Running: Foot Strike and Shoe Type

    As a long time runner and someone who has dealt with various injuries over the years, I thought it would be interesting to explore some of the physics behind the way my feet strike the ground and the pros and cons of various shoe types. The human body in its entirety is more complicated than I feel capable of discussing so I will mainly be considering the foot itself as it interacts with the ground during a runner's stride. In terms of basic facts or observations, people tend to wear clunkier, more cushioned shoes when they are training and putting in a lot of miles, while they tend to opt for sleeker, more form-fitting, and less cushioned shoes when racing. The added mass of a running shoe worn for training requires more work from the person as they are moving along in their stride. Also, a training shoe has more protective benefits because the padding increases the distance and thus time over which the runner's foot decelerates upon hitting the ground. When racing, people often opt for spikes or flats which have considerably less mass and almost no cushioning. These specialty shoes are typically reserved only for racing because the pounding stride is more punishing on the feet. The reasoning behind this is very similar to our discussion of why bike helmets are important. Since F=ma, a faster acceleration (due to less cushioning) results in a greater force being experienced.

    Another thing to consider is the area of the bottom of the shoes. Spikes/flats have much smaller soles which means that there is a pressure differential as compared to training shoes when thinking about P=F/A. All of this is making spikes and flats sound bad, but actually they are beneficial in desired contexts! Increasing speed involves increasing impact on the foot.

    Going back to the P=F/A equation, it is also interesting to think about the foot strike pattern itself. People who hit the ground mainly with their heel (like I do) are putting a lot of force over a relatively small area which results in the experience of a ton of pressure. Those with more of a mid-foot strike have slightly more area to contend with which can be better for avoiding injuries. There is also the mechanics of the feet and legs which include the plantar fascia, achilles tendon, and calf muscle which are involved in some shock absorption. Running shoes commonly have a thicker heal and slope downwards toward the toe-box, which favors heel strikers. It is worth noting that the added protection of the cushioning is not necessarily healthier for the feet because the added comfort is more forgiving of poor form and it limits proprioception and sensory input signaling in the feet.

    Studies have been done to actually compare different stride styles and to look at the impacts of running both with and without shoes. I have done limited barefoot running myself but there are people out there who swear by it! Heel striking is less common among so-called minimalist runners. The figure on the left shows the force experienced over time for a barefoot heel-striking runner. The figure on the right compares the impact force of a rear-foot-striking barefoot runner to a rear-foot-striking runner wearing shoes and then to a forefoot-striking runner who is barefoot.

    Runners can be a little crazy when it comes to optimizing their gait and footwear. I'm sure there is a lot more to be explored about the physics of running shoes and strides. Given that running has been quite central to human survival over evolutionary time and now people turn to it for exercise, fun, and competition, I think it's pretty cool to look into some of the underlying science!

Works Cited:

Shoes Under Pressure - Lesson. (2019, July 01). Retrieved December 04, 2020, from https://www.teachengineering.org/lessons/view/cub_convshoes_lesson01

Hutchinson, A. (2019, February 26). There's New Research on the Science of Barefoot Running. Retrieved December 04, 2020, from https://www.outsideonline.com/2390686/barefoot-running-biomechanics-study

Lieberman, D. E., Venkadesan, M., Werbel, W. A., et al (2010). Foot strike patterns and collision forces in habitually barefoot versus habitually shod runners. Nature. doi:10.1038/nature08723