymmetry in running, compare levels of asymmetry between injured and noninjured novice runners and compare kinetic variables between the injured and noninjured lower limb within the novice runners with an injury.
At baseline vertical ground reaction forces and symmetry angles (SA) were assessed with an instrumented treadmill equipped with three force measuring transducers. Female participants ran at 8 and 9kmh−1 and male runners ran at 9 and 10kmh−1. Participants were novice female and male recreational runners and were followed during a 9-week running program.
Two hundred and ten novice runners enrolled this study, 133 (63.3%) female and 77 (36.7%) male runners. Thirty-four runners reported an RRI. At baseline SA values varied widely for all spatio-temporal and kinetic variables. The inter-individual differences in SA were also high. No significant differences in SA were found between female and male runners running at 9kmh−1. In injured runners the SA of the impact peak was significantly lower compared to noninjured runners.
Natural levels of asymmetry in running were high. The SA of impact peak in injured runners was lower compared to noninjured runners and no differences were seen between the injured and noninjured lower limbs.

The key findings of this study were:

  • The main kinetic variable that was studied was ground reaction force (and not the other kinetic variables of joint moments etc); with the research question being was the impact greater on the injured side and the amount of asymmetry in it.
  • These were novice runners, but this was a prospective study (good ✔ ) rather than the previously weaker retrospective studies that looked at this.
  • A key finding was a higher ‘symmetry angle’ for contact time and a lower ‘symmetry angle’ for impact peak in the injured runners. Stated another way, those with an injury had less asymmetry in the impact peak. This is inconsistent with the assumption that asymmetry is a risk factor for injury.
  • Previous retrospective studies have reported that there is a higher loading rate on the injured side, but in those types of study designs it was not known if the higher loading rate was the cause or the result of the injury. In the above prospective study they report no differences in the loading rate between the injured and uninjured leg. As the authors note, this suggests that the previous observed difference in loading rates may be the result of the injury and not the cause.

Its probably timely to again re-post the below from this article, as the above study further questions the role of impact and loading rates in injury:

I really struggle to understand why there is so much fuss about reducing impact loads and loading rates. Everywhere you go in the crankosphere blogosphere heel impacts are the cause of all evil and have to be eliminated at all costs. As I always go where the evidence takes me, the evidence to me just does not support the strength of that contention:

  • 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“.
  • 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 impacts were not a factor, but the loading rate was. This was only for tibial stress fractures that make up ~4-5% of all running injuries.
  • 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.

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 fractures, 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 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.

As always, I go where the evidence takes me.

Bredeweg, S., Buist, I., & Kluitenberg, B. (2013). Differences in kinetic asymmetry between injured and noninjured novice runners: A prospective cohort study Gait & Posture DOI: 10.1016/j.gaitpost.2013.04.014

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