Variability from step to step is considered a good thing as this means the same tissues are not necessarily loaded to the same extent with each step. There is evidence that shows that injured runners display reduced variability compared to healthy runners. Joe Hamill and colleagues originally looked at this concept in detail. Despite this seminal publication in 1999 the concept does not come up often and considered as a risk factor for overuse injury in runners. The latest publication in Footwear Science by Frank et al develops the concept further. They hypothesized that running in minimal shoes after running in traditional running shoes will result in reduced variability due to the lack of familiarization using this type of product (and hence by implication, that this could be a risk factor for injury during the transition to minimalist footwear). Here is the abstract:
Lower limb kinematic variability associated with minimal footwear during running
Nicholas S. Frank, Jack P. Callaghan & Stephen D. Prentice
Footwear Science (in press)
Purpose: This study investigated lower limb variability when trained runners wore a minimal shoe for the first time. It was hypothesised that initial lower limb variability would be decreased in the minimal shoe condition due to lack of familiarity. It was also hypothesised that variability would increase over time as runners become more familiar with the condition.
Methods: Testing included three 10 minute treadmill running trials conducted in runner’s own running shoes, a pair of minimal shoes followed by runner’s own shoes again. The shoe order was selected so as to establish a baseline value of variability in a runner’s most familiar shoes followed by a perturbation which was the inclusion of minimal shoes. Continuous Relative Phase (CRP) relationships and kinematic values at heel strike were determined which allowed lower limb variability to be quantified.
Results: Kinematic variability values were not statistically different between runner’s own shoes and minimal shoes. CRP relationships did not differ between minimal shoes and runner’s own shoes or over time.
Conclusions: Trained runners did not change lower limb variability while wearing minimal shoes for the first time. Lack of familiarity does not appear to affect lower limb variability. The footwear included in this research study had similar cushioning properties to traditional footwear but with a different construction which may relate to similar values found between conditions. Investigating how runners of different abilities transition to minimal footwear should be focused upon to reduce risk of injury.
The authors found that variability did not differ between running in their own shoes that they were used to and the minimal shoes that were provided with that they were not used to.
This was done on a treadmill rather than overground, but here is the authors justification for that:
One limitation of the current study was the fact that runners conducted testing on a treadmill. There have been measured differences between overground and treadmill variability (Dingwell et al. 2001). With these known differences in mind, the main focus of this study was to compare each runner to themselves across the different shoe conditions. The treadmill was present in all conditions and was used to collect successive kinematic data over a period of time. For these reasons it was believed that the effect of the treadmill on kinematic variability acted as a constant source of error across all conditions. Surface stiffness has also been shown to have an effect on runner’s kinematics (Hardin et al. 2004). To try and reduce treadmill bed compliance, the treadmill structure was made stiffer by reinforcing the midway point of the treadmill bed.
I only raise this, as I have no problem with it, but because a number of studies that I have commented on in previous articles use a treadmill rather than overground and it often comes up in the comments section for discussion (especially from those who don’t like the results of a study!).
Despite the results of the study not finding decreased variability when acutely changing to minimalist shoes, the concept of kinematic variability as a risk for an overuse injury needs to be looked at further. How much reduced variability is a risk factor? Is there a threshold of variability that is protective? Most importantly, if it is a factor, what control can we have over it to retrain to prevent injury?
As always, I go where the evidence takes me until convinced otherwise.
Frank, N., Callaghan, J., & Prentice, S. (2013). Lower limb kinematic variability associated with minimal footwear during running Footwear Science, 1-7 DOI: 10.1080/19424280.2013.797505