title: Subjective and objective measures of relaxed +Gz tolerance following repeated +Gz exposure.
abstract: BACKGROUND
  Repeated exposure to +Gz acceleration provokes cardiovascular adaptations of potential benefit to pilots' +Gz tolerance, but whether such changes actually improve human tolerance to +Gz acceleration is uncertain. This study assessed +Gz tolerance before and after repeated exposure to +Gz at two different intensities as the role of frequency of +Gz exposure in adaptation also remains unknown.
  
  
  METHODS
  In a cross-over design, 10 experienced male centrifuge volunteers completed two experimental conditions separated by at least 3 wk. Subjects completed four simulated air combat maneuvers (SACM) on a human centrifuge, either twice or four times per week, for 3 consecutive weeks. Relaxed +Gz tolerance (RGT) during a gradual onset run (GOR, 0.1 G x s(-1)) and cardiovascular responses to rapid and incremental head-up tilt were assessed before and after each condition.
  
  
  RESULTS
  Rapid and incremental head-up tilt increased both mean arterial and diastolic blood pressures following +Gz exposure. +Gz exposure attenuated the increase in heart rate (+9 +/- 3 vs. +11 +/- 3 mmHg/Gz) and the decrease in eye-level systolic blood pressure (-11 +/- 3 vs. -14 +/- 4 mmHg/Gz) during GOR, but had no effect on RGT (4/wk: +3.88 +/- 0.56 vs. +3.92 +/- 0.63 Gz; 2/wk: +3.89 +/- 0.69 vs. +3.92 +/- 0.69 Gz).
  
  
  CONCLUSIONS
  Frequent +Gz acceleration, either as 2 (8 SACMs) or 4 sessions (16 SACMs) per week for 3 wk, enhances cardiovascular tolerance to orthostatic stress but does not improve RGT measured during a GOR.
authors.affiliation: Human Sciences, QinetiQ Ltd., Farnborough, Hampshire, UK. jscott1@qinetiq.com
authors.forename: Jonathan P R
authors.initials: JP
authors.lastname: Scott
authors.forename: James
authors.initials: J
authors.lastname: Jungius
authors.forename: Desmond
authors.initials: D
authors.lastname: Connolly
authors.forename: Alec T
authors.initials: AT
authors.lastname: Stevenson
mesh_terms: D000054:Acceleration; D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D000337:Aerospace Medicine; D001794:Blood Pressure / Q000502:physiology; D002320:Cardiovascular Physiological Phenomena; D002498:Centrifugation; D018592:Cross-Over Studies; D006185:Habituation, Psychophysiologic / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D018471:Hypergravity / Q000009:adverse effects*; D008297:Male; D055815:Young Adult