Rapid blunting of sympathetic vasoconstriction in the human forearm at the onset of exercise

Differential impacts of body composition on oxygen kinetics and exercise tolerance of HFrEF and HFpEF patients

This study aims to (1) compare the kinetics of pulmonary oxygen uptake (VO2p), skeletal muscle deoxygenation ([HHb]), and microvascular O2 delivery (QO2mv) between heart failure (HF) patients with reduced ejection fraction (HFrEF) and those with preserved ejection fraction (HFpEF), and (2) explore the correlation between body composition, kinetic parameters, and exercise performance. Twenty-one patients (10 HFpEF and 11 HFrEF) underwent cardiopulmonary exercise testing to assess VO2 kinetics, with near-infrared spectroscopy (NIRS) employed to measure [HHb]. Microvascular O2 delivery (QO2mv) was calculated using the Fick principle. Dual-energy X-ray absorptiometry (DEXA) was performed to evaluate body composition. HFrEF patients exhibited significantly slower VO2 kinetics (time constant [t]: 63 ± 10.8 s vs. 45.4 ± 7.9 s; P < 0.05) and quicker [HHb] response (t: 12.4 ± 9.9 s vs. 25 ± 11.6 s; P < 0.05). Microvascular O2 delivery (QO2mv) was higher in HFrEF patients (3.6 ± 1.2 vs. 1.7 ± 0.8; P < 0.05), who also experienced shorter time to exercise intolerance (281.6 ± 84 s vs. 405.3 ± 96 s; P < 0.05). Correlation analyses revealed a significant negative relationship between time to exercise and both QO2mv (ρ= -0.51; P < 0.05) and VO2 kinetics (ρ= -0.63). Body adiposity was negatively correlated with [HHb] amplitude (ρ= -0.78) and peak VO2 (ρ= -0.54), while a positive correlation was observed between lean muscle percentage, [HHb] amplitude, and tau (ρ= 0.74 and 0.57; P < 0.05), respectively. HFrEF patients demonstrate more severely impaired VO2p kinetics, skeletal muscle deoxygenation, and microvascular O2 delivery compared to HFpEF patients, indicating compromised peripheral function. Additionally, increased adiposity and reduced lean mass are linked to decreased oxygen diffusion capacity and impaired oxygen uptake kinetics in HFrEF patients.

Hemodynamic responses to the cold pressor test in individuals with metabolic syndrome: a case-control study in a multiracial sample of adults

Previous research shows that exercise pressor and metaboreflex responses are significantly exaggerated in individuals with metabolic syndrome, but it is unclear if these exaggerated responses extend to the cold pressor test (CPT). This study tested the hypothesis that, contrary to previously reported exaggerated responses during exercise, CPT responses would not be significantly exaggerated in individuals with MetS compared to matched controls. Eleven individuals with MetS and eleven control participants matched by age, race, sex, and ethnicity completed a cardiometabolic prescreening and a CPT. Each CPT required participants to immerse their hand in ice water for two minutes while beat-by-beat blood pressure, heart rate (HR), and leg blood flow (LBF) were continuously measured. Leg vascular conductance (LVC) was calculated as LBF divided by mean arterial pressure (MAP). The precent changes in MAP, systolic blood pressure (SBP), diastolic blood pressure (DBP), HR, LBF, and LVC were compared across time (BL vs. Minutes 1 and 2 of CPT) and between groups (MetS vs. Control) using repeated measures analyses of variance. As expected, MAP (f = 32.11, p < 0.001), SBP (f = 23.18, p < 0.001), DBP (f = 40.39, p < 0.001), and HR (f = 31.81, p < 0.001) increased during the CPT, and LBF (f = 4.75, p = 0.014) and LVC (f = 13.88, p < 0.001) decreased. However, no significant main effects of group or group by time interactions were observed (f ≤ 0.391, p ≥ 0.539). These findings indicate that the hemodynamic responses to the CPT are not significantly exaggerated in MetS, and therefore, previous reports of exaggerated exercise pressor and metaboreflex responses in MetS cannot be attributed to generalized sympathetic overexcitability.

Exercise pressor responses are exaggerated relative to force production during, but not following, thirty-minutes of rhythmic handgrip exercise

PURPOSE

This study tested the hypothesis that blood pressure responses would increase relative to force production in response to prolonged bouts of muscular work.

METHODS

Fifteen individuals performed two minutes of static handgrip (SHG; 35% MVC), followed by three minutes of post-exercise-cuff-occlusion (PECO), before and after thirty minutes of rest (control), or rhythmic handgrip exercise (RHG) of the contralateral and ipsilateral forearms. Beat-by-beat recordings of mean arterial pressure (MAP), heart rate (HR), and handgrip force (kg) were averaged across one-minute periods at baseline, and minutes 5, 10, 15, 20, 25, and 30 of RHG. MAP was also normalized to handgrip force, providing a relative measure of exercise pressor responses (mmHg/kg). Hemodynamic responses to SHG and PECO were also compared before and after contralateral RHG, ipsilateral RHG, and control, respectively. Similar to the RHG trial, areas under the curve were calculated for MAP (blood pressure index; BPI) and normalized to the time tension index (BPInorm).

RESULTS

HR and MAP significantly increased during RHG (15.3 ± 1.4% and 20.4 ± 3.2%, respectively, both p < 0.01), while force output decreased by up to 36.6 ± 8.0% (p < 0.01). This resulted in a 51.6 ± 9.4% increase in BPInorm during 30 min of RHG (p < 0.01). In contrast, blood pressure responses to SHG and PECO were unchanged following RHG (all p ≥ 0.07), and only the mean HR (4.2 ± 1.5%, p = 0.01) and ΔHR (67.2 ± 18.1%, p < 0.01) response to SHG were exaggerated following ipsilateral RHG.

CONCLUSIONS

The magnitude of exercise pressor responses relative to force production progressively increases during, but not following, prolonged bouts of muscular work.

Effects of L-Citrulline Supplementation on Endothelial Function and Blood Pressure in Hypertensive Postmenopausal Women

Aging and menopause are associated with decreased nitric oxide bioavailability due to reduced L-arginine (L-ARG) levels contributing to endothelial dysfunction (ED). ED precedes arterial stiffness and hypertension development, a major risk factor for cardiovascular disease. This study investigated the effects of L-citrulline (L-CIT) on endothelial function, aortic stiffness, and resting brachial and aortic blood pressures (BP) in hypertensive postmenopausal women. Twenty-five postmenopausal women were randomized to 4 weeks of L-CIT (10 g) or placebo (PL). Serum L-ARG, brachial artery flow-mediated dilation (FMD), aortic stiffness (carotid-femoral pulse wave velocity, cfPWV), and resting brachial and aortic BP were assessed at 0 and 4 weeks. L-CIT supplementation increased L-ARG levels (Δ13 ± 2 vs. Δ−2 ± 2 µmol/L, p < 0.01) and FMD (Δ1.4 ± 2.0% vs. Δ−0.5 ± 1.7%, p = 0.03) compared to PL. Resting aortic diastolic BP (Δ−2 ± 4 vs. Δ2 ± 5 mmHg, p = 0.01) and mean arterial pressure (Δ−2 ± 4 vs. Δ2 ± 6 mmHg, p = 0.04) were significantly decreased after 4 weeks of L-CIT compared to PL. Although not statistically significant (p = 0.07), cfPWV decreased after L-CIT supplementation by ~0.66 m/s. These findings suggest that L-CIT supplementation improves endothelial function and aortic BP via increased L-ARG availability.

Sex-related differences in rapid-onset vasodilation: impact of aging

Rapid-onset vasodilation (ROV) in response to a single muscle contraction is attenuated with aging. Moreover, sex-related differences in muscle blood flow and vasodilation during dynamic exercise have been observed in young and older adults. The purpose of the present study was to explore if sex-related differences in ROV exist in young (n = 36, 25 ± 1 yr) and older (n = 32, 66 ± 1 yr) adults. Subjects performed single forearm contractions at 10%, 20%, and 40% maximal voluntary contraction. Brachial artery blood velocity and diameter were measured with Doppler ultrasound, and forearm vascular conductance (mL·min^-1·100 mmHg^-1) was calculated from blood flow (mL·min^-1) and mean arterial pressure (mmHg) and used as a measure of ROV. Peak ROV was attenuated in women across all relative intensities in the younger and older groups (P < 0.05). In a subset of subjects with similar absolute workloads (∼5 kg and ∼11 kg), age-related differences in ROV were observed among both women and men (P < 0.05). However, only older women demonstrated an attenuated peak ROV compared with men (91 ± 6 vs. 121 ± 11 mL·min^-1·100 mmHg^-1, P < 0.05), a difference not observed in the young group (134 ± 8 vs. 154 ± 11 mL·min^-1·100 mmHg^-1, P = 0.15). Additionally, examining the slope of peak ROV across contraction intensities indicated a blunted response in older women compared with their younger counterparts (P < 0.05), with no differences observed between older and young men (P = 0.38). Our data suggest that sex-related differences in the rapid vasodilatory response to single muscle contractions exist in older but not young adults, such that older women have a blunted response compared with older men.NEW & NOTEWORTHY While rapid-onset vasodilation (ROV) has been shown to decrease in older individuals, it is unclear if sex contributes to the decline with aging. We sought to identify if sex-related differences exist in the ROV response to single forearm contractions in young and older adults. Our data suggest sex-related differences are present among older but not young individuals, with women having an attenuated response. These data indicate sex plays a role in decreased vasodilation with aging.

Do interindividual differences in cardiac output during submaximal exercise explain differences in exercising muscle oxygenation and ratings of perceived exertion?

Considerable interindividual differences in the Q˙-V˙O2 relationship during exercise have been documented but implications for submaximal exercise tolerance have not been considered. We tested the hypothesis that these interindividual differences were associated with differences in exercising muscle deoxygenation and ratings of perceived exertion (RPE) across a range of submaximal exercise intensities. A total of 31 (21 ± 3 years) healthy recreationally active males performed an incremental exercise test to exhaustion 24 h following a resting muscle biopsy. Cardiac output (Q˙ L/min; inert gas rebreathe), oxygen uptake (V˙O2 L/min; breath-by-breath pulmonary gas exchange), quadriceps saturation (near infrared spectroscopy) and exercise tolerance (6-20; Borg Scale RPE) were measured. The Q˙-V˙O2 relationship from 40 to 160 W was used to partition individuals post hoc into higher (n = 10; 6.3 ± 0.4) versus lower (n = 10; 3.7 ± 0.4, P < 0.001) responders. The Q˙-V˙O2 difference between responder types was not explained by arterial oxygen content differences (P = 0.5) or peripheral skeletal muscle characteristics (P from 0.1 to 0.8) but was strongly associated with stroke volume (P < 0.05). Despite considerable Q˙-V˙O2 difference between groups, no difference in quadriceps deoxygenation was observed during exercise (all P > 0.4). Lower cardiac responders had greater leg (P = 0.027) and whole body (P = 0.03) RPE only at 185 W, but this represented a higher %peak V˙O2 in lower cardiac responders (87 ± 15% vs. 66 ± 12%, P = 0.005). Substantially lower Q˙-V˙O2 in the lower responder group did not result in altered RPE or exercising muscle deoxygenation. This suggests substantial recruitment of blood flow redistribution in the lower responder group as part of protecting matching of exercising muscle oxygen delivery to demand.

Peripheral blood flow regulation in response to sympathetic stimulation in individuals with Down syndrome

Background

Individuals with Down syndrome (DS) experience autonomic dysfunction, with reduced sympathetic and parasympathetic control. This results in alterations in resting heart rate and blood pressure and attenuated responses to sympathoexcitatory stimuli. It is unknown to what extent this impacts the regulation of peripheral blood flow in response to sympathetic stimuli, which is an important prerequisite to exercise and perform work. Therefore, we aimed to investigate differences in peripheral blood flow regulation in response to lower body negative pressure (LBNP) between individuals with and without DS.

Methods

Participants (n=10 males with DS and n=11 male controls, mean age 23.7 years ± 3.2) underwent 5 min of LBNP stimulations (-20 mmHg), after resting supine for 10 min. One minute steady state blood pressure and blood flow at baseline and during LBNP were obtained for analysis. Mean flow velocity and arterial diameters were recorded with ultrasonography; foreram blood flow (FBF), shear rate and forearm vascular conductance (FVC) were calculated using brachial blood pressure measured right before ultrasound recordings.

Results

Participants with DS responded differently (consistent with reduced vasoconstrictive control) to the LBNP stimulus (significant ConditionxGroup interaction effect) for mean velocity (p=0.02), FBF (p=0.04), shear rate (p=0.02) and FVC (p=0.03), compared to participants without DS.

Conclusion

Young males with DS exhibit reduced peripheral regulation of blood flow in response to LBNP compared to controls, indicating a blunted sympathetic control of blood flow. Further research is necessary to explore the impact of these findings on exercise and work capacity.

Pulmonary oxygen uptake and muscle deoxygenation kinetics during heavy intensity cycling exercise in patients with emphysema and idiopathic pulmonary fibrosis

BACKGROUND

Little is known about the mechanistic basis for the exercise intolerance characteristic of patients with respiratory disease; a lack of clearly defined, distinct patient groups limits interpretation of many studies. The purpose of this pilot study was to investigate the pulmonary oxygen uptake ([Formula: see text] O₂) response, and its potential determinants, in patients with emphysema and idiopathic pulmonary fibrosis (IPF).

METHODS

Following a ramp incremental test for the determination of peak [Formula: see text] O₂ and the gas exchange threshold, six emphysema (66 ± 7 years; FEV_1, 36 ± 16%), five IPF (65 ± 12 years; FEV₁, 82 ± 11%) and ten healthy control participants (63 ± 6 years) completed three repeat, heavy-intensity exercise transitions on a cycle ergometer. Throughout each transition, pulmonary gas exchange, heart rate and muscle deoxygenation ([HHb], patients only) were assessed continuously and subsequently modelled using a mono-exponential with ([Formula: see text] O₂, [HHb]) or without (HR) a time delay.

RESULTS

The [Formula: see text] O₂ phase II time-constant (τ) did not differ between IPF and emphysema, with both groups significantly slower than healthy controls (Emphysema, 65 ± 11; IPF, 69 ± 7; Control, 31 ± 7 s; P < 0.05). The HR τ was slower in emphysema relative to IPF, with both groups significantly slower than controls (Emphysema, 87 ± 19; IPF, 119 ± 20; Control, 58 ± 11 s; P < 0.05). In contrast, neither the [HHb] τ nor [HHb]:O₂ ratio differed between patient groups.

CONCLUSIONS

The slower [Formula: see text] O₂ kinetics in emphysema and IPF may reflect poorer matching of O₂ delivery-to-utilisation. Our findings extend our understanding of the exercise dysfunction in patients with respiratory disease and may help to inform the development of appropriately targeted rehabilitation strategies.

Aging is associated with altered vasodilator kinetics in dynamically contracting muscle: role of nitric oxide

We tested the hypothesis that aging would be associated with slowed vasodilator kinetics in contracting muscle in part due to a reduced nitric oxide (NO) bioavailability. Young (n = 10; 24 ± 2 yr) and older (n = 10; 67 ± 2 yr) adults performed rhythmic forearm exercise (4 min each) at 10, 20, and 30% of max during saline infusion (control) and NO synthase (NOS) inhibition. Brachial artery diameter and velocities were measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated for each duty cycle (1 s contraction/2 s relaxation) from forearm blood flow (FBF; ml/min) and blood pressure (mmHg) and fit with a monoexponential model. The main parameters derived from the model were the amplitude of the FBF and FVC response and the number of duty cycles for FBF and FVC to change 63% of the steady-state amplitude (τFBF and τFVC). Under control conditions 1) the amplitude of the FVC response at 30% maximal voluntary contraction (MVC) was lower in older compared with young adults (319 ± 33 vs. 462 ± 52 ml·min(-1)·100 mmHg(-1); P < 0.05) and 2) τFVC was slower in older (10 ± 1, 13 ± 1, and 15 ± 1 duty cycles) compared with young (6 ± 1, 9 ± 1, and 11 ± 1 duty cycles) adults at all intensities (P < 0.05). In young adults, NOS inhibition blunted the amplitude of the FVC response at 30% MVC and prolonged the τFVC at all intensities (10 ± 2, 12 ± 1, and 16 ± 2 duty cycles; P < 0.05), whereas it did not change in older adults. Our data indicate that the blood flow and vasodilator kinetics in exercising muscle are altered with aging possibly due to blunted NO signaling.

Acute ascorbic acid ingestion increases skeletal muscle blood flow and oxygen consumption via local vasodilation during graded handgrip exercise in older adults

Human aging is associated with reduced skeletal muscle perfusion during exercise, which may be a result of impaired endothelium-dependent dilation and/or attenuated ability to blunt sympathetically mediated vasoconstriction. Intra-arterial infusion of ascorbic acid (AA) increases nitric oxide-mediated vasodilation and forearm blood flow (FBF) during handgrip exercise in older adults, yet it remains unknown whether an acute oral dose can similarly improve FBF or enhance the ability to blunt sympathetic vasoconstriction during exercise. We hypothesized that 1) acute oral AA would improve FBF (Doppler ultrasound) and oxygen consumption (V̇o2) via local vasodilation during graded rhythmic handgrip exercise in older adults (protocol 1), and 2) AA ingestion would not enhance sympatholysis in older adults during handgrip exercise (protocol 2). In protocol 1 (n = 8; 65 ± 3 yr), AA did not influence FBF or V̇o2 during rest or 5% maximal voluntary contraction (MVC) exercise, but increased FBF (199 ± 13 vs. 248 ± 16 ml/min and 343 ± 24 vs. 403 ± 33 ml/min; P < 0.05) and V̇o2 (26 ± 2 vs. 34 ± 3 ml/min and 43 ± 4 vs. 50 ± 5 ml/min; P < 0.05) at both 15 and 25% MVC, respectively. The increased FBF was due to elevations in forearm vascular conductance (FVC). In protocol 2 (n = 10; 63 ± 2 yr), following AA, FBF was similarly elevated during 15% MVC (∼ 20%); however, vasoconstriction to reflex increases in sympathetic activity during -40 mmHg lower-body negative pressure at rest (ΔFVC: -16 ± 3 vs. -16 ± 2%) or during 15% MVC (ΔFVC: -12 ± 2 vs. -11 ± 4%) was unchanged. Our collective results indicate that acute oral ingestion of AA improves muscle blood flow and V̇o2 during exercise in older adults via local vasodilation.

Interactive effect of acute sympathetic activation and exercise intensity on the dynamic response characteristics of vascular conductance in the human calf muscle

PURPOSE

The effect of acute activation of the sympathetic nervous system on the dynamic response of muscle hyperaemia during exercise at different intensities is not clear.

METHODS

To explore this, six men performed 16, 5-min bouts of intermittent calf contractions at two intensities (25 and 50 % MVC) and two levels of sympathetic activation (CPT cold pressor test, CON control). Mean arterial pressure (MAP) and leg vascular conductance (LVC leg blood flow/MAP) were measured during rest and contractions (3 s intervals), and dynamic response characteristics of LVC were estimated using curve-fitting and empirical modeling.

RESULTS

MAP was ~20 % greater (P ≤ 0.05) during CPT than CON before and during initial contractions at both intensities. At 25 % MVC, CPT reduced the exercise-induced change in LVC (0.109 vs 0.125 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), an effect attributed to the reduction in the amplitude of the fast growth phase (0.091 vs 0.128 1 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05). At 50 % MVC, CPT also blunted the fast growth phase (0.147 vs 0.189 ml 100 ml(-1 )min(-1 )mmHg(-1); P < 0.05), but the total change in LVC during exercise was unaffected because of a significant reduction in the amplitude of the rapid decay phase and tendency (P = 0.1) for a lower amplitude of the slow decay phase.

CONCLUSION

Increased sympathetic constraint of vasodilation persists during initial contractions but is overcome at the high intensity by a mechanism apparently related to hyperaemic decay.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Influence of α-adrenergic vasoconstriction on the blunted skeletal muscle contraction-induced rapid vasodilation with aging

We tested the hypothesis that elevated sympathetic tone is responsible for lower peak vasodilation after single muscle contractions in older adults. Young (n = 13, 7 men and 6 women, age: 27 ± 1 yr) and older (n = 13, 7 men and 6 women, age: 69 ± 2 yr) adults performed single forearm contractions at 10%, 20%, and 40% of maximum during 1) control, 2) sympathetic activation via lower body negative pressure (LBNP; -20 mmHg), and 3) intra-arterial infusion of phentolamine (α-adrenergic antagonist). Brachial artery diameter and velocities were measured via Doppler ultrasound, and forearm vascular conductance (FVC; in ml·min(-1)·100 mmHg(-1)) was calculated from blood flow (in ml/min) and blood pressure (in mmHg). Peak vasodilator responses [change in (Δ) FVC from baseline] were attenuated in older adults at 20% and 40% of maximum (P < 0.05). LBNP reduced peak ΔFVC at 10% (98 ± 17 vs. 70 ± 12 ml·min(-1)·100 mmHg(-1)), 20% (144 ± 12 vs. 98 ± 3 ml·min(-1)·100 mmHg(-1)), and 40% (209 ± 20 vs. 161 ± 21 ml·min(-1)·100 mmHg(-1), P < 0.01 vs. control) in younger adults but not in older adults (71 ± 11 vs. 68 ± 11, 107 ± 13 vs. 106 ± 16, and 161 ± 22 vs. 144 ± 22 ml·min(-1)·100 mmHg(-1), respectively, P = 0.22-0.99). With phentolamine, peak ΔFVC was enhanced in older adults at each contraction intensity (100 ± 14, 147 ± 22, and 200 ± 26 ml·min(-1)·100 mmHg(-1), respectively, P < 0.01 vs. control) but not in younger adults (94 ± 13, 153 ± 13, and 224 ± 27 ml·min(-1)·100 mmHg(-1), respectively, P = 0.30-0.81 vs. control). Our data indicate that α-adrenergic vasoconstriction and/or blunted functional sympatholysis might contribute to the age-related decreases in skeletal muscle contraction-induced rapid vasodilation in humans.

Effect of hypoxia on the dynamic response of hyperaemia in the contracting human calf muscle

Although systemic hypoxia increases the muscle hyperaemic response during 'steady-state' exercise, its effect on the dynamic characteristics of this response is not clear. In the present study, we first established that hypoxia increases the steady-state hyperaemic response at low workloads during calf exercise. To study dynamic aspects of this response, eight subjects performed eight exercise trials while breathing a normoxic (fractional inspired O(2) = 0.2094) or hypoxic gas mixture (fractional inspired O(2) = 0.105). Subjects performed intermittent contractions (1 s) of the calf muscle at 20% maximal voluntary contraction, and the leg blood flow (LBF), leg vascular conductance (LVC) and EMG activities of the triceps surae muscles were measured during each contraction-relaxation period (3 s). The LBF and LVC responses were averaged for each subject and fitted using a four-phase, exponential growth and decay function. Hypoxia evoked significant increases in the change in LBF (15%) and LVC (23%) from the start to the end of exercise, as well as the amplitude of the rapid growth phase of LBF and LVC (21%). Similar, but non-significant, effects on the amplitude of the slow growth phase of LBF (P = 0.08) and LVC (P = 0.10) were observed. By contrast, hypoxia had no effect on temporal parameters of these growth phases, parameters defining the decay phases or EMG activities. These results suggest that the effect of hypoxia on exercise hyperaemia is targeted at the rapid and perhaps the slow growth phase of the response, and is not mediated by a change in the level of muscle activation.

Exercise increases the angiotensin II effects in isolated portal vein of trained rats

Training in rats adapts the portal vein to respond vigorously to sympathetic stimuli even when the animal is re-exposed to exercise. Moreover, changes in the exercise-induced effects of angiotensin II, a potent venoconstrictor agonist, in venous beds remain to be investigated. Therefore, the present study aimed to assess the effects of angiotensin II in the portal vein and vena cava from sedentary and trained rats at rest or submitted to an exercise session immediately before organ bath experiments. We found that training or exposure of sedentary animals to a single bout of running exercise does not significantly change the responses of the rat portal vein to angiotensin II. However, the exposure of trained animals to a single bout of running exercise enhanced the response of the rat portal vein to angiotensin II. This enhancement appeared to be territory-specific because it was not observed in the vena cava. Moreover, it was not observed in endothelium-disrupted preparations and in preparations treated with N(omega)-nitro-l-arginine methyl ester hydrochloride, indomethacin, BQ-123 or BQ-788. These data indicate that training causes adaptations in the rat portal vein that respond vigorously to angiotensin II even upon re-exposure to exercise. This increased response to angiotensin II requires an enhancement of the vasocontractile influence of endothelin beyond the influence of nitric oxide and vasodilator prostanoids.

Plasma ATP concentration and venous oxygen content in the forearm during dynamic handgrip exercise

Background

It has been proposed that adenosine triphosphate (ATP) released from red blood cells (RBCs) may contribute to the tight coupling between blood flow and oxygen demand in contracting skeletal muscle. To determine whether ATP may contribute to the vasodilatory response to exercise in the forearm, we measured arterialised and venous plasma ATP concentration and venous oxygen content in 10 healthy young males at rest, and at 30 and 180 seconds during dynamic handgrip exercise at 45% of maximum voluntary contraction (MVC).

Results

Venous plasma ATP concentration was elevated above rest after 30 seconds of exercise (P < 0.05), and remained at this higher level 180 seconds into exercise (P < 0.05 versus rest). The increase in ATP was mirrored by a decrease in venous oxygen content. While there was no significant relationship between ATP concentration and venous oxygen content at 30 seconds of exercise, they were moderately and inversely correlated at 180 seconds of exercise (r = -0.651, P = 0.021). Arterial ATP concentration remained unchanged throughout exercise, resulting in an increase in the venous-arterial ATP difference.

Conclusions

Collectively these results indicate that ATP in the plasma originated from the muscle microcirculation, and are consistent with the notion that deoxygenation of the blood perfusing the muscle acts as a stimulus for ATP release. That ATP concentration was elevated just 30 seconds after the onset of exercise also suggests that ATP may be a contributing factor to the blood flow response in the transition from rest to steady state exercise.

Exercise-induced reduction in systemic vascular resistance: a covert killer and an unrecognised resuscitation challenge?

BACKGROUND

Systemic vascular resistance falls in exercise as a consequence of metabolically-linked vasodilatation in active skeletal muscles. This exercise-induced vasodilatation is closely linked with reduced muscle tissue oxygen tension in and is characterised by reduced response to adrenergic vasoconstrictor mechanisms which is often referred to as functional sympatholysis. Systemic arterial blood pressure in exercise is maintained at normal or, more commonly, at elevated levels by increase in cardiac output and increased sympathetic vasomotor tone. Recovery of normal resting skeletal muscle tissue oxygen tension and skeletal muscle vascular tone after exercise depends on the post-exercise recovery process. This process requires ongoing elevated skeletal muscle perfusion and can therefore be predicted to be impaired in shock and cardiopulmonary resuscitation scenarios. Comprehensive consideration of this exercise physiology and its extrapolation into shock, cardiac arrest and resuscitation scenarios supports the proposal that exercise-induced sympatholytic vasodilatation in skeletal muscle may be of considerable unrecognised significance for resuscitation medicine. MAIN HYPOTHESIS: Reduced systemic vascular resistance due to pre-existing exercise-induced sympatholytic vasodilatation in skeletal muscle can significantly exacerbate systemic arterial hypotension in acute shock states and resuscitation scenarios. SUB-HYPOTHESES: 1. Onset of syncope, clinical shock states and pulseless electrical activity can occur at significantly higher cardiac output levels in subjects who were engaged in immediate pre-morbid exercise as compared to resting subjects. 2. The efficacy of external chest compression in generating coronary and cerebral perfusion in cardiopulmonary resuscitation can be significantly impaired when cardiac arrest has occurred during exercise. 3. The efficacy of adrenergic vasopressor agents in resuscitation scenarios can be significantly impaired in subjects who were engaged in immediate pre-morbid exercise.

CURRENT EVIDENCE

The limited available evidence is compatible with the hypothesis being true but does not provide direct confirmation. There is no evidence available directly supporting or refuting the hypothesis.

IMPLICATIONS

Significant potential clinical implications are outlined relating to the management of cardiopulmonary and trauma resuscitation for patients who were involved in immediate pre-morbid exercise, particularly, but not exclusively, at higher exercise intensities. There are also significant potential prognostic implications.

CONCLUSION

Reduction in systemic vascular resistance due to exercise-induced sympatholytic vasodilatation in skeletal muscle may largely explain the reported poor success rate for cardiopulmonary resuscitation with prompt defibrillation for sudden cardiac arrest in young previously healthy athletes. Investigation of this unexplored area of pathophysiology poses major difficulties but could lead to significant improvements in the outcomes of resuscitation for patients who were involved in immediate pre-morbid exercise.

Kinetics of muscle deoxygenation are accelerated at the onset of heavy-intensity exercise in patients with COPD: relationship to central cardiovascular dynamics

Patients with chronic obstructive pulmonary disease (COPD) have slowed pulmonary O2 uptake (V̇o2p) kinetics during exercise, which may stem from inadequate muscle O2 delivery. However, it is currently unknown how COPD impacts the dynamic relationship between systemic and microvascular O2 delivery to uptake during exercise. We tested the hypothesis that, along with slowed V̇o2p kinetics, COPD patients have faster dynamics of muscle deoxygenation, but slower kinetics of cardiac output (Q̇t) following the onset of heavy-intensity exercise. We measured V̇o2p, Q̇t (impedance cardiography), and muscle deoxygenation (near-infrared spectroscopy) during heavy-intensity exercise performed to the limit of tolerance by 10 patients with moderate-to-severe COPD and 11 age-matched sedentary controls. Variables were analyzed by standard nonlinear regression equations. Time to exercise intolerance was significantly ( P < 0.05) lower in patients and related to the kinetics of V̇o2p ( r = −0.70; P < 0.05). Compared with controls, COPD patients displayed slower kinetics of V̇o2p (42 ± 13 vs. 73 ± 24 s) and Q̇t (67 ± 11 vs. 96 ± 32 s), and faster overall kinetics of muscle deoxy-Hb (19.9 ± 2.4 vs. 16.5 ± 3.4 s). Consequently, the time constant ratio of O2 uptake to mean response time of deoxy-Hb concentration was significantly greater in patients, suggesting a slower kinetics of microvascular O2 delivery. In conclusion, our data show that patients with moderate-to-severe COPD have impaired central and peripheral cardiovascular adjustments following the onset of heavy-intensity exercise. These cardiocirculatory disturbances negatively impact the dynamic matching of O2 delivery and utilization and may contribute to the slower V̇o2p kinetics compared with age-matched controls.

The effect of motor activity on the onset and progression of brachial plexus block with bupivacaine: a randomized prospective study in patients undergoing arthroscopic shoulder surgery

BACKGROUND

A decreased latency of onset of neural blockade has been noted when muscular exercise of the hand was performed after supraclavicular brachial plexus block using lidocaine. In this observational study, we examined the effect of repetitive muscle contraction of the hand on the speed of onset of interscalene brachial plexus block (ISB) using bupivacaine.

METHODS

Forty patients were enrolled, all of whom received an ISB as one component of their anesthetic management for elective arthroscopic shoulder surgery. Patients were asked either to rest their arms after the performance of the ISB (nonexercise group) or to perform a repetitive hand exercise for 5 min (exercise group). Bilateral hand grip strength and tolerance to transcutaneous electrical stimulation were used to quantify the degree of motor and sensory blockade.

RESULTS

Patients in the exercise group had a statistically significant lower tolerance to transcutaneous electrical stimulation 20 min after completion of the block (P < 0.05).

CONCLUSIONS

Our results imply that attempting to use a frequency-dependent conduction block with repetitive motor activity as a clinical adjuvant to brachial plexus block with bupivacaine is without merit.

Dynamics of noninvasively estimated microvascular O2 extraction during ramp exercise

Utilization of near-infrared spectroscopy (NIRS) in clinical exercise testing to detect microvascular abnormalities requires characterization of the responses in healthy individuals and theoretical foundation for data interpretation. We examined the profile of the deoxygenated hemoglobin signal from NIRS {deoxygenated hemoglobin + myoglobin [deoxy-(Hb+Mb)] ≈ O2 extraction} during ramp exercise to test the hypothesis that the increase in estimated O2 extraction would be close to hyperbolic, reflecting a linear relationship between muscle blood flow (Q̇m) and muscle oxygen uptake (V̇o2m) with a positive Q̇m intercept. Fifteen subjects (age 24 ± 5 yr) performed incremental ramp exercise to fatigue (15–35 W/min). The deoxy-(Hb+Mb) response, measured by NIRS, was fitted by a hyperbolic function [ f( x) = ax/( b + x), where a is the asymptotic value and b is the x value that yields 50% of the total amplitude] and sigmoidal function { f( x) = f0 + A/[1 + e−(− c+ dx)], where f0 is baseline, A is total amplitude, and c is a constant dependent on d, the slope of the sigmoid}, and the goodness of fit was determined by F test. Only one subject demonstrated a hyperbolic increase in deoxy-(Hb+Mb) ( a = 170%, b = 193 W), whereas 14 subjects displayed a sigmoidal increase in deoxy-(Hb+Mb) ( f0 = −7 ± 7%, A = 118 ± 16%, c = 3.25 ± 1.14, and d = 0.03 ± 0.01). Computer simulations revealed that sigmoidal increases in deoxy-(Hb+Mb) reflect a nonlinear relationship between microvascular Q̇m and V̇o2m during incremental ramp exercise. The mechanistic implications of our findings are that, in most healthy subjects, Q̇m increased at a faster rate than V̇o2m early in the exercise test and slowed progressively as maximal work rate was approached.

Effect of graded leg cycling on postischaemic forearm blood flow in healthy subjects

This study assessed in healthy subjects, the effect of leg cycling on the forearm vascular responses to ischaemia to confirm previous results showing that exercise-induced sympathetic activation during leg cycling reduced postischaemic forearm hyperaemia. Seven young healthy subjects performed two bouts of cycling exercises at 50% and 80% of their maximal aerobic capacity (Ex(50), Ex(80) respectively) during which forearm arterial blood flow was successively occluded for 40, 90 and 180 s. Control forearm blood flow (FBF) and postischaemic forearm blood flow (pi-FBF) measured at the release of arterial occlusions were assessed using plethysmography. Digital arterial pressure was continuously monitored allowing calculation of control and postischaemic forearm conductance (FC and pi-FC respectively). At rest, pi-FBF increased with the duration of ischaemia (5 +/- 1, 19 +/- 3, 29 +/- 3, 31 +/- 4 ml min(-1) 100 ml(-1) after 0, 40, 90 and 180 s of ischaemia respectively). During Ex(50), FBF and pi-FBF did not change significantly although pi-FC was significantly reduced (Deltapi-FC = -39%, -33%, -27% for 40, 90, 180 s of ischaemia respectively). During Ex(80), there was a further dramatic decrease in pi-FC (-53%, -66%, -62% from rest) and pi-FBF were largely blunted (13 +/- 4 versus 19 +/- 3, 14 +/- 4 versus 29 +/- 3, 17 +/- 5 versus 31 +/- 4 ml min(-1) 100 ml(-1)). These results demonstrated that forearm responses to ischaemia depended on leg activities. It was suggested that exercise-induced sympathetic activation may have interfered on local vasodilatation because of ischaemia.

Do vasoregulatory mechanisms in exercising human muscle compensate for changes in arterial perfusion pressure?

We tested the hypothesis that vasoregulatory mechanisms completely counteract the effects of sudden changes in arterial perfusion pressure on exercising muscle blood flow. Twelve healthy young subjects (7 female, 5 male) lay supine and performed rhythmic isometric handgrip contractions (2 s contraction/ 2 s relaxation 30% maximal voluntary contraction). Forearm blood flow (FBF; echo and Doppler ultrasound), mean arterial blood pressure (arterial tonometry), and heart rate (ECG) were measured. Moving the arm between above the heart (AH) and below the heart (BH) level during contraction in steady-state exercise achieved sudden ∼30 mmHg changes in forearm arterial perfusion pressure (FAPP). We analyzed cardiac cycles during relaxation (FBFrelax). In an AH-to-BH transition, FBFrelax increased immediately, in excess of the increase in FAPP (∼69% vs. ∼41%). This was accounted for by pressure-related distension of forearm resistance vasculature [forearm vascular conductance (FVCrelax) increased by ∼19%]. FVCrelax was restored by the second relaxation. Continued slow decreases in FVCrelax stabilized by 2 min without restoring FBFrelax. In a BH-to-AH transition, FBFrelax decreased immediately, in excess of the decrease in FAPP (∼37% vs. ∼29%). FVCrelax decreased by ∼14%, suggesting pressure-related passive recoil of resistance vessels. The pattern of FVCrelax was similar to that in the AH-to-BH transition, and FBFrelax was not restored. These data support rapid myogenic regulation of vascular conductance in exercising human muscle but incomplete flow restoration via slower-acting mechanisms. Local arterial perfusion pressure is an important determinant of steady-state blood flow in the exercising human forearm.

Muscle blood-flow dynamics at exercise onset: do the limbs differ?

Common approaches to understanding control of muscle blood flow in exercise focus on the contributions of various putative vasoregulatory mechanisms to the magnitude of the steady-state response. The application of systems-control principles offers a unique approach to characterizing and quantifying the non-steady-state adaptation of muscle blood flow with exercise onset. Information gained from this approach provides novel insight into the nature of control mechanisms governing physiological responses to exercise. This review is intended to provide the reader with an understanding of 1) exercise models, methodology for measuring muscle blood flow, and analysis approaches for quantifying muscle blood-flow dynamics; 2) what is currently known about the dynamic response of muscle blood-flow control mechanisms in humans; and 3) the similarities and differences in exercising muscle blood-flow control in the upper versus the lower limbs in humans.

Rapid vasoregulatory mechanisms in exercising human skeletal muscle: dynamic response to repeated changes in contraction intensity

We tested the hypothesis that vasoregulatory mechanisms exist in humans that can rapidly adjust muscle blood flow to repeated increases and decreases in exercise intensity. Six men and seven women (age, 24.4 ± 1.3 yr) performed continuous dynamic forearm handgrip contractions (1- to 2-s contraction-to-relaxation duty cycle) during repeated step increases and decreases in contraction intensity. Three step change oscillation protocols were examined: Slow (7 contractions per contraction intensity × 10 steps); Fast (2 contractions per contraction intensity × 15 steps); and Very Fast (1 contraction per contraction intensity × 15 steps). Forearm blood flow (FBF; Doppler and echo ultrasonography), heart rate (ECG), and mean arterial pressure (arterial tonometry) were examined for the equivalent of a cardiac cycle during each relaxation phase (FBFrelax). Mean arterial pressure and heart rate did not change during repeated step changes ( P = 0.352 and P = 0.190). For both Slow and Fast conditions, relaxation phase FBFrelaxadjusted immediately and repeatedly to both increases and decreases in contraction intensity, and the magnitude and time course of FBFrelaxchanges were virtually identical. For the Very Fast condition, FBFrelaxincreased with the first contraction and thereafter slowly increased over the course of repeated contraction intensity oscillations. We conclude that vasoregulatory mechanisms exist in human skeletal muscle that are capable of rapidly and repeatedly adjusting muscle blood flow with ongoing step changes in contraction intensity. Importantly, they demonstrate symmetry in response magnitude and time course with increasing versus decreasing contraction intensity but cannot adjust to very fast exercise intensity oscillations.

Forelimb postischaemic reactive hyperaemia is impaired by hypotensive low body negative pressure in healthy subjects

Local metabolic conditions adapt blood supply to metabolic requirement by a direct effect on vascular smooth muscles and indirectly by modulating sympathetic vasoconstrictor effectiveness. During exercise, sympathetic nervous activity could in turn interfere on local metabolic control of vascular tone and restrain blood flow to active muscles. In order to investigate that interaction non-invasively, we measured postischaemic reactive hyperaemia (RH) in the forelimb of eight healthy young men (22.7 +/- 2.1 years) at rest and during two levels of sympathetic stimulation using low body negative pressure (LBNP -15 and -30 mmHg). During every stages, RH was measured after 40, 60, 90 and 180 s of arterial occlusion, respectively. In control conditions, RH rose with duration of ischaemia (18.9, 24.2, 30.4, 33.1 ml min(-1) per 100 ml(-1) for 40, 60, 90 and 180 s of ischaemia, respectively). During non-hypotensive LBNP (-15 mmHg) sympathetic activation was associated with decreased forelimb blood flow (6.4 +/- 0.9 versus 3.9 +/- 0.6 ml min(-1) per 100 ml(-1), P<0.01), but RH were not significantly different from control conditions. During hypotensive tachycardia LBNP (-30 mmHg), RH were significantly lower than under the previous LBNP stage. This fall in RH was greater after the shortest gap of ischaemia and tapered off as arterial occlusion gap increased (-22.3, -13.1, -10.5 and -8.7% for 40, 60, 90 and 180 s of ischaemia, respectively). These results suggested that vascular tone adaptation to local metabolic conditions was modified by sympathetic nervous activation. This was particularly marked when an hypotensive-mediated sympathetic stimulation was opposed to short gaps of ischaemia.

Human femoral artery and estimated muscle capillary blood flow kinetics following the onset of exercise

The purpose of this study was to compare the kinetics of estimated capillary blood flow (Qcap) to those of femoral artery blood flow (QFA) and estimated muscle oxygen uptake (VO2m). Nine healthy subjects performed a series of transitions from rest to moderate (below estimated lactate threshold, 6 min bouts) knee extension exercise. Pulmonary oxygen uptake (VO2) was measured breath by breath, (QFA) was measured continuously using Doppler ultrasound, and deoxyhaemoglobin ([HHb]) was estimated by near-infrared spectroscopy over the rectus femoris throughout the tests. The time course of (Qcap) was estimated by rearranging the Fick equation (i.e. Qcap = VO2m/(a-v)O2), (arterio - venous O2 difference) using the primary component of VO2 to represent VO2m and [HHb] as a surrogate for (a - v)O2. The overall kinetics of QFA (mean response time, MRT, 13.7 +/- 7.0 s), VO2m (tau, 27.8 +/- 9.0 s) and Qcap (MRT, 41.4 +/- 19.0 s) were significantly (P < 0.05) different from each other. We conclude that for moderate intensity knee extension exercise, conduit artery blood flow (QFA) kinetics may not be a reasonable approximation of blood flow kinetics in the microcirculation (Qcap), the site of gas exchange. This temporal dissociation suggests that blood flow may be controlled differently at the conduit artery level than in the microcirculation.

Effects of arterial hypotension on microvascular oxygen exchange in contracting skeletal muscle

In healthy animals under normotensive conditions (N), contracting skeletal muscle perfusion is regulated to maintain microvascular O2 pressures (Pmv[Formula: see text]) at levels commensurate with O2 demands. Hypovolemic hypotension (H) impairs muscle contractile function; we tested whether this condition would alter the matching of O2 delivery (Q̇o2) to O2 utilization (V̇o2), as determined by Pmv[Formula: see text] at the onset ofmuscle contractions. Pmv[Formula: see text] in the spinotrapezius muscles of seven female Sprague-Dawley rats (280 ± 6 g) was measured every 2 s across the transition from rest to 1-Hz twitch contractions. Measurements were made under N (mean arterial pressure, 97 ± 4 mmHg) and H (induced by arterial section; mean arterial pressure, 58 ± 3 mmHg, P < 0.05) conditions; Pmv[Formula: see text] profiles were modeled using a multicomponent exponential fitted with independent time delays. Hypotension reduced muscle blood flow at rest (24 ± 8 vs. 6 ± 1 ml−1·min−1·100 g−1 for N and H, respectively; P < 0.05) and during contractions (74 ± 20 vs. 22 ± 4 ml−1·min−1·100 g−1 for N and H, respectively; P < 0.05). H significantly decreased resting Pmv[Formula: see text] and steady-state contracting Pmv[Formula: see text](19.4 ± 2.4 vs. 8.7 ± 1.6 Torr for N and H, respectively, P < 0.05). At the onset of contractions, H reduced the time delay (11.8 ± 1.7 vs. 5.9 ± 0.9 s for N andH, respectively, P < 0.05) before the fall in Pmv[Formula: see text] and accelerated therate of Pmv[Formula: see text] decrease (time constant, 12.6 ± 1.4 vs. 7.3 ± 0.9 s for N and H, respectively, P < 0.05). Muscle V̇o2 was reduced by 71% at rest and 64% with contractions in H vs. N, and O2 extraction during H averaged 78% at rest and 94% during contractions vs. 51 and 78% in N. These results demonstrate that H constrains the increase of skeletal muscle Q̇o2 relative to that of V̇o2 at the onset of contractions,leading to a decreased Pmv[Formula: see text]. According to Fick's law, this scenario will decrease blood-myocyte O2 flux, thereby slowing V̇o2 kinetics and exacerbating the O2 deficit generated at exercise onset.

Dynamic response characteristics of local muscle blood flow regulatory mechanisms in human forearm exercise

We sought to understand the nature of control mechanisms involved in the adaptation of exercising muscle hyperemia. Seven subjects performed rhythmic dynamic forearm exercise under two exercise conditions: small step 1 [step increase from rest to 40% peak forearm vascular conductance (FVC), in ml.min(-1).100 mmHg-1] for 5 min followed by small step 2 (further increase to 80% peak FVC for 5 min), and large step (step increase from rest to 80% peak FVC for 5 min). FVC data were fit with a two- (small step 1) and three-component (small step 2, large step) exponential as appropriate. For the rapid phase I response, FVC dynamic response characteristics (time delay, time constant) were not affected by the magnitude of the work intensity increase when the transition began from rest, but were slower in the 40-80% transition. Rest-80% gain was greater than either rest-40% or 40-80% transitions but represented the same proportion of the phase I + phase II gain across all transitions (57 vs. 56 vs. 57%, respectively, P = 0.975). For the slower phase II response, dynamic response characteristics were not affected by the magnitude of the work intensity increase when initiated from rest. The time constant was not altered when the transition began from exercise vs. rest. We conclude that 1) dynamic response characteristics of exercise hyperemia control mechanisms are not affected by the magnitude of work rate increase when forearm exercise is initiated from rest, 2) phase I but not phase II dynamic response characteristics are sensitive to baseline exercise intensity, and 3) the mechanisms contributing to phase I result in the same relative response magnitude, regardless of the size of the step increase in exercise intensity or the baseline from which it is initiated.

Comparison of forearm blood flow responses to incremental handgrip and cycle ergometer exercise: relative contribution of nitric oxide

The contribution of endothelium-derived nitric oxide (NO) to exercise hyperaemia remains controversial. Disparate findings may, in part, be explained by different shear stress stimuli as a result of different types of exercise. We have directly compared forearm blood flow (FBF) responses to incremental handgrip and cycle ergometer exercise in 14 subjects (age +/-s.e.m.) using a novel software system which calculates conduit artery blood flow continuously across the cardiac cycle by synchronising automated edge-detection and wall tracking of high resolution B-mode arterial ultrasound images and Doppler waveform envelope analysis. Monomethyl arginine (L-NMMA) was infused during repeat bouts of each incremental exercise test to assess the contribution of NO to hyperaemic responses. During handgrip, mean FBF increased with workload (P < 0.01) whereas FBF decreased at lower cycle workloads (P < 0.05), before increasing at 120 W (P < 0.001). Differences in these patterns of mean FBF response to different exercise modalities were due to the influence of retrograde diastolic flow during cycling, which had a relatively larger impact on mean flows at lower workloads. Retrograde diastolic flow was negligible during handgrip. Although mean FBF was lower in response to cycling than handgrip exercise, the impact of L-NMMA was significant during the cycle modality only (P < 0.05), possibly reflecting the importance of an oscillatory antegrade/retrograde flow pattern on shear stress-mediated release of NO from the endothelium. In conclusion, different types of exercise present different haemodynamic stimuli to the endothelium, which may result in differential effects of shear stress on the vasculature.

Neural control of muscle blood flow during exercise

Activation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure. Thus the interaction between somatic and sympathetic neuroeffector pathways underlies blood flow control to skeletal muscle during exercise. Muscle blood flow increases in proportion to the intensity of activity despite concomitant increases in sympathetic neural discharge to the active muscles, indicating a reduced responsiveness to sympathetic activation. However, increased sympathetic nerve activity can restrict blood flow to active muscles to maintain arterial blood pressure. In this brief review, we highlight recent advances in our understanding of the neural control of the circulation in exercising muscle by focusing on two main topics: 1) the role of motor unit recruitment and muscle fiber activation in generating vasodilator signals and 2) the nature of interaction between sympathetic vasoconstriction and functional vasodilation that occurs throughout the resistance network. Understanding how these control systems interact to govern muscle blood flow during exercise leads to a clear set of specific aims for future research.

Muscle Contraction-Induced Limb Blood Flow Variability during Dynamic Knee Extensor

PURPOSE

To evaluate whether muscle contraction-induced variability of limb femoral arterial blood flow (FABF) can be reduced with longer sampling durations. This was assessed in relation to muscle contraction-relaxation cycles (CRcycles) during steady-state, one-legged, dynamic knee-extensor exercise (KEE) at varying "exercise intensities" and "contraction frequencies."

METHODS

Eleven male subjects performed steady-state KEE at 10-40 W at 30 and 60 contractions per minute (cpm). FABF (Doppler ultrasound) and contraction-relaxation-induced variability in FABF was determined for 1-, 2-, 5-, 10-, 15-, 20-, and 30-CRcycles during approximately 4-min steady-state KEE. Variability was determined as coefficients of variation (CV).

RESULTS

During KEE at 30 and 60 cpm CVFABF was significantly higher for 1-CRcycles (12.3% and 15.5%) and 2-CRcycles (9.6% and 11.8%) than for 30-CRcycles (4.0% and 5.2%), but similar for 10-CRcycles to 30-CRcycles at all work rates and contraction frequencies. The CVFABF between work rates at 30 and 60 cpm did not statistically differ (P = NS) for any of the CRcycle measurements. However, the single CRcycles-induced CVFABF at 60 cpm was significantly higher (P < 0.05) than that at 30 cpm at the lower exercise intensities of 10 and 20 W, but with no significant difference at 30 and 40W.

CONCLUSION

Limb blood flow variability was markedly reduced with a longer sampling measurement of at least 10-CRcycles, which had a CVFABF of approximately 5%. Furthermore, the 1-CRcycle-induced FABF variability was similar at each exercise intensity, but significant variations were seen between contraction frequencies at lower exercise intensities. It is speculated the difference between the contraction frequencies at lower exercise intensities may be due to the muscle contraction-relaxation-induced variations in muscle force (intramuscular pressure), along with the superimposed blood pressure waves.