I've come to the conclusion that sprint work is dangerous, for me anyways. I thought it would be good to mix some in w/ my normal running. I tried to warm up well, and gradually increase speed w/ the sprints. All was well for a couple of sessions and then I must have pushed too hard and pulled a groin muscle- badly. Felt like someone had shot me w/ a rifle. In a couple of days I was bruising from the inside out, almost the full length of my inner quad.
I still do speed work, but that is totally different than sprinting. I'll leave the sprinting to high school and college athletes
200m x6 @100% with 3 min active recovery each repeat
400m x 6 @ 100% with 1:2 work rest ratio
It was totally my fault for doing too much too soon. I should have gradually worked some quick sprints in rather than doing that much in one workout... live and learn i guess