The effect of head and body position on +Gz acceleration tolerance.

It has been suggested there is a relationship between acceleration-induced loss of consciousness (G-LOC) and head/body position. A two-part investigation was conducted to determine whether head and body position affects acceleration tolerance. A retrospective analysis of high-G training data (N = 1,914) compared G-LOC occurrence during straight-ahead exposure to a "check-6" exposure [10 s at +9 Gz; 6 G/s onset rate; G-suit inflated; anti-G straining maneuver (AGSM) performed]. A prospective study (N = 12) was conducted with acceleration exposures using light loss criteria with subjects in straight-ahead, above, over-the-right shoulder, or over-the-left shoulder positions. Profiles consisted of 0.1 G/s onset-rate runs (no G-suit inflation; relaxed) to a maximum of +9 Gz and 0.5 G/s onset-rate runs (G-suit inflated; AGSM performed) to +9 Gz for up to 26 s. In the retrospective study, no significant difference existed between G-LOC occurrence during straight-ahead (22/1914) and check-6 (32/1914) positions. During the prospective study with AGSM runs, there was no significant difference in the time at maximum G among any of the positions. During the relaxed runs, several comparisons yielded significant differences in peak G attained. These results indicate there may be an underlying physiologic effect of head and body position on acceleration tolerance; however, the AGSM and the G-suit overcame this effect. Although task saturation and distraction may compromise performance of the AGSM and subsequently predispose acceleration-related hazards, a proper AGSM, combined with effective protective systems, remains essential components of a protection strategy.