The Science of Injury Invariability


by Dr. Reed Ferber, Ph.D. CAT(C)

In my last article, I described how rotating multiple shoes and engaging in cross-training activities are both strategies that reduce your risk of injury, based on increasing the variability in the loads being applied to your body. Here, we’ll continue this discussion and give you some strategies to help reduce your injury risk based on biomechanical research.

My research group first introduced the concept of stride-to-stride variability as an indicator of a running injury nearly 10 years ago. We showed that when a runner has reduced gluteus medius muscle strength (the muscles on the side of your hip), your knee is not properly controlled when you run. Subsequently, the weakness leads to increased variability and an unpredictable running pattern. For example, the knee might slightly collapse outwards during one footfall and inwards for the next. However, once those muscles get stronger, a more predictable pattern and reduction in stride-to-stride variability occurs, so your body knows what to expect during the next footfall.

We’ve conducted a lot of research to confirm these findings. We repeated the above study with a larger group of runners and with runners who have different injuries. In one study, we fatigued a key ankle stabilizing muscle: the tibialis posterior. This muscle runs on the inside of your ankle bone and fans out under your foot to support the arch. Its job is to control arch deformation and foot pronation. When this muscle became fatigued, the otherwise predictable motions between the heel bone, the arch and the lower leg become very unpredictable. In another study, we injected lidocaine into the superior gluteal nerve to effectively “knock out” the gluteus medius muscle. Our results were similar to what happens when the muscle is weak—the variability in the gait pattern increases when the muscle cannot produce adequate force.

Other researchers have investigated variability in an individual’s training program. Some excellent research out of Luxembourg followed 264 recreational runners during a 22-week learn-to-run program. About half of the runners completed 91% of their mileage in the same pair of shoes and only went through 1.3 pairs of shoes in 22 weeks. Sound like you? The other half of the group completed 58% of their mileage in a “main shoe” but rotated among an average of 3.6 pairs of shoes.

Interestingly, the multi-shoe runners had a 39% lower risk of developing a running injury as compared to the single-shoe runners.

There are plenty of reasons why running in multiple pairs of shoes is beneficial. Different shoes distribute impact forces differently, and varying the midsole height and firmness properties of your footwear creates slight changes in your running gait.

What’s also interesting is that the same researchers also found that increasing the average distance of your runs, but reducing the number of weekly runs by participating in other recreational sports, reduced the likelihood of injury by 15 to 20%. They concluded that the “variation of the load applied to the musculoskeletal system” is injury protective.

This is a complex topic, so here is abasic summary of implications for injured and healthy runners. When you’re injured, muscles may be weak and/or injured and your stride-to-stride variability will subsequently be high, resulting in an unpredictable running gait pattern. As such, you want to focus on lowering your variability in all facets of your training. I recommend minimizing variability by running in one pair of shoes, running along a consistent route, keeping your mileage low and pain-free, and focusing on your rehabilitation. You should, of course, be seeking professional help from a sport physiotherapist or a clinician who is a running specialist in order to strengthen the relevant muscles and reduce your variability.

However, when you’re healthy, your stride-to-stride variability is low and you have a predictable running biomechanical gait pattern. In this case, you want to focus on increasing the variability of your training program to reduce your chances of injury. If you’ve ever heard me give a talk, I always say: “Swim, bike, run, lift weights—and buy at least two pair of shoes and switch between them.” I always follow this by clearly stating that I have no financial interest, shares, salary or stocks in the Running Room. Based on the research, I suppose I should change the take-home message to “buy at least 3.6 pairs of shoes.” Good luck explaining that to the store employee on your next shopping trip!



Dr. Reed Ferber is the director of the Running Injury Clinic, a world leader in running-related research and 3D gait analysis technology. For more information, visit

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