• Blood pressure variability [ Time Frame: inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• Spontaneous baroreflex activity [ Time Frame: inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• Pulmonary diffusion [ Time Frame: inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• Upper limbs vascular resistance [ Time Frame: inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• post ischemic forearm hyperemia [ Time Frame: inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• Hematocrit rate [ Time Frame: Inclusion, one week, two weeks ] [ Designated as safety issue: No ]
• Cardiac haemodynamic [ Time Frame: Inclusion, one week, two weeks ] [ Designated as safety issue: No ]

• Blood pressure variability [ Time Frame: inclusion, one week, two weeks ]
• Spontaneous baroreflex activity [ Time Frame: inclusion, one week, two weeks ]
• Pulmonary diffusion [ Time Frame: inclusion, one week, two weeks ]
• Upper limbs vascular resistance [ Time Frame: inclusion, one week, two weeks ]
• post ischemic forearm hyperemia [ Time Frame: inclusion, one week, two weeks ]
• Hematocrit rate [ Time Frame: Inclusion, one week, two weeks ]
• Cardiac haemodynamic [ Time Frame: Inclusion, one week, two weeks ]

Head down tilt (HDT) was widely used to simulate microgravity effects on cardiovascular system. HDT could be a suitable model of water immersion (WI) which is also used to simulate the cardiovascular effects of microgravity and which is not easy to study in laboratory. To define the possibility to simulate immersion by HDT, a comparison between these models is required. A comparison between WI and few angles during HDT seems necessary to understand which angle is more adapt.

The immersion induces an increase of the central blood volume. This increase is caused by a redistribution of blood from peripheral portions of the body to the intrathoracic circulation. It seems to load cardiopulmonary and arterial baroreceptors. These baroreceptors bring into play autonomic nervous system (ANS) activation and induce a bradycardia.

HDT induces an increase in central blood volume as supported by the central venous pressure and cardiac volume increase and in return, MSNA and heart rate decrease. These cardiovascular effects seem to be the same as the thermoneutral immersion and suggest that the ANS activation is the same during HDT and WI.

Nevertheless, a few previous studies about ANS in HDT indicate some discordant results: a sympathetic decrease was reported but several results show an increase of parasympathetic activity linked with a trend of increase of arterial baroreflex. The aim of this study is to assess ANS activity in HDT on different angles (-6° and -15°) and WI. The investigators suppose an increase of parasympathetic activity during WI corresponding to parasympathetic activation during HDT especially at -6°.

• Gabrielsen A, Johansen LB, Norsk P. Central cardiovascular pressures during graded water immersion in humans. J Appl Physiol. 1993 Aug;75(2):581-5.
• Shiraishi M, Schou M, Gybel M, Christensen NJ, Norsk P. Comparison of acute cardiovascular responses to water immersion and head-down tilt in humans. J Appl Physiol. 2002 Jan;92(1):264-8.
• Ueno LM, Miyachi M, Matsui T, Takahashi K, Yamazaki K, Hayashi K, Onodera S, Moritani T. Effect of aging on carotid artery stiffness and baroreflex sensitivity during head-out water immersion in man. Braz J Med Biol Res. 2005 Apr;38(4):629-37. Epub 2005 Apr 13.

Inclusion Criteria:

• Sportsman
• None coffee, alcohol, cigarettes 6 hours before the start of each visit
• Each meal must have been taken 4 hours before each visit
• No intensive sport 48 hours before ech visit
• Written inform consent

Exclusion Criteria:

• Subject with cardiovascular,renal disease or with metabolic syndrome
• Subject with medication
• Subject who participated to an other medical research