The actor sat down with People for an interview published Saturday ... and, he revealed he suffered a pretty serious injury while filming the movie almost 15 years ago.

The way RC tells it ... he was performing a stunt where he jumped off a castle gate onto uneven ground. It was late, and Crowe said they probably rushed the prep before the stunt -- deciding not to bury a crash pad to land on.

Lo and behold, Russell landed and said he felt something akin to an electric shock shoot through his body ... but he managed to struggle through the scene since the epic battle sequence was already in full swing.

And, Russ said he didn't stop struggling on the movie ... saying he had trouble even walking over the final month of shooting for the action-adventure film, though he didn't take a day off.

Fast forward about a decade, and Crowe said some pain in his legs sent him to the doctor ... who X-rayed him and asked when exactly he'd broken his legs 'cause he could see the residual damage in his shin bones.

Crowe said he thinks a year off he took between finishing "Robin Hood" and training for "Man of Steel" let his legs naturally heal enough to overlook the pain ... proving the man's kinda superhuman himself.