Rapid onset and prolonged/sustained accelerations are often encountered in military aviation. The centrifugal force causes blood mass volume displacements. The pilot counters this phenomenon by means of anti-G straining manoeuvres (AGSM). However, the physiology of such prolonged manoeuvres is not clear. Here we have evaluated the eff ects of 4G and 6G accelerations lasting 10s from 1.41Gz baseline on stroke volume (SV) and cardiac output (CO) and data quality, as well as changes in frontal brain oxygenation (OX) as a function of duration of the acceleration. We further compared the pilots' physiological responses to 4G and 6G.Ten military pilots (six active, with various amount of fl ight experience) performed the rapid onset rate (ROR) profi le characterized by rapid onsets of Gz to 4G and 6G that lasted for 10 seconds, each without Anti-G trousers. The pilots' SV, CO were evaluated with bioimpedance cardiography, while their OX -with near infrared spectroscopy. ECG was constantly monitored.