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Background Footwear characteristics have been implicated in fatigue and foot pain. The
recommended time for changing running shoes is every 500 miles. The aim of our study was to
assess and compare plantar peak pressures and pressure time integrals in new and old running
shoes.
Findings This was a prospective study involving 11 healthy female volunteers with no previous
foot and ankle problems. New running shoes were provided to the participants. Plantar pressures
were measured using the Novel Pedar system while walking with new and participants’ personal
old running shoes. Plantar pressures were measured in nine areas of the feet. Demographic data,
age of old running shoes, Body Mass Index (BMI), peak pressures and pressure-time integral
were acquired. The right and left feet were selected at random and assessed separately. Statistical
analysis was done using the paired t test to compare measurements between old and new running
shoes.
The mean peak pressures were higher in new running shoes (330.5±79.6 kiloPascals kPa)
when compared to used old running shoes (304±58.1 kPa) (p=0.01). The pressure-time integral
was significantly higher in the new running shoes (110±28.3 kPa s) compared to used old
running shoes (100.7±24.0 kPa s) (p=0.01).
Conclusion Plantar pressure measurements in general were higher in new running shoes. This could
be due to the lack of flexibility in new running shoes. The risk of injury to the foot and ankle
would appear to be higher if running shoes are changed frequently. We recommend breaking into
new running shoes slowly using them for mild physical activity.

Full text: http://www.biomedcentral.com/1756-0500/4/307/abstract

 

A study involving 11 females, with no clarification of individual activity levels/running mileage, analysis performed when only walking not running, and in no way investigating injury.

You can't get from there to here:

 

When I read the first half of the abstract, I was sure that they were going to have the subjects back after a certain number of miles in their shoes. It sounds like they were comparing two different kinds of shoes.

On the other hand, worn shoes will compress more in areas of high pressures. That might explain why newer shoes had higher pressures.

Eric