Last year I had a remarkably injury-free run for most of the season and while its no doubt mainly attributable to my low mileage in that period (as they say "training causes injuries!") I have been curious whether my regular sprint sessions had anything to do with it.
There wasn't much logic behind this thought at first until I was readying Kevin Beck's "Running Strong" (one of four running books I'm currently devouring) and noticed that when sprinting your body is subjected to ground forces up to 6 or more times your body-weight whereas normal running is closer to 2.5-3x your body-weight.
I've always known sprinting is great for your form, your leg speed and muscular coordination, for working your fast-twitch fibres without damaging the slow-twitch, and for getting a good old stretch, but this was if anything more important. When you run downhill the ground forces hitting your legs increase many-fold over the normal flat impact. Could it be that by subjecting ourselves to 6 or more times our body-weight during sprints, we build a durability in muscles and joints (e.g. by applying this stress in a controlled manner) that transfers into injury-resistance for endurance running? Its a fascinating thought.
In any case, as only 3 weeks remain of my marathon conditioning (or base training if you will) I've started doing 10x100m sprints once per week midway through one of my easier runs. While I won't be Usain Bolt anytime soon, I was glad to record a PB in my second attempt taking my 100m time down to 14.88 seconds (the pace you run at if you are a 4-minute miler) which is a good start. I'd like to develop the leg speed to do these in about 12-12.5 seconds. All I need now is a Mussabini who can "find me those two extra yards, mr. Borg".