I've taken Muay Thai for 5 days a week for 3 years now and too say that some bum who has no experience is going to be able to properly defend himself is just stupid. If i were to kick you in the shin you would probably drop like a girl, I'm not trying to act tough its just my shins are solid and yours are jelly. I'm not even going to go into the rest like clinching or striking.
http://www.youtube.com/user/hmtmma#p/u/1/Y5hb85B1wXE
My instructor in his prime hes close to 60 now but his shin is made of concrete and is still in great condition.