Let’s say you are out for a night on the town. Your significant other turns to you and says how do you like my outfit? Unless you answer this question perfectly (still not sure how to answer this even after many years of marriage!) you may notice that your significant other withdraws for a while to cool off. Could be a few minutes, could be for the night, but this time away allows things to mellow and return to a state of bliss yet again. What does this have to do with shoes?
It’s common practice for a lot of runners to have multiple pairs of shoes at one time. This though process behind this has been that running in a shoe breaks down the cushioning properties of the shoe, and it takes time for it to rebound before your next run.
Well, Let’s look at some objective information on this subject. This is not going to turn into a discussion of minimal shoes vs. traditional construction. We are simply going to look at what happens to the properties of the midsole itself. The midsole is the squishy part of the shoe that lies between your foot and the tread. Its commonly made of a chemical compound called ethyl-vinyl acetate (EVA). Shoe manufacturers manipulate properties of the midsole to get their shoe to perform a certain way. Obviously a minimal shoe has less of this midsole material in it then a traditionally constructed shoe, but they both have some type of cushioning material between the foot and the shoe tread.
As you run, you are applying mechanical forces to the shoe itself. These forces physically break down the midsole. In fact, lets look at this at the microscopic level. The following picture is from a study (1) that looked at the state of the midsole at various points in the shoe’s lifespan. In fig A, you are looking at an electron micrograph cross section of a brand new shoe. It’s easy to see the outline of the well defined air pockets in the midsole. This intact formation allows the midsole to perform as it was designed. Fig B shows a cross section of the midsole after 500 kilometers (310 mi). This type of magnification allows you to see that the edges of the former well defined air pockets are now frayed and weakened. Finally, Fig C shows the midsole after 750 Kilometers (466 mi). It is now possible to see that the majority of the air pores are frayed, and in fact some of them have actually deformed enough to create holes. Thus, the structural properties of this midsole material are now very different from what they originally were when new. While your body can repair tissues that have been affected from mechanical stresses in running, your shoes cannot. Resting your shoes by the front door between runs won’t reverse these changes.
While it’s probably a good thing to be nice to your shoes (running in wet environments with no chance to dry out may accelerate breakdown of the midsole), they don’t have feelings. You can pound on them day in and day out – even 2 runs in the same day. The breakdown of the material in your shoe is cumulative. So what happens to our gait as shoes breakdown?
A 2009 article (2) revealed that running in worn shoes caused the runner to increase their stance time (time spent on the leg) and alter their lower leg range of motion in order to keep forces on the body somewhat constant. What does this mean? As your shoes break down, the body will slightly alter its gait style adapt to the gradual changes that occur in the shoe itself. When do these gait alterations reach critical mass (causing injury if you don’t buy new shoes)? Shoe breakdown is variable depending on the runner’s mass, running surfaces, and gait style. I know runners who note that they become injured if they put more than 250 miles on their shoes, and I know runners that put well over 1200 miles on a single pair. The old school rule of thumb states 400-500 miles, and is likely a good starting point based on the research stated above.
- Verdejo, R. , Mills NJ. Heel-shoe interactions and the durability of EVA foam running-shoe midsoles. J Biomech. 2004. Sept; 37(9):1379-86.
- Kong, PW., Candelaria, NG., Smith, DR. Running in new and worn shoes: a comparison of three types of cushioning footwear. Br J Sports Med 2009 43: 745-749