HEMODYNAMIC AND HORMONAL RESPONSES TO LOWER-BODY NEGATIVE-PRESSURE INMEN WITH VARYING PROFILES OF STRENGTH AND AEROBIC POWER

Hemodynamic, cardiac, and hormonal responses to lower-body negative pr essure (LBNP) were examined in 24 healthy men to test the hypothesis t hat responsiveness of reflex control of blood pressure during orthosta tic challenge is associated with interactions between strength and aer obic power. Subjects underwent treadmill tests to determine peak oxyge n uptake (VO2max) and isokinetic dynamometer tests to determine knee e xtensor strength. Based on predetermined criteria, subjects were class ified into one of four fitness profiles of six subjects each, matched for age, height, and body mass: (a) low strength/average aerobic fitne ss, (b) low strength/high aerobic fitness, (c) high strength/average a erobic fitness, and (d) high strength/high aerobic fitness. Following 90 min of 0.11 rad (6 degrees) head-down tilt (HDT), each subject unde rwent graded LBNP to -6.7 kPa or presyncope, with maximal duration 15 min, while hemodynamic, cardiac, and hormonal responses were measured. All groups exhibited typical hemodynamic, hormonal, and fluid shift r esponses during LBNP, with no intergroup differences between high and low strength characteristics. Subjects with high aerobic power exhibit ed greater (P<0.05) stroke volume and lower (P<0.05) heart rate, vascu lar pe ripheral resistance, and mean arterial pressure during rest, HD T, and LBNP. Seven subjects, distributed among the four fitness profil es, became presyncopal. These subjects showed greatest reduction in me an arterial pressure during LBNP, had greater elevations in vasopressi n, and lesser increases in heart rate and peripheral resistance. Neith er VO2max nor leg strength were associated with fall in arterial press ure or with syncopal episodes. We conclude that interactions between a erobic and strength fitness characteristics do not influence responses to LBNP challenge.