Subatmospheric pressure applied to the human body from the iliac crest caudally (Lower Body Negative Pressure, LBNP) has been known to cause various central and peripheral cardiovascular reactions. We investigated the effects of such a LBNP maneuver on energy metabolism of human skeletal leg muscle. We tested the hypothesis whether an altered fluid distribution induced by LBNP may cause any effect on aerobic and/or anaerobic energy metabolism and cellular pH-regulation during isometric contraction monitored by 31P-MRS.