We conducted a randomized prospective cross-over experimental study of @ healthy male volunteers ( @-@ years of age ) placed in @ different body positions : supine ( SU ) , prone ( PR ) , prone maximal restraint with no weight force ( PMR-@ ) , prone maximal restraint with @lbs added to the subject 's back ( PMR-@ ) , and prone maximal restraint with @lbs added to the subject 's back ( PMR-@ ) for @min .