St-Pierre is the best athlete I've seen in the UFC for his size. He has a potent combination of speed, strength, agility, flexibility, balance, cardio and explosiveness. He was never known for his power (in his hands) but he wasn't the type to swing for the fences either. His wrestling shot is the fastest/quickest I've seen to date and he covers a crazy amount of distance in a very short amount of time with it (as well as his lunging jab and Superman punch). I'm talking before he tore his ACL, since he's obviously declined in springiness/explosiveness since, unfortunately. This is really evident because he relies so much on his wrestling, takedowns specifically. GSP was named the Canadian Athlete of the Year three years in a row from 2008-2010.
There have also been many people that've been in awe of the guy, including past opponents. Josh Koscheck was a hell of a collegiate wrestler and very athletic himself, he's one of the most explosive athletes to ever fight in the UFC. He accused GSP of experimenting with steroids years back only to apologize and take back his statement. He made this interesting comment back in early 2012 about GSP...
Koscheck: Hopefully George is out for more than 10 months and they make that interim title holder fight twice.
Helwani: You're rooting against him?
Koscheck: Yeah absolutely. Hopefully he never comes back. Basically ACL surgeries take a lot out of you. I've had friends who've had the surgery and they say they're never the same. Hopefully it evens out the playing field and we can get another crack at Georges.
Helwani: There's still bad blood there?
Koscheck: No. Me and George have always been cool. He's a respectable guy outside of the cage when we're not fighting. He's a great person. I'm sure we would probably hang out. Well maybe not, he'd probably say no but I'd be willing to kick it with him.
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Talking about GSP's complete ACL tear and surgery, he actually says "equals out the playing field" but he's talking specifically about GSP's ability to use his high level of athleticism, and this is coming from a very good athlete himself in Koscheck. It's telling, and he'd know, GSP beat him twice handily and outwrestled him, and he's a very accomplished wrestler. KOS is a 4x Division-I All-American, a NCAA Division-I national wrestling champion (and Runner-Up another year), and has several other impressive awards that he won as a collegiate wrestler.
Here is Eddie Bravo practically losing it while talking about GSP's "incredible" wrestling in MMA while teaching at 10th Planet Jiu-Jitsu. He can't understand 'how' GSP is able to take everyone down at will, especially considering that GSP never wrestled in high school or collegiately. He wants to figure out "what the FUCK" GSP is doing that allows him to do this to some of the best wrestlers in MMA (and in American collegiate wrestling history), as he says in the video.
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The most impressive feats of athleticism that I've seen in MMA have also come from GSP, on several different occasions throughout his career.
GSP hitting an explosive scissor sweep from the botton...
GSP stuffing KOS' explosive shot as he tries to drive through him for the TD...
GSP's own double leg shot, covering a lot of distance with lightning quickness...
GSP sprawling hard on Trigg's double leg attempt and then reversing him for the takedown...