Upright posture reduces forearm blood flow early in exercise

The hypothesis that upright posture could modulate forearm blood flow (FBF) early in exercise was tested in six subjects. Both single (2-s duration) a nd repeated (1-s work/2-s rest cadence for 12 contractions) handgrip contra ctions (12 kg) were performed in the supine and 70 degrees head-up tilt (HU T) positions. The arm was maintained at heart level to diminish myogenic ef fects. Baseline brachial artery diameters were assessed at rest in each pos ition. Brachial artery mean blood velocity (MBV; Doppler) and mean arterial pressure (MAP) (Finapres) were measured continuously to calculate FBF and vascular conductance, MAP was not changed with posture. Antecubital venous pressure (P-v) was reduced in HUT (4.55 +/- 1.3 mmHg) compared with supine (11.3 +/- 1.9 mmHg) (P < 0.01). For the repeated contractions, total excess FBF (TEF) was reduced in the HUT position compared with supine (P < 0.02). With the single contractions, peak FBF, peak vascular conductance, and TEF during 30 s after release of the contraction were reduced in the HUT posit ion compared with supine (P < 0.01). Sympathetic blockade augmented the FBF response to a single contraction in HUT (P < 0.05) and tended to increase this response while supine (P = 0.08). However, sympathetic blockade did no t attenuate the effect of HUT on peak FBF and TEF after the single contract ions. Raising the arm above heart level while supine, to diminish P-v, resu lted in FBF dynamics that were similar to those observed during HUT. Altern atively, lowering the arm while in HUT to restore P-v to supine levels rest ored the peak FBF and vascular conductance responses, but not TEF response, after a single contraction. IL was concluded that upright posture diminish es the hyperemic response early in exercise. The data demonstrate that symp athetic constriction restrains the hyperemic response to a single contracti on but does not modulate the postural reduction in postcontraction hyperemi a. Therefore, the attenuated blood flow response in the HUT posture was lar gely related to factors associated with diminished venous pressures and not sympathetic vasoconstriction.