forearm muscles are involved in grabbing/holding, highly involved. if you can grab and hold 400+ lbs., your forearms will have to be big.
and on another note, i used to have a bad posture when i first started lifting. deadlifts/squats were what straightened me up. and i haven't done either for some time now (well, not using high weight and just doing them unaltered), and i notice a bit of a curving coming back in my upper shoulders, a droop if you will. well, i'm happy to say that i am BACK on them shlts. today i did:
4x10, 8, 6, 5 deads
4x15, 12, 10, 8 squats
and i loved it. like old friends.