Effect of Different Body Postures on the Pressures Generated During an L-1 Maneuver

Changes in blood pressure, intrathoracic pressure, heart rate and the electromyographic activity of various muscle groups were determined while nine male subjects performed 15-s L-1 straining maneuvers at four spine-to-thigh angles (70, 84, 94, and 105°) and two seatback angles (30 and 60°). There was no significant difference between the changes in these variables due to the different body positions. At the onset of the L-1, arterial pressure immediately increased to 195 ± 5 mm Hg, but fell progressively during the next 5 s to 160 ± 5 mm Hg. It remained constant during the next 5 s of the maneuver and then recovered to 180 ± Hg during the last 5 s of the maneuver. Esophageal pressure followed essentially the same pattern of response, but heart rate progressively increased during the entire L-1. No one muscle group was utilized more than another. Inflation of an anti-G suit to 4 PS1 had no effect on the variables measured. Generation of high arterial pressures during L-1 maneuvers is transitory and not affected either positively or negatively by altering subject body position