The guy was out cold, and it was pretty obvious at the moment of the hit. If you watch the replay Martin was a little freaked out by it, too - he backed away like he was scared of the guy.
Both players lowered their shoulders to take the impact, it's just that Sparty got lower and took the brunt of Martin's helmet in the back of his head. Not much Martin could have done. That whole sequence of approach, lowering and bracing for impact probably took less than a quarter of a second. If anyone gripes about helmet-to-helmet from this, they need to stop watching football. This was totally unintentional, and the blame is more on Sparty than Martin.