Running Shoe Anatomy and The Evidence
Systematic Review of the Role of Footwear Construction in Running Biomechanics: Implications for Running-Related Injury and Performance - Sun et al., 2020
Summery of Main Findings:
- Softer and thicker stacks tend to decrease impact parameters likely increasing comfort levels; mixed results lean to hard midsoles improving performance parameters
- Decreased drop increases relative demands on the ankle and calf and increased drop increases relative work of the knee; selection may therefore be largely influenced by injury history
- Minimalist shoes have the same effects on relative joint loading as decreased drop, as well as documented improvements in performance which are contributed to by their light weight and tendency to cause runners to shift to more mid/forefoot striking
- Shoe bending stiffness is significantly and reliably associated with improvements in variables associated with performance and should therefore be one of the main criteria when selecting shoes for performance
-Heel cups are effective for reducing plantar pressure in the heel and other variables associated with heel fat pad irritation and are therefore effective in managing this type of pain
This study is a good read to orient yourself to the various features of a running shoe and the evidence base to support their respective biomechanical effects. This information may contribute to shoe selection choices for performance and/or injury risk reduction. Below the results of each shoe characteristic investigated are interpreted.
Shoe bending stiffness - Shoe bending stiffness was found to reliably and significantly improves performance whereby a stiff shoe absorbs and releases energy that would otherwise have to be attenuated/generated by the joints and muscles in the feet and primarily the first metatarsal phalangeal joint (MTPJ). In fact, the Nike Vaporfly 4% - which has a carbon plate built into the midsole to increase stiffness - has been demonstrated to improve running economy by 4% (hence the name) and has been widely hailed as the shoe that broke running due to its role in the the worlds top 5 marathon times (Hoogkammer et al., 2017). Since decreased MTPJ joint work is consistently measured in a stiff shoe, this feature is also likely to benefit those with MTPJ pathology such as turf-toe, hallux rigidus, or osteoarthritis in this joint. If you’re wondering what shoe stiffness actually is, just try bending the toes of one of your shoes up to touch the heel, and then compare to another shoe.
Midsole Characteristics - Of the 13 studies that looked at midsole hardness, 9 found positive effects on performance characteristics and 4 found no effect. Studies that found positive effect through increased hardness found this to be due to decreased energy lost in the MTPJ and decreased peak rearfoot eversion, a similar mechanism as the previously discussed with shoe bending stiffness. With regards to sole thickness and cushioning, all studies that investigated these characteristic found decreases in impact parameters implying improved comfort. Although not mentioned in the study the trade off of adding material to a shoe is increased weight, and weight is known to be one of the primary influencers of performance (Fuller et al., 2015). Therefore this section supports my heuristic that cushioned shoes with thick stacks are often good for comfort and are especially important for long distance training runs; whereas you may choose lighter variants for speed work and race day when comfort is not as important.
Heel Flare - The single study examining heel flare (25 degrees, 0 degrees and rounded edge) found no kinematic differences between conditions. However there is a reasonable conceptual basis to believe that a lateral posterior heel flare in heel strikers would generally increase pronatory forces (through an increase in the lever arm at initial strike); therefore manipulation of this parameter may still be indicated in certain clinical scenarios.
Heel-Toe Drop - The results here were consistent and what one would expect, the larger the drop the more relative force was attenuated at the knee vs the ankle. Therefore considering medical history when selecting drop is very reasonable; if you have a history of knee pain perhaps go for lower drop and vis versa for ankle/foot pain. Interestingly the one clinical study comparing drop heights found no difference in injury rate, supporting my stance that although lay runners often attribute shoes to injury the nuances discussed in these biomechanical studies often have very small effects on overall injury risk (training load management, recovery, and injury history being much stronger determinants).
Minimalist Shoes - Although there is likely only a small - if any - overall injury-risk difference between shoe types what is evident through the research on minimalist shoes is that that the profile of injuries may change based off footwear characteristics. In the studies review they found minimalist shoes vs “conventional” running shoes caused increased load adaptation of the Achilles and showed other biomechanical indications of increasing relative load on the ankle and decreasing relative load on the knee. If this sounds similar to the heel-toe drop section that’s because a smaller drop is one of the characteristics a minimalist shoe. Another primary characteristic of a minimalist shoe is light weight, which we know to be associated with performance (Fuller et al., 2015). Minimalist shoe use was also associated with a transition to a forefoot strike technique which has also been associated with improved performance. For further discussion of the role of footstrike on performance you can read more in my article running gait biomechanics. Considering these observations its unsurprising that this study also found a positive effect of minimalist shoes on performance.
Heel Cups - This was the first time I encountered scientific investigation of the heel cup and I was pleased to find that the two studies included in this review supported my clinical success using heel cups to decrease pain with plantar heel pain (I usually tell patients to buy a 5-15$ product off Amazon). And indeed, decreases in the the biomechanical stresses associated with plantar heel pain (such as plantar pressure) and self reported pain were reduced. It’s important to note that this benefit applies to irritation of the plantar heel tissues (ie plantar fat pad and possibly the calcaneal periosteum) and not the plantar fascia. Although I have always prescribed gel or rubber heel cups, one study compared rubber to plastic and plastic did a better job of preserving heel pad thickness, something that I will have to experiment with in my clinical practice moving forward.
Although this article exclusively focused on the biomechanical consequences of various shoe characteristics it is important to keep in mind the overall role of running shoes in performance and injury-risk reduction in order to prioritize our resources accordingly. Currently there is not sufficient evidence to support the idea that traditional shoe prescription methods can reduce injury, and the varience of performance effects are also quite small. Therefore, for the vast majority of un-injured runners, comfort and cost should be the primary considerations with shoe selection. For further discussion and substantiation of these points please read my running shoe selection bible.
Citations
Hoogkamer, W., Kipp, S., Frank, J. H., Farina, E. M., Luo, G., & Kram, R. (2017). A Comparison of the Energetic Cost of Running in Marathon Racing Shoes. Sports Medicine, 48(4), 1009–1019. doi: 10.1007/s40279-017-0811-2
Fuller, J. T., Bellenger, C. R., Thewlis, D., Tsiros, M. D., & Buckley, J. D. (2014). The Effect of Footwear on Running Performance and Running Economy in Distance Runners. Sports Medicine, 45(3), 411–422. doi: 10.1007/s40279-014-0283-6
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