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

Data analysis technique influences blood flow kinetics parameter estimates for moderate- and heavy-intensity exercise transitions

NEW FINDINGS

What is the central question of this study? During exercise, there are fluctuations in conduit artery blood flow (BF) caused by both cardiac and muscle contraction-relaxation cycles. What is the optimal method to process Doppler ultrasound-measured BF for the purpose of characterizing the dynamic response of BF during step-transitions in exercise? What is the main finding and its importance? Continuous BF data were processed in relation to either cardiac or muscle contraction-relaxation cycles and computed based on 'binned' or 'rolling' averages over one, two or five consecutive cycles. Kinetics characterization revealed no data processing technique-specific differences in steady-state BF, but variability in the rapidity at which BF attained steady-state (i.e., mean response time) was observed.

ABSTRACT

The overall rate of blood flow (BF) adjustment (i.e., kinetics) from the onset of an exercise transition can be quantified by the mean response time (MRT). However, the BF response profile can be distorted during rhythmic, dynamic exercise consequent to variations caused by the cardiac cycle (HR) and the muscle contraction-relaxation (CR) cycle. We examined the extent to which distortions imposed by HR and CR cycles affected BF kinetics. Eight healthy, young men (27 (4) years; mean (SD)) performed transitions of alternate-leg knee-extension exercise from 3 W to either a moderate- (MOD) or heavy-intensity (HVY) power output. Femoral artery BF was continuously measured by Doppler ultrasound and averaged over one, two or five 'binned' (e.g., HR2b, etc.) or 'rolling' (e.g., CR5r, etc.) HR and CR cycles. Among analysis techniques, there were no differences for steady-state BF values at the 3 W baseline. In MOD, MRT using contraction-relaxation cycle (CR1) was smaller than most other analysis techniques. For both MOD and HVY, the 95% confidence interval for MRT was generally larger when using HR- compared to CR-related methods, and monoexponential fits based on 'rolling' averages (HR2r, HR5r, CR2r, CR5r) had a poorer ability to estimate the true end-exercise BF in HVY than in MOD. When modelling BF kinetics, we conclude that the CR1 method is a good option because of its ability to accurately estimate the 'data-determined' end-exercise BF value from the 'model-derived' response, maintain a relatively high density of data points during the transition and yield a relatively small 95% CI.

Mechanical compression during repeated sustained isometric muscle contractions and hyperemic recovery in healthy young males

BACKGROUND

An elevated intramuscular pressure during a single forearm isometric muscle contraction may restrict muscle hyperemia. However, during repeated isometric exercise, it is unclear to what extent mechanical compression and muscle vasodilatation contribute to the magnitude and time course of beat-to-beat limb hemodynamics, due to alterations in leg vascular conductance (LVC).

METHODS

In eight healthy male subjects, the time course of both beat-to-beat leg blood flow (LBF) and LVC in the femoral artery was determined between repeated 10-s isometric thigh muscle contractions and 10-s muscle relaxation (a duty cycle of 20 s) for steady-state 120 s at five target workloads (10, 30, 50, 70, and 90% of maximum voluntary contraction (MVC)). The ratio of restricted LBF due to mechanical compression across workloads was determined by the formula (relaxation LBF--contraction LBF)/relaxation LBF (%).

RESULTS

The exercise protocol was performed completely by all subjects (≤ 50% MVC), seven subjects (≤ 70% MVC), and two subjects (≤ 90% MVC). During a 10-s isometric muscle contraction, the time course in both beat-to-beat LBF and LVC displayed a fitting curve with an exponential increase (P < 0.001, r (2) ≥ 0.956) at each workload but no significant difference in mean LBF across workloads and pre-exercise. During a 10-s muscle relaxation, the time course in both beat-to-beat LBF and LVC increased as a function of workload, followed by a linear decline (P < 0.001, r (2) ≥ 0.889), that was workload-dependent, resulting in mean LBF increasing linearly across workloads (P < 0.01, r (2) = 0.984). The ratio of restricted LBF can be described as a single exponential decay with an increase in workload, which has inflection point distinctions between 30 and 50% MVC.

CONCLUSIONS

In a 20-s duty cycle of steady-state repeated isometric muscle contractions, the post-contraction hyperemia (magnitude of both LBF and LVC) during muscle relaxation was in proportion to the workload, which is in agreement with previous findings. Furthermore, time-dependent beat-to-beat muscle vasodilatation was seen, but not restricted, during isometric muscle contractions through all target workloads. Additionally, the relative contribution of mechanical obstruction and vasodilatation to the hyperemia observed in the repeated isometric exercise protocol was non-linear with regard to workload. In combination with repeated isometric exercise, the findings could potentially prove to be useful indicators of circulatory adjustment by mechanical compression for muscle-related disease.

Repeatability of popliteal blood flow and lower limb vascular conductance at rest and exercise during body tilt using Doppler ultrasound

We tested the data repeatability for popliteal blood flow velocity (PBV), popliteal arterial diameter (AD(pop)), popliteal blood flow (PBF) and lower limb vascular conductance (VC) at rest and exercise in three body positions, two work rates and two inspired oxygen fractions. Fifteen, eleven and ten healthy volunteers participated in the three phases of the studies. Resting protocols were performed in horizontal (HOR), 35° head-down tilt (HDT) and 45° head-up tilt (HUT) for 5 min in each body position. Participants also exercised at lower and higher power outputs (repeated plantar flexion contractions at 20% and 30% maximal voluntary contraction, respectively) in HOR, HDT and HUT and in normoxia (21%O2) and hypoxia (14%O2) with the same work rates and body positions. PBV and AD(pop) were measured by ultrasound to determine PBF, and VC was estimated by dividing PBF by muscle perfusion pressure (MPP). PBV, AD(pop), PBF and VC were not different, demonstrated good agreement and consistency between the two days of testing during both rest and exercise conditions regardless of body position. Therefore, these data support the utilization of Doppler and echo Doppler ultrasound as a reproducible method to measure PBV and AD(pop) and consequently estimate PBF and VC responses in such conditions.

Physiological aspects of the determination of comprehensive arterial inflows in the lower abdomen assessed by Doppler ultrasound

Non-invasive measurement of splanchnic hemodynamics has been utilized in the clinical setting for diagnosis of gastro-intestinal disease, and for determining reserve blood flow (BF) distribution. However, previous studies that measured BF in a "single vessel with small size volume", such as the superior mesenteric and coeliac arteries, were concerned solely with the target organ in the gastrointestinal area, and therefore evaluation of alterations in these single arterial BFs under various states was sometimes limited to "small blood volumes", even though there was a relatively large change in flow. BF in the lower abdomen (BF(Ab)) is potentially a useful indicator of the influence of comprehensive BF redistribution in cardiovascular and hepato-gastrointestinal disease, in the postprandial period, and in relation to physical exercise. BF(Ab) can be determined theoretically using Doppler ultrasound by subtracting BF in the bilateral proximal femoral arteries (FAs) from BF in the upper abdominal aorta (Ao) above the coeliac trunk. Prior to acceptance of this method of determining a true BF(Ab) value, it is necessary to obtain validated normal physiological data that represent the hemodynamic relationship between the three arteries. In determining BF(Ab), relative reliability was acceptably high (range in intra-class correlation coefficient: 0.85-0.97) for three arterial hemodynamic parameters (blood velocity, vessel diameter, and BF) in three repeated measurements obtained over three different days. Bland-Altman analysis of the three repeated measurements revealed that day-to-day physiological variation (potentially including measurement error) was within the acceptable minimum range (95% of confidence interval), calculated as the difference in hemodynamics between two measurements. Mean BF (ml/min) was 2951 ± 767 in Ao, 316 ± 97 in left FA, 313 ± 83 in right FA, and 2323 ± 703 in BF(Ab), which is in agreement with a previous study that measured the sum of BF in the major part of the coeliac, mesenteric, and renal arteries. This review presents the methodological concept that underlies BF(Ab), and aspects of its day-to-day relative reliability in terms of the hemodynamics of the three target arteries, relationship with body surface area, respiratory effects, and potential clinical usefulness and application, in relation to data previously reported in original dedicated research.

Relationship between reduced lower abdominal blood flows and heart rate in recovery following cycling exercise

AIM

To examine the blood flow (BF) response in the lower abdomen (LAB) in recovery following upright cycling exercise at three levels of relative maximum pulmonary oxygen consumption (VO(2max)) and the relationship of BF(LAB) to heart rate (HR) and target intensity.

METHODS

For 11 healthy subjects, BF (Doppler ultrasound) in the upper abdominal aorta (Ao) above the coeliac trunk and in the right femoral artery (RFA) was measured repeatedly for 720 s after the end of cycling exercises at target intensities of 30%, 50% and 85% VO(2max), respectively. Blood flow in the lower abdomen (BF(LAB)) can be measured by subtracting bilateral BF(FAs) (≈twofolds of BF(RFA)) from BF(Ao). Change in BF(LAB) (or BF(LAB) volume) at any point was evaluated by difference between change in BF(Ao) and in BF(FAs). Heart rate and blood pressure were also measured.

RESULTS

At 85% VO(2max), significant reduction in BF(LAB) by approx. 89% was shown at 90 s and remained until 360 s. At 50% VO(2max), reduction in BF(LAB) by approx. 33% was found at 90 s although it returned to pre-exercise value at 120 s. On the contrary at 30% VO(2max), BF(LAB) showed a light increase (<20%) below 70 bpm of HR. There was a close negative relationship (P < 0.05) between change in BF(LAB) and recovery HR, as well as between change in BF(LAB) volume and both recovery HR and % VO(2max).

CONCLUSION

This study suggests that the lower abdominal BF in recovery may be influenced by sympathetic-vagus control, and dynamics of BF(LAB) may be closely related to the level of relative exercise intensities.

Determination of comprehensive arterial blood inflow in abdominal-pelvic organs: impact of respiration and posture on organ perfusion

Background

Arterial blood flow (BF) to all abdominal-pelvic organs (AP) shows potential for an indicator of comprehensive splanchnic organ circulation (reservoir of blood supply for redistribution) in cardiovascular disease, hepato-gastrointestinal disease or hemodynamic disorders. Our previous assessment of splanchnic hemodynamics, as magnitude of BF_AP [measuring by subtracting BF in both femoral arteries (FAs) from the upper abdominal aorta (Ao) above the celiac trunk] using Doppler ultrasound, was reported as the relationship between Ao and FAs, day-to-day variability and response to exercise. For accurate determination of BF_AP, it is important to consider the various factors that potentially influence BF_AP. However, little information exists regarding the influence of respiration (interplay between inspiration and expiration) and posture on BF_AP.

Material/Methods

Ten healthy males were evaluated in sitting/supine positions following a 12 hr fast. Magnitude of BF_AP was determined as measurement of Ao and FAs hemodynamics (blood velocity and vessel diameter) using pulsed Doppler with spectral analysis during spontaneous 4-sec inspiration/4-sec expiration phases.

Results

BF/blood velocity in the Ao and FAs showed significant lower in inspiration than expiration. BF_AP showed a significant (P<0.005) reduction of ~20% in inspiratory phase (sitting, 2213±222 ml/min; supine, 2059±215 ml/min) compared with expiratory phase (sitting, 2765±303 ml/min; supine, 2539±253 ml/min), with no difference between sitting and supine.

Conclusions

Respiratory-related to alterations in BF_AP were observed. It may be speculated that changes in intra-abdominal pressure during breathing (thoracic-abdominal movement) is possibly reflecting transient changes in blood velocity in the Ao and FAs. Respiratory effects should be taken into account for evaluation of BF_AP.

Femoral artery blood flow and its relationship to spontaneous fluctuations in rhythmic thigh muscle workload

BACKGROUND AND AIM

Limb femoral arterial blood flow (LBF) is known to increase linearly with increasing workload under steady-state conditions, suggesting a close link between LBF and metabolic activity. We, however, hypothesized that sudden physiological and spontaneous changes in exercise rhythm, and consequently workload temporarily alter blood flow to the working muscle. LBF and its relation to fluctuations in the contraction rhythm and workload were therefore investigated.

METHODS

LBF, measured by Doppler ultrasound, and the achieved workload, were continuously measured in nine subjects, aiming to perform steady-state, one-legged, dynamic knee-extensor exercise at 30 and 60 contractions per minute (cpm), at incremental target workloads of 10, 20, 30 and 40 W.

RESULTS

In agreement with previous findings, LBF increased positively and linearly (P<0.05) with increasing target workload. However, LBF was inversely and linearly related (P<0.05) to the actually achieved workload, when measured over 60 consecutive contraction-relaxation cycle bouts, for each target intensity at 30 and 60 cpm respectively. Thus any sudden spontaneous increase or decrease in the achieved workload transiently altered the relationship between LBF and the achieved workload. The influence upon the magnitude of LBF, due to fluctuations in the achieved workload from the target workload, was furthermore similar between target workload sessions at 30 and 60 cpm respectively. LBF was, however, not associated with variations in the contraction frequencies.

CONCLUSIONS

These findings indicate that a transient sudden increase in the workload more rapidly impedes LBF and that vasodilatation may be elicited to restore the intensity related steady-state LBF response in relation to the average metabolic activity.

Resting mechanomyography before and after resistance exercise

A number of mechanisms have been proposed to explain the elevation in oxygen consumption following exercise. Biochemical processes that return muscle to its pre-exercise state do not account for all of the extra oxygen consumed after exercise (excess post-exercise oxygen consumption, EPOC). Muscle at rest after aerobic exercise produces mechanomyographic (MMG) activity of increased amplitude, compared to the pre-exercise state, which declines exponentially with the same time constant as EPOC. The purpose of this study was to determine how the resting MMG is affected by resistance exercise, and whether any change is related to oxygen consumption (VO(2)). Ten young male subjects (22.9 years) performed 30 min of resistance exercise consisting of one set of 10 repetitions at 50% 1-repetition maximum (1-RM) followed by five sets of eight repetitions at 75% of 1-RM for leg press and leg (knee) extension, with 1 min rest between sets. Oxygen consumption was measured by indirect calorimetry, MMG by an accelerometer placed over the rectus femoris, and surface electromyogram (EMG) with electrodes placed distal to the accelerometer. Recordings were made before exercise and for 5.5 h after exercise. MMG activity, expressed as mean absolute acceleration, was significantly elevated after exercise (P = 0.0006), as was EMG activity expressed as root-mean-square voltage (P = 0.03). MMG and VO(2) demonstrated exponential decay after exercise with similar time constants of 7.5 +/- 2.2 and 7.2 +/- 1.0 min, respectively. We conclude that resting muscle is more mechanically active following resistance exercise and that this may contribute to an elevated VO(2).

Arterial blood flow of all abdominal-pelvic organs using Doppler ultrasound: range, variability and physiological impact

The pulsed Doppler method theoretically enables human arterial blood flow (BF) to be determined in all of the abdominal-pelvic organs (BF(AP)) by subtracting the bilateral proximal femoral arterial BF from the upper abdominal aorta BF above the coeliac trunk. Evaluation of BF(AP) is a potentially useful indicator of exercise or food intake related flow distribution to organs; however, there is a lack of information regarding the physiological significance of BF(AP), and the measurements are yet to be validated. The aims of the present study are to examine the range in BF(AP) among subjects, monitor physiological day-to-day variability in BF(AP) over three different days and then determine whether mean BF(AP) (averaged over the three different measurement days) is related to body surface area (BSA). Forty healthy males (19-39 years) with a wide range of body weights (51-89 kg) were evaluated in a sitting position following a 12 h fast. The above-mentioned three conduit arteries were measured to determine BF(AP) using pulsed Doppler with spectral analysis. The mean BF(AP) was 2078 +/- 495 ml min(-1) (mean +/- SD) (range, 1153-3285 ml min(-1)), which is in agreement with a previous study that measured the sum of BF in the major part of the coeliac, mesenteric and renal arteries. The physiological day-to-day variability (mean coefficient of variation) was 14.5 +/- 10.0%. Significant (p < 0.05) positive linear relationships were observed between BF(AP) and BSA as well as body weight, which is in good agreement with the results of a previous study. The present data suggest that BF(AP) determined by three-conduit arterial hemodynamics may be a valid measurement that encompasses physiologic flow to multiple abdominal-pelvic organ systems.

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.

Femoral and Axillary Ultrasound Blood Flow during Exercise

PURPOSE

To use Doppler ultrasound 1) to assess the relationship between exercise intensity and changes in femoral and axillary artery diameter, 2) to determine whether volume blood flow (BF) measured during early recovery accurately reflects exercise BF, and 3) to assess the influence of artery caliber and/or site as well as exercise intensity on BF measurement reproducibility.

METHODS

Thirteen healthy subjects (mean age 25.9+/-7.7 yr) performed progressive and maximal leg-extension (LE) and elbow-flexion (EF) exercises in the supine position. The duration of each stage was 150 s, followed by a 30-s recovery period. Arterial diameter and blood flow velocity were recorded simultaneously and continuously during the last 30 s of exercise as well as 30 s into recovery.

RESULTS

Arterial dilation was 3.5 and 6.5% at maximal effort in femoral and axillary arteries, respectively. A significant increase was observed for both arteries from workload 2 to peak exercise when arterial cross-sectional area was calculated. Blood flow velocity during the recovery period was significantly different from end-exercise values, depending on time and workload. The coefficients of variation of BF measurement during exercise were 7.1-12.1% and 6.4-9.5% in LE and EF, respectively.

CONCLUSION

This study showed that BF measurement with Doppler ultrasound during exercise is reproducible but requires measurement of arterial diameter at each workload. Measurements performed immediately after exercise cannot be used as a surrogate for blood flow velocity during exercise.

Frequency-domain characteristics and filtering of blood flow following the onset of exercise: implications for kinetics analysis

We examined the validity and usefulness of a low-pass filter (LPFILTER) to reduce point-to-point variability and enhance parameter estimation of the kinetics of blood flow (BF). Computer simulations were used to determine the power spectrum of simulated responses. Moreover, we studied the leg BF response to a single transition in four subjects during supine knee-extension exercise using three methods of data processing [beat-by-beat, average of 3 cardiac cycles (AVG3 BEATS), and LPFILTER]. The power spectrum of BF containing the kinetics information (≤0.2 Hz) did not overlap with the oscillations due to muscle contraction and cardiac cycle (simulations and Doppler measurements). There were no significant differences between the parameter estimates for a two-exponential model using Beat-by-Beat, AVG3 BEATS, and LPFILTER ( P > 0.05; n = 4). However, LPFILTER (cutoff = 0.2 Hz) resulted in a significantly lower standard error of the estimate for all parameters ( P < 0.05). The means ± SD for the standard error of the estimate for Beat-by-Beat, AVG3 BEATS, and LPFILTER were, respectively, time constant- phase 1 = 5.0 ± 1.1 s, 4.5 ± 2.1 s, and 0.3 ± 0.2 s; time delay- phase 2 = 17.8 ± 7.9 s, 12.8 ± 7.5 s, and 1.4 ± 1.4 s; time constant- phase 2 = 15.8 ± 4.6 s, 9.9 ± 2.9 s, and 1.1 ± 0.5 s. In conclusion, LPFILTER appeared to be a valid procedure providing a high signal-to-noise ratio and data density and thus LPFILTER resulted in the smallest confidence interval for parameter estimates of BF kinetics.

Alterations in the Blood Velocity Profile Influence the Blood Flow Response during Muscle Contractions and Relaxations

The present study examined the influences of the muscle contraction (MCP) and relaxation (MRP) phases, as well as systole and diastole, on the blood velocity profile and flow in the conduit artery at different dynamic muscle contraction forces. Eight healthy volunteers performed one-legged dynamic knee-extensor exercise at work rates of 5, 10, 20, 30, and 40 W at 60 contractions per minute. The time- and space-averaged, amplitude-weighted, mean (V(mean)) and maximum (V(max)) blood flow velocities were continuously measured in the common femoral artery during the cardiosystolic (CSP) and cardiodiastolic (CDP) phases during MCP and MRP, respectively. The V(max)/V(mean) ratio was used as a flow profile index where a ratio of approximately (~) 1 indicates a "flat" velocity profile, and a ratio significantly greater than (>>) 1 indicates a "parabolic" velocity profile. At rest, a "steeper" parabolic velocity profile was found during the CDP (ratio: 1.75 +/- 0.06) than during the CSP (ratio: 1.31 +/- 0.02). During the MRP of exercise, the V(max)/V(mean) ratio shifted to be less steep (p < 0.05) than at rest during the CDP (ratio: 1.41-1.54) at 5, 10, 20, 30, and 40 W; whereas it was slightly higher (p < 0.05) at 30 and 40 W than at rest during the CSP (ratio: 1.43-1.46). During the MCP, the parabolic blood velocity profile was enhanced (p < 0.05) at higher contraction forces, 20 W during the CDP (ratio: 2.15-2.52) and 30 W during the CSP (ratio: 1.49-1.77), potentially because of a greater retrograde flow component. A higher blood flow furthermore appeared during the MRP compared to during the MCP, coinciding with a greater uniformity of the red blood cells moving at higher blood velocities during the MRP. Thus part of the difference in the magnitude of blood flow during the MRP vs. MCP may be due to the alterations of the blood velocity flow profile.

Alterations in the rheological flow profile in conduit femoral artery during rhythmic thigh muscle contractions in humans

The present study examined the rheological blood velocity profile in the conduit femoral artery during rhythmic muscle contractions at different muscle forces. Eight healthy volunteers performed one-legged, dynamic knee-extensor exercise at work rates of 5, 10, 20, 30, and 40 W at 60 contractions per minute. The time and space-averaged, amplitude-weighted mean (V(mean)) and maximum (V(max)) blood flow velocities in the common femoral artery were measured during the cardiosystolic phase (CSP) and cardiodiastolic phase (CDP) by the Doppler ultrasound technique. The V(max)/V(mean) ratio was used as a flow profile index, in which a ratio of approximately 1 indicates a "flat velocity flow profile" and a ratio significantly >1 indicates a "parabolic velocity flow profile." At rest, the V(max)/V(mean) ratio was approximately 1.3 and approximately 1.8 during the CSP and CDP, respectively. The V(max)/V(mean) ratio was higher (p < 0.01) during the CDP than during the CSP, both at rest and at all work rates. The V(max)/V(mean) ratio during the CSP was higher (p < 0.01) at 30 and 40 W compared to at rest. The V(max)/V(mean) ratio during the CDP was lower (p < 0.05) at 5 and 10 W compared to at rest. There was a positive linear correlation between blood flow and incremental work rates during both the CSP and CDP, respectively. Thus under resting conditions, the findings indicate a "steeper" parabolic velocity profile during the CDP than during the CSP. The velocity profile during the CDP furthermore shifts to being less "steep" during rhythmic muscle contractions at lower intensities, but to being reelevated and normalized as at rest during higher intensities. The "steepness" of the parabolic velocity profile observed during the CSP at rest increased during muscle contraction at higher intensities. In conclusion, the blood velocity in the common femoral artery is parabolic both at rest and during exercise for both the CSP and CDP, indicating the persistence of laminar flow. The occurrence of any temporary slight disturbance or turbulence in the flow at the sight of measurement in the common femoral artery does consequently not induce a persisting "disturbed" and fully flat "plug-like" velocity profile. Instead, the "steepness" of the parabolic velocity profile is only slightly modified, whereby blood flow is not impaired. Thus the blood velocity profile, besides being influenced by the muscle contraction-relaxation induced mechanical "impedance," seems also to be modulated by the cardiac- and blood pressure-phases, consequently influencing the exercise blood flow response.