The first question is really how often should one change their running shoe? Basically, every six months or 300 miles, whichever comes first. The reason is because as the shoe wears, specifically the midsole, you will start developing little aches and pains. Sometimes, they are not so little. These aches can show up as plantar fasciitis, Achilles tendonitis, shin splints and even lower back pain.
Even stubborn doctors, like me, forget to check their shoes. Today I ran with a new pair of shoes, and my back does not ache. On one hand I felt rejuvenated, but on the other hand, I felt stupid. I tell my patients this all the time. I have two pairs of shoes, and I just figured the second pair was still good, until one day I decided to take that split second and look at the soles of my shoes. The tread was gone. Even with my orthotics, I was not getting the control I needed when I ran.
So, please alternate your shoes, because it takes 24 hours for the midsole to rebound. In addition, check your shoes for wear and tear, or just mark the mileage you have on each pair and the date you started using these shoes.