WHAT IS THE BLOOD-FLOW TO RESTING HUMAN MUSCLE

1. An investigation was carried out in five healthy lean adults to ass ess whether forearm and calf plethysmography largely reflect muscle bl ood flow as measured by Xe-133 and whether there is substantial variab ility in the blood flow to muscles located at different sites in the b ody. 2. Blood flow to forearm and calf flexors and extensors, biceps, triceps and quadriceps was assessed using the Xe-133 clearance techniq ue. Blood flow to forearm skin and subcutaneous adipose tissue was als o measured using the Xe-133 clearance technique, whereas blood flow to the forearm and calf was measured using strain gauge plethysmography. 3. The mean blood flow to different muscles ranged from 1.4+/-0.6 (ga strocnemius) to 1.8+/-0.7 (forearm extensor) ml min-1 100 g-1 muscle ( 1.4+/-0.6 and 1.9+/-0.8 ml min-1 100 ml-1 muscle, respectively) but th ere were no significant differences between them. Forearm and calf blo od flows (2.7+/-0.3 and 3.0+/-0.7 ml min-1 100 ml-1 limb tissue, respe ctively) were about 50% to more than 100% greater (P<0.025) than blood flow to the muscles within them (1.7+/-0.5 and 1.4+/-0.5 ml min-1 100 g-1 muscle, respectively, or 1.8+/-0.6 and 1.5+/-0.4 ml min-1 100 ml- 1 muscle, respectively). In contrast, the blood flows to 100 g of fore arm skin (9.1+/-2.6 ml min-1 100 g-1) and adipose tissue (3.8+/-1.1 ml min-1 100 g-1) were higher than the blood flow to 100 g of forearm (P <0.01 and not significant, respectively). 4. Although several possibil ities can explain the discrepancy between muscle blood flow measured b y Xe-133 and blood flow to the distal limbs measured by plethysmograph y, the results suggest that nonmuscular blood flow, especially that to skin, is substantially greater than muscular blood flow. Indeed, the overall blood flow to the forearm could be accounted for by summation of blood flows to individual constituent tissues, which were assumed t o be present in proportions typical of lean subjects. The results have important implications in the use of arteriovenous catheterization st udies for assessing flux of oxygen, carbon dioxide and metabolites acr oss muscle.