G tolerance and vascular sympathetic reflex responses as affected by repeated prolonged exposures to increased force field.

The capacity to tolerate high G loads in the head-to-seat direction (+Gz tolerance) is critical for pilots flying high-performance aircraft. The adaptive effects of repeated +Gz loading on relaxed +Gz tolerance and G-protective sympathetic reflex pressor responses were investigated. Twelve men were exposed to increased +Gz loads in a relaxed state, during 15 × 40 min sessions across 5 wk. Before and after the training regimen, relaxed +Gz tolerance was investigated during rapid onset-rate (ROR) and gradual onset-rate (GOR) G exposures, and cardiovascular responses were investigated during exposures to 2.5 G in the belly-to-back direction (+Gx) as well as during orthostatic provocations and pressure manipulations of the carotid baroreceptors. The G training increased ( = 0.04) the ROR G tolerance by 17% but did not affect GOR G tolerance, orthostatic tolerance, or the sensitivity and operational pressure range of the carotid baroreflex pressor response. The training reduced ( < 0.001) the arterial pressure response to +Gx exposure. The results suggest that repeated high +Gz exposures do not improve the overall vascular sympathetic response to high +Gz nor the responsiveness of the vascular branch of the carotid baroreflex, but, judging by the arterial pressure responses to +Gx loads, reduces the responsiveness of the vestibulosympathetic reflex. That the G training improved the ROR +Gz tolerance is attributable to local vascular adaptation, in terms of increased stiffness in dependent precapillary vessels resulting from the iterative increments in local transmural pressures. Five weeks of repeated exposures to head-to-seat directed G load in the relaxed state increased the capacity to tolerate rapid but not gradual G elevations, suggesting unaffected overall vascular sympathetic responses to high G. The G training did not affect the responsiveness of the vascular branch of the carotid baroreflex but reduced the vestibulosympathetic reflex response. The improved tolerance to rapid G elevations is attributable to increased stiffness in dependent precapillary vessels.