Muscle tensing during standing - Effects on cerebral tissue oxygenation and cerebral artery blood velocity

Background and Purpose-When standing up causes dizziness, tensing of the le g muscles may alleviate the symptoms. We tested the hypothesis that leg ten sing improves orthostatic tolerance via enhanced cerebral perfusion and oxy genation. Methods-In 10 healthy young adults, the effects of leg tensing on transcran ial Doppler-determined middle cerebral artery (MCA) mean blood velocity (V, ean) and the near-infrared spectroscopy-determined frontal oxygenation (O(2 )Hb) were assessed together with central circulatory variables and an arter ial pressure low-frequency (LF) (0.07 to 0.15 Hz) domain evaluation of symp athetic activity. Results-Standing up reduced central venous pressure by (mean +/- SEM) 4.3 /-2.6 mm Hg, stroke volume by 49 +/-7 mt, cardiac output by 1.9 +/-0.4 L/mi n, and:mean arterial pressure at MCA level by 9 +/-4 mm Hg, whereas it incr eased heart rate by 30 +/-4 beats per minute (P <0.05). MCA V-mean declined from 67 +/-4 to 56 +/-3 cm/s, O(2)Hb decreased by 7 +/-2.8%, and LF spectr al power increased (P <0.05). Leg tensing increased central venous pressure by 1.4 +/-2.7 mm Hg and cardiac output by 1.8 +/-0.4 L/min with no signifi cant effect on blood pressure, whereas head rate decreased by 11 +/-3 beats per minute (P <0.05). MCA V-mean increased to 63 +/-3 cm/s and O(2)Hb incr eased by 2.1 +/-2.6%, whereas LF power declined (P <0.05). Within 2 minutes after leg tensing, these effects had disappeared. Conclusions-During standing, tensing of the leg muscles attenuates a reduct ion in cerebral perfusion and oxygenation as it stabilizes central circulat ory variables and reduces sympathetic activity.