Do Softer Running Shoes Cause More Injuries?

Vaporfly from Nike

Let's say you have been chosen as a volunteer in a running shoe study. The researcher assigns you a pair of running shoes with very thick midsoles to run with. Your expectations may be affected by how you run compared to being assigned an ultra thin and ultra light shoe.

It will also affect how you perceive aches and pain received from running during the study. It will probably not be possible to tell whether it was the softer midsoles, heel drop, lightness or other differences that gave you grief.

To avoid this, researchers in this study collaborated with Decathlon to make custom running shoes that cannot be visually distinguished and only has one single technical detail different for each experiment. Both researcher and subject were not aware (double blind) which type of shoe was used.

I have written before that runners, new to wearing shoes with thicker midsoles, tend to land more heavily and with greater force. This is even after running with the shoe for more than 6 weeks. This has become what is known as the shoe cushioning paradox, where more cushioning leads to runners landing more heavily, with a lesser chance of getting injured.

Decathlon made 2 prototypes, each with an inch thick of midsole. Half the soles had soft cushioning while the other half had firmer midsoles, which were randomly assigned to 848 runners. These runners completed a treadmill test to assess stride characteristics and were then monitored for 6 months.

Stride analyses showed that runners land more heavily when wearing the shoes with the softer midsoles.This is similar to other studies, which may cause more injuries (not less).

However, the runners wearing the shoes with firmer midsoles were 52 percent more likely to get injured during the follow up period (perhaps to suggest injury protection offered by the softer cushioning). This is similar to the shoe cushioning paradox. How is this so?

What the researchers found was the timing of impact matters. When your foot strikes the ground, there are 2 distinct impacts. The first impact occurs when your lower leg slows down suddenly after striking the ground. The second impact which occurs a few milliseconds later is the larger force caused by the rest of your body. The first impact is the one researchers suspect is linked to injuries. 

This force is greater in runners with the softer midsoles, but the softer midsoles slow down that first jolt and spreads it out over a longer period of time causing it to overlap with the second impact. This combined impact makes the total force appear greater thus giving the impression that softer midsoles cause runners to land harder.

After separating the first and second impacts into 2 differerent values, the authors found that the first impact (the one linked to injuries) was actually smaller with the softer midsoles.

Does this mean we should now run with shoes with the thickest and softest midsoles? Afterall, they do lighten the load on your joints and reduce injury risk. At least that's what the study by Malisoux et al (2022) suggest.

Do note that these Decathlon made shoes for the study are different from the ones available in stores. Morever, with each different brand, they have different technologies and use different materials. 

Just like the newest generation of Vaporfly (with its super thick soles and carbon plate), it still remains questionable whether it reduces injury risk, with the carbon fiber plate(s) still untested.

The midsole thickness definitely makes your foot more unstable especially when going around sharp corners or while making a u-turn in an out and back route. Only time and another specifically designed study can tell us more.

Even the authors concede that very little is known about the complex links between running shoes and injuries. I do like their suggestions, that is to stick with whatever you're running with, if you are happy with them. If you do switch shoes, consider carefully why you're switching, do try several designs  and take you time making the switch. Also consider alternating types of running shoes to vary the stress on your body. 

Just like what I've been telling my patients.

Reference

Malisoux L, Gette P, Backes A et al (2022). Lower Impact Forces But Greater Burden For The Musculoskeletal System In Running Shoes With Greater Cushioning Stiffness. Eur J Sp Sci 19: 1-11. DOI: 1080/17461391.2021.2023655