The Monday Shakeout: What Happens to Midsoles As They Wear Out?
By Matthew Klein
This week Matt takes a look at the life of a midsole, and what happens as they are run in over time. We release a Shakeout each Monday focusing on a new topic from the team. Have an idea for us? Email us at doctorsofrunning@gmail.com
A common question received not only by us but at running stores around the country is: "How do I know when my shoes have worn out." Most companies have gone by an untested 300-500 mile rule, but those of us who have run through many pairs of shoes know that numbers varies greatly (and it appears to be decreasing over the years). That variation can depend heavily on the individual and their unique biomechanics. Some people who are light on their feet can get tons of miles out of their shoes. Others (like me) with a heavy or scuffing footstrike can shred or wear through shoes quickly. This also depends on the individual shoe. Shoes that are lighter with no outsole material or less durable midsoles can break down quickly, while those with heavier, more dense midsoles and outsoles may last forever. This week's Shakeout addresses the sole as uppers vary just as much for similar and different reasons.
The Life of a Sole
Given the variability in how long shoes may take to break down, let's talk about how they break down. Typically, most of the wear experienced by a shoe occurs at the midsole and outsole. The midsole refers to the primary layer of foam between the foot and the ground. The purpose of the midsole is to provide cushioning and protection. This is usually where the majority of foam exists within the shoe with various types, including EVA, TPU, PEBA, combinations of those, and many others. All foams share a few basic properties, particularly the ability to deform under pressure. This is where the perception of cushioning comes from underfoot (also influenced by the sensory component of your nervous system, which may also vary depending on the individual). That deformation, commonly referred to as "compression", has a few variations we talk about, including compliance and resilience. Compliance refers to how much the foam compresses, whereas resilience refers to how much it compresses and reforms to its original shape.
Most people will refer to compliance as how "soft" a shoe feels, while resilience is often referred to as how much "responsiveness" or "bounce back" a midsole has. When a midsole foam is brand new, it has a certain amount of compliance and resilience. As it breaks down, it slowly loses the amount of compliance and resilience it has. The foams begin to lose their ability to reform to their original shape. This is often described to people as the shoe losing its cushioning properties. So as shoe midsoles wear down more, they lose their cushioning abilities because the foam has been more permanently compressed. Instead of having a soft cushioned feel, shoes that are more worn down will often be described as "hard" or "firm" compared to when they were new.
The Issue with Soles as They Get Worn
Thanks to the unique movement patterns of humans (which are often asymmetrical from side to side as well), the parts of the midsole that will compress are rarely uniform. A certain part of the shoe, usually specific to the individual's preferred movement pathway through the shoe, will wear down more relative to other parts of the shoe. This means part of the midsole maintains its cushioning while other parts will not. The parts that begin to break down and compress more may create geometries within the sole that push the individual to move in a more exaggerated version of their normal motion pathway. An example of this is the lateral part of the heel being compressed because people tend to land there. The more it is compressed it is relative to the medial side, the more lateral the person may go. This can happen at any part of the shoe and can also be influenced by outsole wear.
The outsole refers to the thin layer(s) of rubber on the bottom of the shoe that provides traction and resistance to abrasion (outsoles are not always present). Outsole abrasion is rarely uniform and often occurs in specific places. The lateral heel is a common spot (because a majority of runners land there). As this wears away relative to the medial side, it creates a larger wedge that predisposes the ankle to move more lateral at heel strike. The more lateral it translates, the more stress there is on lateral structures like the fibular tendons, lateral ankle ligaments, lateral calcaneus bone, etc. Some people may be fine with this, but others who do better with uniform stress on the medial and lateral sides of the foot may find themselves at risk for overuse injuries where that greater amount of stress may be occurring. This concept of wear and excessive motion can happen at any part of the shoe. The medial and lateral forefoot are also common areas, but again this wear can be seen anywhere in the sole depending on the person.
How Long Should I Wear My Shoes Then?
Although different people will be able to get different amounts of mileage out of their shoes, all shoes do break down. Whether that is the midsole losing its cushioning properties (compliance and resilience), abnormal geometries being created as the individual wears their unique movement pattern into the shoe, the outsole being rubbed off in certain places or the upper tear/losing the ability to hold the foot, shoes do not last forever. Different people and training will impact the durability of a shoe. The shoe type will also impact the durability, as racing flats tend to not last as long as training shoes. The best way to determine whether a shoe is worn out is to pay attention to how it feels. Random aches and pains that show up for no reason (ie no change in training or other variables) are a potential sign the shoe may be wearing out. Excessive wear on the outsole or visual compression of the midsole may be other signs that it is time to replace them.
For those who want a specific number, you should begin at least thinking about a new pair of training shoes every 200-250 miles (there is no research behind this, just experience). That does not mean you need to retire the previous shoes immediately, but you should begin to rotate a new pair in (if you only typically use one shoe) to help your body adjust to something brand new. Switching between the two of them will also potentially make them last longer as the more time a midsole foam has to recover from a run, the longer it may last. As your older shoes starts to feel less cushioned, you notice larger amounts of wear on it or it becomes less comfortable, it may be time to think about retiring them permanently. As mentioned, shoes do not last forever. So as one of the many parts of the complex equation of reducing injury risk in runners, we suggest keeping your shoes relatively up to date. That means paying attention and learning about how long shoes wear out for you. When they start to feel worn out, it is time to begin introducing a new pair of either the same shoe or something different. Which one will depend on what you want next.
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NEXT: Do Super Shoes Reduce Muscle Soreness?