The vascular endothelium plays an important role in the modulation of vascular tone and function by synthesizing and releasing nitric oxide (15). Under physiological conditions, nitric oxide is released in response to increases in blood flow and shear stress. Exercise training provides repeated bouts of hyperemia in the blood vessels. The increased blood flow and the elevated shear stress associated with exercise are thought to augment vasodilatation through the release of nitric oxide. Regular exercise augments endothelium-dependent vasodilation in both animals (8,27) and humans (9,13). Interestingly, exercise-induced increases in endothelium-dependent vasodilation have been observed in sites that are not primary working muscles (e.g., forearm in response to cycling or walking exercise) (9,13,18). Because of these findings, many investigators speculate that the adaptation in endothelium in response to regular exercise may be systemic in nature, and the elevations in shear stress may not be a stimulus for augmenting endothelium-dependent vasodilation with regular exercise.
This reasoning appears to stem from the prevailing view that blood flow to inactive tissue decreases during exercise. This concept has been widely described in review articles and exercise physiology textbooks (20,21,24,25). Indeed, animal studies have consistently demonstrated reductions in blood flow to inactive muscles (e.g., temporalis muscle) during exercise (2,14,22). Classic studies conducted in humans are in agreement with these animal studies showing reductions in blood flow in nonexercising limbs during low-intensity submaximal exercise (5,6). From the standpoint of redistribution of systemic blood flow to the contracting muscles, the background supporting this view appears to be solid. However, more recent human studies have yielded inconsistent findings with reports of a reduction (3,28,29), no difference (17,26,29), or an increase (1,11) in blood flow to nonworking muscles during exercise. These variable results may reflect differences in methodologies used, limbs studied, unintentional movements in nonworking muscles, and variable exercise intensities at which blood flow was assessed. Indeed, increases in blood flow to nonworking limbs previously observed in some studies have been attributed to muscular activities in inactive muscles (1). Additionally, most previous studies used low-intensity submaximal exercise of short duration, and blood flow during strenuous exercise is less studied. This is an important consideration because blood flow to nonworking limbs may behave in biphasic fashion during exercise (6).
Accordingly, we determined whether blood flow and shear stress would increase in inactive limbs during incremental aerobic exercise to volitional exhaustion. To address this aim, first we constructed a modified arm cycle ergometer and a recumbent cycle ergometer such that the movements in inactive muscles would be minimized. In order to ensure that there would be no muscular activity in inactive musculature during exercise, EMG activity was measured. We chose high-resolution Doppler ultrasonography for the measurement of whole-limb blood flow as it allows accurate determination of “absolute” levels of limb blood flow as well as the subsequent calculation of shear stress. To address this issue more comprehensively, we assessed blood flow in nonworking limbs during both arm and leg exercise as there appear to be differences in vasodilator responses between these vascular beds (23). Based primarily on the results of previous animal studies (2,14,21,22) and classic human studies (5,6), we hypothesized that blood flow to nonworking limbs would decrease during incremental exercise.
Eight young healthy female subjects (mean ± SEM; 23 ± 1 yr) were studied. Subjects were either sedentary or recreationally active, but none participated in moderate- or high-intensity aerobic exercise >3 d·wk−1. All subjects were normotensive (<140/90 mm Hg), nonobese (body mass index < 30 kg·m−2), nonmedicated, nonsmokers, and free of overt chronic diseases as assessed by a medical history questionnaire. Because the present study did not involve serial measurements, the testing at any given menstrual cycle phase (i.e., early follicular phase) was not strictly enforced. However, most subjects were tested during the follicular phase of their menstrual cycle. Their mean (±SE) height and weight were 158 ± 1 cm and 53 ± 2 kg, respectively. All subjects gave their written informed consent to participate, and all procedures were approved by the institutional review board at Japan Women's College of Physical Education.
Before they were tested, subjects abstained from caffeine and fasted for at least 4 h. Each test was conducted in the thermoneutral condition in the environmental chamber. After a short period of quiet rest, subjects performed two graded exercise tests to volitional exhaustion: one on a modified arm cycle ergometer and another on a recumbent (leg) cycle ergometer. The arm cycle ergometer and recumbent (leg) cycle ergometer were modified to minimize any involuntary movements of nonworking limbs by subjects and to expose the upper arm and the groin for placement of Doppler transducers. During both arm and leg exercises, subjects' upper body was recumbent at an angle of approximately 120°. Subjects were instructed to minimize any involuntary movements of nonworking limbs during exercise, and familiarization sessions were conducted to eliminate any significant EMG activity. The two exercise trials were performed on different days in randomized order. Work rate was increased every 3 min by 10 W during leg cycling and 5 W during arm cycling until subjects reached volitional exhaustion. As the peak work rates achieved by subjects were different, the exercise data were presented up to the work rate that most subjects were able to attain (30 W in arm exercise and 100 W in leg exercise). On average, these exercise intensities represented, respectively, 94 and 91% of the peak arm and leg work rates. During the exercise, the subjects were reminded to minimize any movement in the nonworking limbs. Moreover, to verify that there was no muscular movement in inactive limbs, EMG activity (Nihon Koden WEB-5000) of vastus lateralis and medial gastrocnemius muscles (during arm cycling) and on ulnar and radial muscles (during leg cycling) was monitored.
Duplex Doppler ultrasound machines equipped with high-resolution linear-array transducers were used to measure whole-limb blood flow on the common femoral artery (Hewlett Packard 8500-GP) and on the brachial artery (GE Yokogawa Medical Systems Vivid 7 Pro) as previously described (16). Multiple ultrasound machines were used to minimize the frequent movement of machines and personnel in the small laboratory space during the experiments. These machines yielded quantitatively similar blood flow values. Arterial diameter and mean blood flow velocity were captured simultaneously. Mean blood velocity measurements were performed with the insonation angle <60° and were corrected for the insonation angle. The sample volume gate was adjusted to cover the width of the vessel. To minimize turbulence from the bifurcation, the measurements on the common femoral and common carotid arteries were performed approximately 2–3 cm proximal to its bifurcation. Brachial artery blood flow was measured in the distal third of the upper arm. The anatomical sites, at which ultrasound probes were placed, were standardized. All the data were digitally recorded directly on the hard drive and were analyzed by the software provided by the ultrasound machine manufacturer. The data reported were time averages of the last 30 s at each work load (19). Wall shear stress was then calculated using the equation 4ηMBV/ID, where η is the blood viscosity (assumed as 4.0 cp), MBV is the mean blood velocity, and ID is the time-averaged (mean) internal diameter (10). HR (via ECG) and oxygen consumption (via metabolic cart) were measured continuously during the exercise. Arterial blood pressure was measured using a mercury sphygmomanometer every minute during the leg cycling exercise.
Data were analyzed using repeated-measures ANOVA. In the case of a significant F-value, a post hoc test using the Newman–Keuls method was used to identify significant differences among mean values. All data were reported as mean ± SEM. Statistical significance was set a priori at P < 0.05 for all comparisons.
In both arm and leg cycle exercise, HR and oxygen consumption increased in a linear fashion (Tables 1 and 2). No EMG activity was detected in nonworking limbs during exercise. Femoral blood flow and calculated femoral shear stress increased linearly and progressively during graded arm cycle exercise (Fig. 1). There was an approximately 3.5-fold increase in femoral blood flow from rest to 30 W (286 ± 62 vs 982 ± 252 mL·min−1; P < 0.05). The increase in femoral blood flow was primarily due to increases in mean blood velocity as there were no significant changes in arterial diameter (P > 0.05). During graded leg cycle exercise, brachial blood flow increased progressively in curvilinear fashion (Fig. 2). There was an approximately fourfold increase in blood flow on the brachial artery from rest to 100 W (19 ± 6 vs 77 ± 16 mL·min−1; P < 0.05). The increase in brachial blood flow was mainly attributed to increases in mean blood velocity, and brachial artery diameter did not change significantly (P > 0.05).
The primary finding of the present study is that blood flow to the nonactive limbs increased markedly in proportion to the work intensity. At peak exercise intensities, there were approximately fourfold increases in blood flow (P < 0.05) in both the brachial and femoral arteries during leg and arm exercises. These results suggest that the conduit arteries in the nonworking limbs are exposed to the increases in blood flow and shear stress during exercise, which may lead to enhanced endothelium-dependent vasodilation in the nonworking limbs that was previously observed in exercise intervention studies (9,13,18).
In contrast to the working hypothesis, we found that the blood flow as well as calculated shear stress to the inactive limbs increased significantly during incremental exercise. Our present findings are consistent with those of previous studies (1,4) using a dye dilution or thermodilution technique, gold standards for the measurement of limb blood flow during exercise in humans. These studies demonstrated that blood flow in the inactive leg was several fold higher during moderate arm cycle exercise (1,4). More recently, studies using a Doppler ultrasound technique reported that forearm blood flow increased significantly during lower limb exercise (11,12). However, increases in blood flow in nonworking limbs observed in these studies may be attributed to involuntary muscle contractions of “inactive” limbs. In the present study, the increases in blood flow to nonworking limbs were not a consequence of involuntary muscle contraction during exercise. The EMG recordings during exercise confirmed that the nonworking limbs were indeed “inactive.” This was made possible by the use of a recumbent cycle ergometer and a customized arm cycle ergometer as well as the repeated familiarization sessions that we implemented. A striking finding of the present study is that blood flow to the nonworking limbs increased markedly (as much as three- to fourfold) and significantly (P < 0.05) during graded exercise.
The present experiments do not provide any insight into the mechanisms underlying the increased perfusion in nonworking limbs, and we could only speculate the underlying mechanisms. However, teleologically, why does blood flow increase in nonactive limbs during exercise? One possibility is the removal of lactate by nonworking muscles. During one-leg exercise, the inactive leg takes up lactate (1), and the lactate uptake by inactive muscles is closely related to blood flow to these tissues (4).
Studies that used pharmacological manipulations demonstrated that intraarterial infusion of l-NMMA, an inhibitor of nitric oxide synthase, significantly decreased forearm blood flow during leg exercise (11,12). Their results suggest that production of nitric oxide occurs during exercise even in nonworking vessel beds that do not feed metabolically active tissue. Our present study findings are consistent with their findings and indicate that blood vessels of inactive limbs are exposed with markedly elevated blood flow and shear stress during exercise. Taken together, these results suggest that so-called inactive limbs may be receiving sufficient stimuli from increased blood flow and shear stress, which might result in the modulation of endothelial function in these vascular beds. In addition to nitric oxide described above, it is likely that other endothelium-derived dilator substances in the inactive muscle (e.g., endothelium-derived hyperpolarizing factors and prostanoids) and/or substances released from the active muscle into the general circulation may have contributed to the increases in blood flow in the inactive limbs.
We cannot exclude the possibility that much of the increased perfusion in the nonactive limbs during exercise was directed toward skin and bone rather than to skeletal muscle (7,25). If that is the case, the human and animal literature may not be at odds as skeletal muscle blood flow was directly measured in animal studies (2,14,22). Currently, there is no methodology to assess skeletal muscle blood flow during exercise directly and accurately in humans. However, regardless of where the increased blood flow is directed (to skeletal muscle or to the skin and bone), a more imperative issue to consider for the purpose of the present study is that the conduit arteries of inactive limbs, where endothelial functions were often evaluated (9,13,18), were experiencing increases in blood flow and shear stress during each exercise bout. Long-term or repeated increases in flow exert their effects on endothelial vasodilation by modulating the expression of nitric oxide synthase (8,27). The expression of mRNA for nitric oxide synthase is upregulated when endothelial cells are exposed to shear stress. Thus, these results suggest that the conduit arteries, which do not feed into metabolically active tissues, may be exposed to stimuli that enhance endothelium-dependent vasodilation.
Brachial artery blood flow and shear stress increased curvilinearly with increasing intensity during leg cycling, and blood flow at the initial work rate was below the baseline (resting) values. Previous studies have also observed similar biphasic changes in brachial blood flow with the initial decrease in arm blood flow followed by gradual return to the baseline during fixed low-intensity submaximal leg exercise (6). This might explain the apparent discrepancy between the present study and those findings obtained in classic human studies (5,6). Because short bouts of low-intensity submaximal exercise were used in these previous studies, exercise bouts may not have been strenuous enough or long enough in these studies to show the increase in blood flow to nonworking limbs.
Interestingly, leg blood flow increased linearly, rather than curvilinearly, with increasing exercise intensity during arm cycling. We do not have a reasonable explanation for the site-specific differences, but the different patterns of blood flow response in nonworking limb may be related to the differential impact of retrograde blood flow during diastole (12) and/or differences in local cutaneous vasodilation between upper body (smaller muscle mass and metabolic heat production) and lower body (larger muscle mass and high metabolic demand). Nevertheless, our present findings are consistent with the heterogeneous limb-specific vascular responses to endothelium-dependent vasodilation recently observed in humans (23).
In summary, we found that blood flow and shear stress increase in inactive limbs during incremental exercise. These results suggest that the conduit arteries in the nonworking limbs are exposed to the increases in blood flow and shear stress during exercise and that improvement of endothelial vasodilatory capacity in conduit arteries of nonworking limbs previously observed in exercise intervention studies (9,13,18) may be due to the elevated blood flow and shear stress within these vessels.
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Keywords:©2006The American College of Sports Medicine
ENDOTHELIUM; VASODILATION; DOPPLER ULTRASONOGRAPHY; INACTIVE MUSCLE; HEMODYNAMICS