Subjective and objective measures of relaxed +Gz tolerance following repeated +Gz exposure.

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). DISCUSSION: 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.