Why should I replace my running shoes?

Why should I replace my running shoes?

As runners we often read that we ought to be replacing our running shoes more frequently. Is this just a marketing ploy by shoe manufacturers? Actually there are some very solid reasons that shoes wear out, and when they do you're at a greater risk of injury.
Learning to love hills Reading Why should I replace my running shoes? 6 minutes Next So this is how we run
As runners we often read that we ought to be replacing our running shoes more frequently. We're chided for forgetting to update them, or getting too attached to a given pair of shoes after all the miles we ran together. Is there any science behind this, or is it just a marketing ploy by shoe manufacturers and retailers? Indeed, in a recent ReRunnr survey more than 80% of runners said they had no pattern to replacing shoes - most just wait until they see obvious signs of wear and tear. Actually there are some very solid reasons that shoes wear out before then, and when they do you're at a greater risk of injury.


Lets start with the shoe itself - how do they wear out anyway? Unsurprisingly, it's the repeated impact caused by running (After all, 10,000 steps a day is the equivalent number of impacts on a shoe) and this repetitive stress is most damaging to the midsole foam. Most often, this squishy layer between the hard rubber sole and the fabric upper is made from stuff like ethylene-vinyl acetate which contains tiny cells to trap air and cushion your foot fall. These cells flatten over time and distance, reducing the protection they offer to your body - in short, your own soft tissues have to take over the work that the midsole was designed to do.


Unfortunately that's not even the end of the story of shoe decay. The rubber soles can also wear thin, losing traction and sometimes even creating holes. The heel-counter (a hard cup at the back of the shoe) starts to break down and allows your feet to slide inside your shoes. Finally, the fabric upper can also stretch, holding the foot less firmly and allowing more lateral movement.


These issues will all vary by runner and shoe - how far and where do you run? What kind of shoes are you using? The most important factors on shoe ageing are:
  • How far you're running is definitely the #1 driver of your shoes' performance life. Experts generally estimate a life of 300-500 miles, depending on the other factors.
  • What's your run style? Most road runners are heel strikers which has a larger effect on midsole foam compression. Conversely, hill and trail runners are often forefoot strikers leading to wear around the toe section of the rubber role.
  • Your build is also important. Compression is driven by force, which (as we all learned at high school) is the product of mass and acceleration. Simply put, heavier (and taller) bodies generally put more stress on the shoe.
  • Finally, an appropriate shoe for your running that has been professionally fitted for your gait will usually last longer because you have already removed the additional stress factors of pronation etc. There's early indication that the very lightweight 'super shoes' may have a shorter life but realistically more data is needed to prove this.
Ok - so everything wears out, right? But does it really matter? Can't I save some money by keeping going until my shoes literally fall apart? Well as our friends at On Running say "Just like driving a car on bald tires: you could keep going, but the chance of something bad happening increases each time you hit the road."


Time to talk about the impact (no pun intended) of running in shoes that are past their performance lifetime. There is science here, although it's not always exact. For the research geeks there is an excellent paper by Nicolas Chambon et al in The Journal of Sport Sciences on this subject. They determined that a "result of this study was that shoe ageing also affects muscular activity at impact"


Just like the worn tires, over-worn running shoes are no longer delivering the performance that they were designed for. At the extreme that could lead to you turning an ankle on the sidewalk or losing your footing on the trail. In either case the resulting injury can be very painful and take a long time to heal. More common are the running overuse injuries that occur in tendons and ligaments, such as:
  • Plantar Fasciitis - a stabbing pain on the bottom of the foot often caused by excessive impact from poor running mechanics.
  • Achilles Tendinitis - an inflammation of the achilles tendon that connects your heel to your calf muscle and more common in the over 40s. Its frequently caused by a change in your running style or routine - worn shoes can do that without you even noticing until its too late.
  • Shin Splints - an inflammation of the connective tissue that attaches this muscle to the tibia (shin) bone. This is a classic overuse injury that can be accelerated when your shoes do not provide adequate cushioning.
So the good news is that you can reduce all of these injury risks by replacing your running shoes at the right time. How to know that time? Well, tracking your mileage is a good start - but also listen to your body. Are you feeling new or unusual pains in your legs or back? They may be signs that your running style is having to compensate for shoes that are wearing out.


A final thought that matters to all of us - what to do with the old shoes? Don't just throw them in the trash - recycle them instead. And this is what ReRunnr is all about...

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