When comparing the biomechanics of different foot strike patterns, these reseachers set out to answer the somewhat ambiguous question of whether the biomechanical differences are due to changes in footwear, foot strike, or a combination of the two:
Rearfoot and Midfoot/Forefoot Impacts in Habitually Shod Runners
Boyer, Elizabeth R.; Rooney, Brandon D.; Derrick, Timothy R.
Medicine & Science in Sports & Exercise: 2 December 2013
Purpose: Shear loading rates have not been investigated in runners with a mid/forefoot strike (FFS) versus rearfoot strike (RFS). The purpose of this study was to compare three-dimensional ground reaction forces (GRF) and loading rates (LR) during impact in habitual RFS (hRF) and habitual FFS (hFF) strikers.
Methods: Thirty competitive runners performed 10 overground running trials with both foot strike styles. Peak three-dimensional and resultant GRFs and instantaneous LRs during impact were compared.
Results: Vertical LR significantly decreased for hRF using a FFS (RFS: 148+/-36, FFS: 98+/-31 BW/s) but was similar for hFF running with either foot strike (FFS: 136+/-35, RFS: 135+/-28 BW/s). Posterior impact forces were present during FFS but not RFS, and posterior LR was significantly greater for both groups during FFS (-58+/-17 versus -19+/-6 BW/s). Medial impact forces were also present during FFS but not RFS, and medial LR was significantly larger for both groups during FFS (-21+/-7 versus -6+/-6 BW/s). Interestingly, hFF had greater impact peaks and LRs in all directions compared to hRF during FFS. This may be explained by hFF using a smaller strike index (hFF: 62+/-9%, hRF: 67+/-9%; P=0.02), which was significantly inversely related to vertical LR and impact peak.
Conclusion: Peak resultant and vertical LRs are not ubiquitously lower when using a shod FFS versus RFS despite an absence of resultant and vertical impact peaks. Furthermore, there were impact peaks in the posterior and medial directions, leading also to greater LRs in these directions during FFS. Therefore, transitioning from RFS to FFS in traditional running shoes may not offer long-term protection against impact-related running injuries since hFF running with a FFS demonstrated many GRFs and LRs similar to or greater than RFS.
What they did was take 15 habitual rearfoot strikers and 15 habitual forefoot/midfoot strikers (and the other characteristics of two groups were comparable). In the study they all used same brand and model of shoe (unusually it was not-specified as to what it was!). For first set of trials they ran ‘naturally’ in their habitual foot strike pattern and in the second condition, given instructions on how to foot strike opposite to first condition. A whole range of 3d ground reaction forces and loading rate parameters were then compared between the groups and the foot strike conditions.
The results were somewhat “technical” and are somewhat complicated for me to present here in a brief enough fashion to still make them understandable without just regurgitating the content of the whole paper, so those interested can just go and read it rather than me get into all sorts of copyright issues. They are summarized in the above abstract.
I will however, pull out one paragraph from the discussion by the authors as it does nicely summarize the implications:
Shod rearfoot strikers wishing to decrease their resultant or vertical loading rates may consider switching to a forefoot strike since these variables decreased in our study as well as others and elevated vertical LR is associated with a history of stress fracture. These decreases in loading rates, however, may be temporary as habitual forefoot striking running with a forefoot strike had higher loading rates than rearfoot strike. Runners must also be aware of the increased shear ground reaction forces and loading rates associated with a forefoot strike, which may be important from an injury perspective since bones cannot withstand shear forces as well. If choosing to convert from a rearfoot strike to forefoot strike for long distance running (especially if also changing to minimalist footwear or barefoot), it should be done progressively and with caution to avoid injuries. As we have discussed, the metatarsals may be more susceptible to injury if converting to a rearfoot strike since the mid/forefoot regions are loaded continuously throughout stance and experience greater shear forces at higher loading rates based on our findings. However, shod runners who are plagued by knee injuries or anterior compartment syndrome may benefit by converting. Therefore, whether converting is appropriate may depend on which joint/segment is currently experiencing excessive loading.
In other words, as I keep saying:
1. Different running techniques load different tissues differently
2. You can not offload one tissue without increasing the load in another¹
This is a nice contribution to our understanding of the differences in foot strike patterns. Nothing jumps out at me regarding any shortcomings of the study, except perhaps that foot strike alterations were acute and not habituated. It is however, also based on the premise that impact related parameters are important and as far as I am concerned the evidence supporting that is far from compelling.
I do have one minuscule concern, that I will consider a fatal flaw, until I know the answer. What was the make and model of shoe they used? Most other studies like this name the make and model and I just curious! Was it a minimalist shoe, a lightweight shoe, a stability shoe or a cushioned shoe?
As always, I go where the evidence takes me, until convinced otherwise.
Boyer, Elizabeth R.; Rooney, Brandon D.; Derrick, Timothy R (2013). Rearfoot and Midfoot/Forefoot Impacts in Habitually Shod Runners Medicine & Science in Sports & Exercise DOI: 10.1249/MSS.0000000000000234