i think it's in the materials/ Old shoes were probazbly compressed PVC in the midsole not compressed EVA like now.
From what i remember from the gumf you get in running magazines on a recurring basis. EVA is lighter than PVC
but it's not a solid, it's a very very ...very thick liquid. It's more sensible to temperature changes than the old PVC. Could be i remember it wrong, EVA is definitely technically better but people who don't hit the ground very hard got by just as well with the old stuff.
Did very well for about 1.5 years using a light neutral shoe for more than 500 miles and after wearing the shoes just a little longer, after those tell tale soreness had started, went from healthy to gimpy in only 2 days and was away from running for almost two months.
I knew a guy in high school who liked to drive really fast down dark country roads with the lights out and never crashed but most folks do better with the beams.
I have also put 2000 miles on a pair of DS trainers, and a similar amount on my nike zoom streaks. I don't really need too much cushioning, and i HATE running in shoes with high heels. Usually the only reason I get new shoes is because the old ones get torn up -- eventually my socks start showing. When I was younger and stupider (and possibly weaker) and wore shoes with high heels and lots of cushioning, I would start to get different aches and pains if I used the same pair of shoes for too long, so I'd get new ones every 600 miles or so.
i love the ds trainers. the reason i'd usually have to go out and get a new pair was because of holes. Even when the soles were long gone i'd still run in em
Jason Rexing does all his training and racing in vapors. He puts 20,500 miles a year on them
I once saw a graph that shows the degradation of midsole material over time. After the first 100 miles you lose I think 50% of the shock absorption and then it more or less levels off to around 30% after 300-500 miles. I never count miles on my shoes. Just run until you can feel the stones under your feet or you feel your feet becoming more sore after a run. Also, be careful of lateral wear of the shoe. If the lateral side of the shoe (look at the heel as well) becomes really worn you will basically be landing on your foot at a more pronounced angle, requiring your foot to recover to the midline more (takes more time for your foot to roll toward the midline), reducing speed, increasing the stress on your foot, which could lead to injury. When I see the lateral side becoming really worn then I know it's time for new shoes.
Think of it this way...in new shoes you're still landing on the outside edge but with old shoes the edge is beveled so the landing is at a more pronounced angle. Because this angle is less in new shoes you should be able to run FASTER in new shoes because the foot will take less time to pronate over to the midline.
Alan