To determine whether conditions for O 2 utilization and O2 off-loading from the hemoglobin are different in exercising arms and legs, six cross-country skiers participated in this study. Femoral and subclavian vein blood flow and gases were determined during skiing on a treadmill at ϳ76% maximal O 2 uptake (V O2 max) and at V O2 max with different techniques: diagonal stride (combined arm and leg exercise), double poling (predominantly arm exercise), and leg skiing (predominantly leg exercise). The percentage of O 2 extraction was always higher for the legs than for the arms. At maximal exercise (diagonal stride), the corresponding mean values were 93 and 85% (n ϭ 3; P Ͻ 0.05). During exercise, mean arm O 2 extraction correlated with the PO2 value that causes hemoglobin to be 50% saturated (P 50: r ϭ 0.93, P Ͻ 0.05), but for a given value of P 50, O2 extraction was always higher in the legs than in the arms. Mean capillary muscle O 2 conductance of the arm during double poling was 14.5 (SD 2.6) ml ⅐ min Ϫ1 ⅐ mmHg Ϫ1 , and mean capillary PO 2 was 47.7 (SD 2.6) mmHg. Corresponding values for the legs during maximal exercise were 48.3 (SD 13.0) ml ⅐ min Ϫ1 ⅐ mmHg Ϫ1 and 33.8 (SD 2.6) mmHg, respectively. Because conditions for O 2 off-loading from the hemoglobin are similar in leg and arm muscles, the observed differences in maximal arm and leg O 2 extraction should be attributed to other factors, such as a higher heterogeneity in blood flow distribution, shorter mean transit time, smaller diffusing area, and larger diffusing distance, in arms than in legs. diffusing capacity; fatigue; oxygen extraction; performance; training MUSCULAR OXYGEN UPTAKE depends on extrinsic factors such as O 2 delivery and the intrinsic factors that regulate both the transfer of O 2 from the erythrocytes to the mitochondria and the subsequent utilization of O 2 in the mitochondria. However, the diffusive transfer of O 2 is not only determined by intrinsic factors, because it also depends on mean capillary O 2 tension. It is currently assumed that during exercise with a small muscle mass, intrinsic factors are the main determinants of peak local muscular V O 2 , because the O 2 delivery is extraordinary high (3,44,61). During exercise with a large muscle mass, the V O 2 peak of the lower extremities appears to be O 2 delivery dependent (6,7,16,33,35,57). O 2 extraction across the lower extremities may reach maximal values between 90 and 92% of the arterial O 2 content (Ca O 2 ), and the PO 2 in the femoral vein may be close to 10 mmHg in active subjects (6,7,16), leaving little room for further extraction. However, in sedentary subjects, the maximal O 2 extraction across the legs lies close to 70% of the Ca O 2 (59), implying that their peak muscular V O 2 also may be limited by intrinsic factors (20). In physically active but nonarm-trained subjects, a low O 2 extracting capacity has been reported for the arms (1,11,51,70). Moreover, arm training resulted in only a marginal improvement in the O 2 extraction of the arms (51). Therefore, ...

That muscular blood flow may reach 2.5 l kg −1 min −1 in the quadriceps muscle has led to the suggestion that muscular vascular conductance must be restrained during whole body exercise to avoid hypotension. The main aim of this study was to determine the maximal arm and leg muscle vascular conductances (VC) during leg and arm exercise, to find out if the maximal muscular vasodilatory response is restrained during maximal combined arm and leg exercise. Six Swedish elite cross-country skiers, age (mean ± S.E.M.) 24 ± 2 years, height 180 ± 2 cm, weight 74 ± 2 kg, and maximal oxygen uptake (V O 2 ,max ) 5.1 ± 0.1 l min −1 participated in the study. Femoral and subclavian vein blood flows, intra-arterial blood pressure, cardiac output, as well as blood gases in the femoral and subclavian vein, right atrium and femoral artery were determined during skiing (roller skis) at ∼76% ofV O 2 ,max and atV O 2 ,max with different techniques: diagonal stride (combined arm and leg exercise), double poling (predominantly arm exercise) and leg skiing (predominantly leg exercise). During submaximal exercise cardiac output (26-27 l min −1 ), mean blood pressure (MAP) (∼87 mmHg), systemic VC, systemic oxygen delivery and pulmonaryV O 2 (∼4 l min −1 ) attained similar values regardless of exercise mode. The distribution of cardiac output was modified depending on the musculature engaged in the exercise. There was a close relationship between VC andV O 2 in arms (r = 0.99, P < 0.001) andlegs(r = 0.98,P < 0.05).PeakarmVC(63.7 ± 5.6 ml min −1 mmHg −1 )wasattainedduring double poling, while peak leg VC was reached at maximal exercise with the diagonal technique (109.8 ± 11.5 ml min −1 mmHg −1 ) when arm VC was 38.8 ± 5.7 ml min −1 mmHg −1 . If during maximal exercise arms and legs had been vasodilated to the observed maximal levels then mean arterial pressure would have dropped at least to 75-77 mmHg in our experimental conditions. It is concluded that skeletal muscle vascular conductance is restrained during whole body exercise in the upright position to avoid hypotension.

To study the role of muscle mass and muscle activity on lactate and energy kinetics during exercise, whole body and limb lactate, glucose, and fatty acid fluxes were determined in six elite cross-country skiers during roller-skiing for 40 min with the diagonal stride (Continuous Arm ϩ Leg) followed by 10 min of double poling and diagonal stride at 72-76% maximal O2 uptake. A high lactate appearance rate (Ra, 184 Ϯ 17 mol ⅐ kg Ϫ1 ⅐ min Ϫ1 ) but a low arterial lactate concentration (ϳ2.5 mmol/l) were observed during Continuous Arm ϩ Leg despite a substantial net lactate release by the arm of ϳ2.1 mmol/min, which was balanced by a similar net lactate uptake by the leg. Whole body and limb lactate oxidation during Continuous Arm ϩ Leg was ϳ45% at rest and ϳ95% of disappearance rate and limb lactate uptake, respectively. Limb lactate kinetics changed multiple times when exercise mode was changed. Whole body glucose and glycerol turnover was unchanged during the different skiing modes; however, limb net glucose uptake changed severalfold. In conclusion, the arterial lactate concentration can be maintained at a relatively low level despite high lactate Ra during exercise with a large muscle mass because of the large capacity of active skeletal muscle to take up lactate, which is tightly correlated with lactate delivery. The limb lactate uptake during exercise is oxidized at rates far above resting oxygen consumption, implying that lactate uptake and subsequent oxidation are also dependent on an elevated metabolic rate. The relative contribution of whole body and limb lactate oxidation is between 20 and 30% of total carbohydrate oxidation at rest and during exercise under the various conditions. Skeletal muscle can change its limb net glucose uptake severalfold within minutes, causing a redistribution of the available glucose because whole body glucose turnover was unchanged. lactate dehydrogenase; cross-country skiing; tracers AS EARLY AS 1907, Fletcher and Hopkins (11) not only provided definitive evidence of the relation between muscle activity and production of lactic acid in the amphibian skeletal muscle, but they also concluded that skeletal muscles possess the requisite chemical mechanisms for the removal of lactic acid once formed. Despite this early finding, lactate was long considered a metabolic end product, that is, lactate produced during muscle contraction and released into the circulation for subsequent uptake by the liver for recycling via gluconeogenesis. The importance of skeletal muscle in lactate clearance in humans became clear from experiments starting in the late 1950s. It was shown that, during exercise, lactate was taken up by nonactive skeletal muscles (1,7,12). Furthermore, when the arterial lactate concentration was also elevated, active skeletal muscles cleared lactate (12,26,30), and when two-legged cycle ergometer exercise was performed with one leg having a normal and the other a low glycogen content, the leg with the normal glycogen content released lactate, whereas lactate was taken up ...

To identify a perfusion flow at which the interstitial fluid completely equilibrates with the microdialysis perfusion fluid, a protocol with successively lower perfusion flows was used. A colloid was included in the perfusion fluid to make sampling possible at the lowest perfusion flows. At 0.16 μl/min, the measured metabolites had reached a complete equilibration in both tissues, and the measured concentrations of glucose, glycerol, and urea were in good agreement with expected tissue-specific levels. The glucose concentration in adipose tissue (4.98 ± 0.14 mM) was equal to that of plasma (5.07 ± 0.07 mM), whereas the concentration in muscle (4.41 ± 0.11 mM) was lower than in plasma and adipose tissue ( P < 0.001). The concentration of lactate was higher ( P< 0.001) in muscle (2.39 ± 0.22 mM) than in adipose tissue (1.30 ± 0.12 mM), whereas the glycerol concentration in adipose tissue (233 ± 19.7 μM) was higher ( P< 0.001) than in muscle (40.8 ± 3.0 μM) and in plasma (68.7 ± 3.97 μM). The concentration of urea was equal in the two tissues. Overall, the study indicates that microdialysis at a low perfusion flow may be a tool to continuously monitor tissue interstitial concentrations.