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 (P = 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 (P < 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.NEW & NOTEWORTHY 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.