Thanks for your reply, CouchWarrior, The Muay-Thai Roundhouse must generate tremendous power, either that or that guy's bones must be thin from cancer. Why did that guy have such a weak shin? I saw on the "Fight Science" video that a Muay-Thai's shins get lots of bone strengthening from repeated practice striking on the bags.
By the way, I know that girl is not showing a Mauy-Thai kick. It's a Karate Kick. But it appears to be faster than a Mauy-Thai style. If the Mauy-Thai is always so slow, and if the defense is as easy as lifting your knee, then why bust your shin?
It's a lot more common that a fighter successfully blocks a leg kick in kickboxing fights than in MMA fights, it seems. A good fighter can kick fast, and you need good reflexes to block the kick. It might also be smarter to open up first with a combination of punches and then throw the leg kick, so that the opponent hopefully is distracted and doesn't react fast enough to block the kick in time.
I don't know what was wrong with his leg. I've seen a video of a show wrestler jumping from the ropes and breaking his leg when he hit the canvas. Sometimes crazy things like that happen, I don't know the reasons.