We conducted a randomized prospective cross-over experimental study of 25 healthy male volunteers ( 22-43 years of age ) placed in 5 different body positions : supine ( SU ) , prone ( PR ) , prone maximal restraint with no weight force ( PMR-0 ) , prone maximal restraint with 50lbs added to the subject 's back ( PMR-50 ) , and prone maximal restraint with 100lbs added to the subject 's back ( PMR-100 ) for 3min .