Mean diameters ( SD ) decreased significantly under compression ( p < 0.0001 ) in the standing position : in the antero-posterior ( ap ) plane of the GV by 6.5 ( 16.6 ) % under VU and 8.2 ( 16.5 ) % under VM , in the SSV 9.7 ( 15.8 ) % under VU and 5.7 ( 19.2 ) % under VM .