When to replace a pair of running shoes.

I have been holding out on this for quite some time. But at some point, a running blog needs to mention running shoes.

To limit the information overflow, this week will zoom (hehe) in on when you should start looking for a box-fresh new pair of speed slippers.

NO, no, no.

Not yet, patience!

For now, here is some light reading to keep you from ordering new shoes straight away.


Sports science talk

The running motion is all about opposite forces.

As your foot touches the ground, it will first experience a brief yet poignant braking force*. To then push yourself forwards, your body has to deal with a surface (read: the ground) that will generate a fitting reaction force.

*(this would not be the case if you have an hyper-efficient running style)

The faster you run, the harder your foot’s downward push will have to be. At high running speeds, you can expect ground reaction forces of over twice your body weight making their way back up through your body with every single step!

So what do running shoes have to do with that?

Virtually all the major shoe running brands pride themselves in selling the softest, fluffiest and most energy-efficient shoes your feet will ever know. Until the next model launches of course.

The idea that running on 150 euros worth of cloud fabric will somehow shield you from injury is sadly only half-right.

While a modern running shoe can help you with both the braking and the ground reaction forces, we need to sprinkle a dash of nuance here.

Braking force
  • Most of the braking force trauma leads back to running with poor form. This means it can initially be avoided almost completely by putting those hips to work and landing directly under your centre of mass. Much like this barefoot runner would.

 

  • When you or your hip muscles eventually get fatigued, either due to a lack of mileage or eagerness to commit to supplemental hip strength exercises, the harmful braking force will seep back into your running to wreak havoc. In this case, some extra cushioning will keep you from blowing your legs out.
  • TL;DR: You need either stronger legs or more cushioning for longer distances.
Ground reaction force
  • Since Abebe Bikila won the 1960s Olympic Marathon in Rome (2h15m16s), no runner has ever finished a marathon faster while also being barefoot.
  • Running shoes with a high-energy return foam midsole can rebound up to 70+% of the energy your downward push puts on the insole. In the grand scheme of things this will net you a glorious 1% performance boost .
  • TL;DR: Running with shoes is more comfortable and faster. Not too much though.

Burning rubber

Naturally, both these forces induce the wear and tear that sends us back to running stores most seasons.

Durability seems to vary greatly between different brands and shoe models. From personal experience, I have deduced two key points that can explain most of the damage:

  • Running style:
    • running in a light and efficient fashion (get a coach here) will make you use less oxygen and rubber per kilometre.
    • Mimicking an elephant stampede will scare away oncoming traffic but also wear out your cushioning in no time.
  • Weight:
    • MEN: I personally see the 75 kg as an upper limit for lightweight shoes with little cushioning. Even with good technique, I would pick a more supportive shoe to prevent impact injuries.
    • WOMEN: I’d reckon 65 kg would count as an equal top ceiling. This is merely an educated guess, so please consult your shoe salesperson before your next buy 🙂

Litmus test for used running shoes

1) Track mileage

I use a free Strava function to warn me when I run over 700km on a pair of shoes.

To me, this is an arbitrary sweet spot to start looking for shoe updates.

You can expect to squeeze out of your daily training pair anything between 350 km for a pair of feather-light track racers to over 1000 km for bulky supportive marathon shoes.

2) Damage analysis

Much like the state of your party pants, going hard all weekend will leave some marks on the outside of your running shoes.

Case in point:

After 10 km
After 300+ km
After 700+ km

Three things to notice:

  1. As expected, the oldest shoes lost the most rubber.
  2. Based on the wear patterns, we can safely assume my landing strike pattern ignores the heel completely.
  3. Strong hint of supination, as the pinky toe edge of my shoe donated the most rubber to mother Earth.
3) running Economy analysis

After step 1 & 2 have given you a positive indication whether or not your shoes are ready for retirement, I suggest you keep using them for as long as possible.

Why?

Running shoes with worn out cushioning are often a painful reminder of what parts of your running technique are lacking.

That being said, as soon as you start feeling recurring injuries (e.g. blisters, hot spots, bruises,..) STOP USING THOSE SHOES.

Not worth it to lose two months of running just to be cheap.


Wrap up

Looking to replace your old running shoe?

  • Check mileage
  • Check wear and tear on the outside of the shoe
  • Stubbornly keep using them till it hurts
  • Replace

Run well,

Simon

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