Reduction in LBNP tolerance following prolonged endurance exercise training

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Abstract
Eight young men underwent an 8-month endurance exercise training program. Prior to and following the training program, the subjects' maximal oxygen uptake (V̇O2max, total blood volume (TBV) and plasma volume (PV), tolerance to lower body negative pressure (LBNP) assessed by the cumulative stress index (CSI) to presyncope, and their hemodynamic responses to 0 to −45 torr LBNP was determined. Hemodynamic measures included rebreathe carbon dioxide cardiac output (𝑄̇c), heart rate (HR), directly measured arterial blood pressures (ABP), and strain gauge determination of forearm blood flow (FBF) and leg volume changes (δLgV). Calculated values of stroke volume (SV), forearm, vascular resistance (FVR), and peripheral vascular resistance (PVR) were made. Following training, each subject had an increased V̇O2max (x̄ = +27.4%, P < 0.001), TBV (x̄ = +15.8%, P < 0.02), and PV (x̄ = +16.5%, P < 0.02) and each subject had a decreased tolerance to LBNP (x̄ CSI = −24%, P < 0.001). Stepwise linear regression identified that the major factors to significantly predict the decreased CSI pre- to post-training were a reduced response of PVR to LBNP from −15 to −45 torr (Model R2 = 0.853), the A TBV (model R2 = 0.981), and the greater post-training reduction in SBP to LBNP of 0 to −45 torr (model R2 = 1.0). These data suggest that physiologic adaptations associated with the increased V̇O2max and TBV resulting from a prolonged endurance exercise training program can alter the reflex control of vasomotion and cardiac output during LBNP and reduce the LBNP tolerance.

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