The World Building subforum and the General Science thread in OTC would be good places to ask.
As for your question, people break their legs all the time under the maximum known human speed of Usain Bolt's 27.44 mph. I imagine even Bolt can't go much faster than that before breaking his leg bones. Somewhere between 30 and 40 mph, I'd guess, probably on the lower side.
That isn't to say humans can't be insanely tough.
But, there's probably a reason we tend to have the same limits. We can't really do truly insane stuff unmodified.
On the other hand, your speedster will likely have a full Super Package Set of Super-Strength, Super-Toughness, and probably Super-Reflexes, along with a likely more efficient calorie-to-energy conversion rate if your accounting for Required Secondary Powers (although the first two don't really have to overshadow the speed in the wow factor, like Spider-Man and his spidey reflexes).
Not sure about the air friction, but I'm guessing you'll have to be more worried about your loose clothes flying off once you reach 100+ mph (skin-tight suits suddenly make more sense). Smacking into bugs would be annoying as well, and you'll probably want to wear goggles like early plane pilots to protect your eyes. As for the air friction, there have been a few guys who have gone almost and more than supersonic after 100,000+ foot skydiving drops and made it out okay (with pressurized suits, admittedly, but okay nonetheless). Your speedster will probably not be going that fast if your asking about breaking legs, so they'll probably just need a sturdy suit (and all the other requirements I listed) for long-term use.
edited 6th Feb '15 6:06:48 PM by Ekuran
Zzzzzzzzzz
Knock it off, you two. The fact that this thread is in Yack Fest does not mean that it's a free-for-all for your nonsense.
deadpool616 I'm moving this to Writer's Block, but I think your best results might come from reposing your question in "The Medical Thread" in On-Topic Conversations.
...if you don’t love you’re dead, and if you do, they’ll kill you for it.
A question that I didn't know where else to bring: a person has super speed and decideds to go for a run. How fast can he run before his leg bones break from hitting the sidewalk and air friction starts burning him? I realize that there are other problems the speedster would face, but I'm only concerned with those two.