I've only recently started running and while it was fine at first now it's fucking killing my knees. I feel like frankenstein and can only run flat flooted (i'm not flat footed). I did some sprinting in the local park and i usually like to sprint on ball of my foot (front + toes) but my knees just wouldn't have it. Anyone else have this problem, is it the hardness of the pavement that does this or have i just got bad knees. I never really had a problem with this until now.