Georges St-Pierre -- Beach Muscle Training ... With Obliterated ACL

File this Under: "Impressive ... or Really Stupid?"

He's got a TORN ACL in his left knee, but "semi-retired" UFC legend Georges St-Pierre didn't let that stop him from working out on the beach in FL ... and even pulling off a headstand.

It's a risky move for the fighter, considering if he came down wrong on his left leg, he could've done even more damage to his knee ... and ruined any chance of a UFC comeback.

In fact, GSP recently said his ACL is so badly torn, he's undergoing surgery in the next couple of days.

So again we ask ... is it impressive that GSP pulled off some risky training moves on a bum knee IN THE SAND ... or really stupid?