The materials in the shoe will eventually wear out. boninstudio/Shutterstock^[6]

To our best knowledge, there’s only one study on running performance and shoe wear, but unfortunately it did not involve carbon fibre plate shoes. A University of Connecticut 2020 master’s thesis^[7] investigated eight college-level runners over 400 miles (643km) of Nike Pegasus shoe use.

Large reductions in running economy were reported at 240km, and this was statistically significant at 320km. No reduction was observed at 160km.

So, if you’re chasing personal best times, the evidence above suggests that for peak performance, shoes should be replaced somewhere between 160 and 240km (although this is not directly based on carbon fibre plate shoe research).

It appears that minimising training kilometres for your favourite racing shoes – keeping them “fresh” – could contribute to peak performance on race day, compared to racing in a pair of old shoes.

Injury or discomfort

The link between shoe wear and injury is unclear, and based on minimal and often conflicting evidence^[8].

One study did find that runners who alternate their running shoes^[9] have a lower risk of injury than runners who run only in the same pair of shoes over a 22-week period. Runners who alternated shoes throughout the study period would have accumulated less wear in each shoe.

This provides some support for the notion that accumulating too many kilometres in your shoes may increase risk of injury. Unfortunately, the exact age of running shoes was not reported in this study.

However, based on the running characteristics reported, the single-shoe pair users completed an average of 320km in their shoes (after adjusting for a small fraction who had to replace shoes during the study).

This was compared to the multi-shoe pair users who used an average of 3.6 pairs of shoes, ran more total kilometres, but accumulated an average of only 200km per shoe pair.

For race day, it might make sense to invest in a pair of shoes you keep ‘fresh’. Real Sports Photos/Shutterstock^[10]

Comfort

Comfort and fit are the two most important factors^[11] to runners when selecting running shoes. Evidence linking improved shoe comfort to reduced injury rates^[12] or improved running economy^[13] is mixed, but reducing harms from poorly fitting and uncomfortable shoes is clearly a priority for runners.

Most runners land on their heel^[14]. The repeated compression of the midsole causes the material to harden, possibly after as little as 160km^[15], according to one study from 2017. However, there was virtually no change in the amount of cushioning runners perceived under their heel after 160km. Even after using the shoes for 640km, they felt little difference – about 3%.

While at first this might seem like runners are not very good at judging when shoes lose their cushioning, it also tells us changes in perceived shoe cushioning are very gradual and may not be important for runners until they reach a certain threshold.

This amount will differ from person to person, and from shoe to shoe, but research suggests it’s not until perceived cushioning reaches about a 10% change^[16] that runners consider it meaningful.

We must be careful when applying these findings to the latest running shoes which use newer materials.

But you can use it as a rule of thumb – once you notice a drop in comfort, it’s time to get a new pair.

When to choose new shoes

Ultimately, there’s no one simple answer for when you should get new running shoes. You may also not keep close track of how many kilometres your favourite pair has racked up.

Overall, we believe the most practical advice is to keep your racing shoes “fresh” (under 240km), alternate a couple of other pairs during regular training, and replace them when you detect a notable drop in comfort.

Read more https://theconversation.com/how-do-i-know-when-its-time-to-replace-my-running-shoes-238997