Ex-NFL Star James Harrison -- OLD MAN STRENGTH ... Destroys Steelers Weight Room

He just retired, but ex-Super Bowl hero James Harrison is still out to prove he can outlift EVERYONE in the NFL ... by crashing the Pittsburgh Steelers gym and dominating the bench press.

With current Steelers Ike Taylor and Lawrence Timmons spotting him (shirtless for some reason), the former linebacker repped out 405 POUNDS like it was nothing.

The video is pretty hilarious ... and a tad frightening ... with 36-year-old Harrison explaining that he doesn't plan on shrinking down in his retirement.