NFL Star James Harrison -- Bangs Out Push Ups ... With 300 lb. Man On His Back!!

Age means NOTHING to James Harrison -- especially when it comes to brute strength -- 'cause the Steelers linebacker just knocked out a set of push-ups with a 304 lb. lineman on his back!!

The 36-year-old superstar -- one of the oldest guys in the NFL -- let teammate Maurkice Pouncey sit on his back while he got some chest work in during a gym session early Tuesday morning.

Do the math ... 275 (Harrison's weight) + 304 (Pouncey's weight) = STRONG.

Of course, Harrison came out of retirement to rejoin the Steelers this season and absolutely KILLED IT this weekend ... single-handedly terrorizing Ravens QB Joe Flacco all game long.

Then again, James never really stopped working out ... the guy was pushing serious weight in the Steelers gym even before he officially resigned with the team.

#Beast