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Department of Exercise Science, University of Iowa, Iowa City, Iowa 52242
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We imposed opposing oscillations in
treadmill speed and grade on nine rats to test for direct mechanical
coupling between stride frequency and hindlimb blood flow. Resting
hindlimb blood flow was 15.5 ± 1.7 ml/min. For 90 s at 7.5 m/min, rats alternated walking at 
10° for 10 s and +10° for
10 s. This elicited oscillations in hindlimb blood flow having an
amplitude of 4.1 ± 0.5 ml/min (18% of mean flow) with a delay
presumably due to metabolic vasodilation. Similar oscillations in speed
(5.5-9.5 m/min) elicited oscillations in hindlimb blood flow
(amplitude 3.4 ± 0.5 ml/min, 15% of mean flow) with less of a
delay, possibly due to changes in vasodilation and muscle pump
function. We then simultaneously imposed these speed and grade
oscillations out of phase (slow uphill, fast downhill). The rationale
was that the oscillations in vasodilation evoked by the opposing
oscillations in speed and grade would cancel each other, thereby
testing the degree to which stride frequency affects hindlimb blood
flow directly (i.e., muscle pumping). Opposing oscillations in speed
and grade evoked oscillations in hindlimb blood flow having an
amplitude of 3.3 ± 0.6 ml/min (16% of mean flow) with no delay
and directly in phase with the changes in speed and stride
frequency. The finding that hindlimb blood flow changes
directly with speed (when vasodilation caused by changes in speed and
grade oppose each other) indicates that there is a direct
coupling of stride frequency and hindlimb blood flow (i.e.,
muscle pumping).
exercise hyperemia; metabolic vasodilation; rat
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE ONSET OF LOCOMOTION and most forms of dynamic exercise are accompanied by a rapid increase in the blood flow to the muscles engaged in producing movement. The rise in blood flow is largely attributable to a rise in the calculated vascular conductance across muscle, which in turn is attributable to the muscle pump (7, 9, 12, 14, 15, 18) and to the inhibition of arteriolar smooth muscle after the production, release, diffusion, and transduction of vasodilator chemicals (2, 6, 11, 13, 14, 16). The relative contribution of each of these two mechanisms is unclear. A number of investigators employing a broad mixture of exercise conditions have concluded that the muscle pump can augment blood flow across muscle (3, 9, 12, 14, 15, 18). A common assumption is that contraction frequency constitutes a major determinant of muscle pump efficacy (3, 5, 12, 14, 17) just as cardiac frequency can constitute a major determinant of cardiac pump efficacy, and several studies have provided evidence in support of this idea (5, 12, 14). However, there is also evidence against the idea that the muscle pump directly improves muscle blood flow (8), and thus the hypothesis remains controversial.
The aim of the present study was to test whether there is a direct, mechanical coupling (muscle pumping) between treadmill speed and muscle blood flow. The overall approach was to impose cyclic changes in treadmill speed and grade during locomotion. The rationale was that relatively rapid, cyclic changes in grade during locomotion at a steady speed should induce only delayed, vasodilator-mediated, cyclic changes in muscle blood flow. Conversely, relatively rapid, cyclic changes in speed during locomotion at a steady grade will tend to induce both 1) immediate cyclic changes in blood flow owing to alterations in muscle pumping, and 2) delayed, vasodilator-mediated, cyclic changes in muscle blood flow. Thus, if cyclic changes in speed and grade were imposed in phase with one another (high speed at high grade and low speed at low grade), then the vasodilator drive from both factors should add together. Alternatively, if relatively rapid, cyclic changes in speed and grade were imposed out of phase with one another (high speed at low grade and low speed at high grade), then the vasodilator drive from each factor should oppose and cancel each other (if they are similar in magnitude), thereby revealing any muscle pump effect in isolation.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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All procedures met National Institutes of Health guidelines and were reviewed and approved by the Institutional Animal Care and Use Committee of the University of Iowa.
Thirteen Sprague-Dawley rats (7 males and 6 females; 301 ± 11 g) were selected for their willingness to walk on a motor-driven treadmill (Modular Treadmill 1010, Columbus Instruments; Columbus, OH). The rats were familiarized with treadmill walking before the following aseptic surgical procedures were performed.
Surgical preparation. Rats were anesthetized with isoflurane. Each animal had an ultrasonic transit-time blood flow transducer (model 1.5RB, Transonic; Ithaca, NY) implanted on the terminal aorta through a mid-line abdominal incision. The probe cable was tunneled to an exit site on the back. The animal was given nalbuphine HCl (1 mg/kg sc) for control of postoperative pain. The animal was allowed to recover until an acceptable blood flow signal was acquired (usually 2-3 days).
Data collection. The animal was lightly anesthetized (1% isoflurane), and the flow transducer was connected to a flowmeter (T106, Transonic). The animal was then placed on the treadmill and, after it regained full consciousness, was allowed at least 30 min to recover. A pressure transducer (PE10 EZ, Ohmeda; Madison, WI) connected to a length of water-filled tubing was mounted parallel to the walking surface of the treadmill. The pressure transducer was connected to a signal conditioner (Gould 6600, Gould Instrument Systems; Valley View, OH). Treadmill speed was measured continuously. To verify that stride frequency varied with treadmill speed as treadmill speed was varied between 5.5 and 9.5 m/min every 10 s, a signal from a spike generator was manually triggered with each right rear foot strike in six rats. This signal was fed to the data-collection software for calculation of stride frequency. Signals were displayed on a chart recorder (MT95K2, Astro-Med; West Warwick, RI), digitized at 1 kHz, and written to a fixed disk of a microcomputer using commercially available software (PONEMAH Physiology Platform, P3, Gould Instrument Systems).
Experimental protocols.
Nine rats completed the following protocols. The animals performed four
bouts of treadmill exercise. For each bout, exercise was performed at
7.5 m/min and 0° for 90 s. As the animal continued to walk on
the treadmill, one of the four following procedures was then imposed.
In one trial treadmill speed was maintained constant at 7.5 m/min, and
the treadmill incline was altered between 10° and +10° every
10 s for 90 s. In another trial treadmill the incline was
maintained constant (0°), and treadmill speed was varied between 5.5 and 9.5 m/min every 10 s for 90 s. In another trial treadmill
speed was varied between 5.5 and 9.5 m/min, and the treadmill incline
was varied between 10° and +10° every 10 s in phase with one
another (fast uphill and slow downhill) for 90 s. Finally,
treadmill speed was varied between 5.5 and 9.5 m/min, and treadmill
incline was varied between 10° and +10° every 10 s out of
phase with one another (slow uphill and fast downhill) for 90 s.
These four bouts were performed in no regular order. At least 2 min of
recovery time was imposed between bouts.
Data analysis. Terminal aortic flow was averaged over 1-s periods. The response of blood flow to the perturbations was analyzed graphically. The multiple cycles under each condition in each rat was ensemble averaged into one composite cycle, and the average value of blood flow over the entire composite cycle was subtracted from each data point such that only the cyclic variation of flow about zero remained. The composite cycles from each rat for each condition were then averaged together.
To test the nature of the summation [linear (additive), inhibitory, or facilitatory] of vasodilator drive from changes in speed and incline, the changes in blood flow from the trials in which speed and incline were altered in isolation were summed digitally and compared graphically to the trials where blood flow was measured directly while the changes in these two factors were imposed simultaneously. This was done by subtracting the average blood flow over the entire cycle from each data point such that only the variation above (positive values) and below (negative values) the mean remained. This was done for the trials in which grade was altered in isolation and in which speed was altered in isolation. The resulting relationships were aligned so they were in phase with one another, and the two relationships were summed into a single composite relationship. The resulting composite relationship generated by digital summation of the individual alterations in grade and speed was directly compared graphically to the relationship measured when the two factors were simultaneously imposed in phase with one another during locomotion. The responses to changes in grade and speed in isolation were then realigned so that they were out of phase with one another and then summed. The resulting relationship was directly compared graphically to the relationship measured when these two factors were simultaneously imposed out of phase with one another during a single bout of locomotion.Statistical analysis.
The average peak blood flow was compared with the average trough blood
flow for each condition by a paired t-test. The data presented in Table 1 were analyzed
statistically by repeated-measures ANOVA, and adjustment for multiple
comparisons was done by the Bonferroni procedure. A P value
<0.05 was selected to indicate statistical significance. Data are
presented as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Resting blood flow was 15.5 ± 1.7 ml/min.
Stride frequency varied directly with treadmill speed with only a
slight lag (Fig. 1).
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Figure 2 provides an illustrative example
of the changes in hindlimb blood flow that are elicited when treadmill
grade is altered between 10° and +10° every 10 s during
locomotion at 7.5 m/min in a single rat. It can be seen that the rise
and fall of blood flow lag the rise and fall of treadmill grade. The
four complete cycles shown in Fig. 2 were ensemble averaged into the single composite cycle shown in Fig. 3,
and again it can be seen that the rise and fall of blood flow lag the
rise and fall of treadmill grade. The open circles in Fig. 3 depict the
average of the four peaks in blood flow observed in Fig. 2 and the
average of the four nadirs in blood flow observed in Fig. 2. These
symbols are plotted at the four times at which they occurred relative to the cycle, averaged together. It can be seen that ensemble-averaging cycles in which the peaks and troughs occur at different times relative
to the cycle lead to a flattening of the composite curve and a
reduction of the apparent amplitude of the curve.
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The composite curve from Fig. 3 (less the average flow over the entire
cycle) was averaged together with similar curves from the remaining
eight rats to generate the curve shown in Fig.
4. The average peak flow (25.6 ± 2.6 ml/min) was significantly greater (P < 0.001) than
the average trough flow (21.5 ± 2.3 ml/min). Again, the changes
in blood flow lag the changes in treadmill grade. Again,
ensemble-averaging composite curves from rats having peaks and troughs
occurring at different times relative to the cycle lead to a further
reduction in the apparent amplitude of the changes in flow. That is,
the amplitude of the composite curve (Fig. 4) is 3.1 ml/min, whereas
the actual average amplitude was 4.1 ml/min (Table 1).
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The preceding procedures were applied to the blood flow measured when
treadmill speed was altered in isolation, and the resulting curve is
presented in Fig. 5. The average peak
flow (24.4 ± 2.4 ml/min) was significantly greater
(P < 0.001) than the average trough flow (21.0 ± 2.1 ml/min). The changes in blood flow elicited by changes in treadmill
speed appear characteristically different from those elicited by
changes in treadmill grade. Blood flow rises much sooner after an
increase in treadmill speed (Fig. 5) compared with the changes seen in
Fig. 4, causing the nadir flow to occur earlier in the cycle compared
with when isolated changes in grade were imposed (Table 1). Also, the
"plateauing" effect of ensemble averaging appears to be more
pronounced for speed than for grade. That is, the amplitude of the
"speed" curve in Fig. 5 is 1.6 ml/min, whereas the actual average
amplitude was 3.4 ml/min (Table 1).
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The responses of blood flow to cyclic changes in grade and speed
imposed in phase with one another are shown in Fig.
6. The average peak flow (27.3 ± 2.1 ml/min) was significantly greater (P < 0.001) than
the average trough flow (22.0 ± 2.2 ml/min). Overall, blood flow
lags the changes in speed and grade. The largest changes in blood flow
were seen under this condition.
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The responses of blood flow to cyclic changes in grade and speed
imposed out of phase with one another are shown in Fig.
7. The average peak flow (24.3 ± 2.3 ml/min) was significantly greater (P < 0.001) than
the average trough flow (21.1 ± 2.4 ml/min). It can be seen that
the changes in blood flow correspond directly to the changes in speed
(other than the period from about second 2 to about
second 8 of the cycle) with a slight lag, as was seen in
Fig. 1.
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Basic characteristics of the cycles shown in Figs. 4-7 are provided in Table 1. The average blood flows over each cycle were grossly similar. The average flow when changes in speed and grade were imposed in phase with one another was significantly greater than the average flow during isolated changes in speed or when changes in speed and grade were imposed out of phase with one another. The highest amplitude was observed when changes in speed and grade were imposed in phase with one another, and the lowest amplitude was observed when these two factors were imposed out of phase with one another.
The results of the tests of the nature of summation of vasodilator
signals are presented in Figs. 8 and 9.
The thick line in Fig. 8 replots the
blood flow data from Fig. 6 in which blood flow was directly measured
while treadmill speed and grade were altered in phase with one another.
The thin line in Fig. 8 is equal to the digital summation of the curves
in Figs. 4 and 5 in which the same changes in treadmill speed and grade
were imposed independent of one another in separate exercise bouts. It
can be seen that the two curves in Fig. 8 correspond to one another very closely in terms of both the timing and magnitude of the changes
in flow. The thick line in Fig. 9 replots the blood flow data from Fig.
7 in which blood flow was directly measured while treadmill speed and
grade were altered out of phase with one another. The thin line in Fig.
9 is equal to the digital summation of the data in Figs. 4 and 5 after
the data in Fig. 5 were shifted to be out of phase with the data in
Fig. 4. It can be seen that the two curves in Fig. 9 correspond to one
another very closely in terms of both the timing and magnitude of the
changes in flow.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study provides three major new findings. First, cyclic alterations in treadmill speed elicit cyclic alterations in hindlimb blood flow, just as cyclic changes in treadmill grade elicit cyclic alterations in hindlimb blood flow (16). Second, the summed blood flow responses to cyclic changes in treadmill speed and treadmill grade imposed independently in separate exercise bouts correspond directly to the blood flow responses elicited by combined alterations in treadmill speed and treadmill grade imposed simultaneously in a single exercise bout. This finding holds whether the changes in treadmill speed and treadmill grade are imposed in phase or out of phase with one another. Finally, when treadmill speed and treadmill grade are altered cyclically out of phase with one another such that the vasodilator signals from the competing perturbations inhibit one another, muscle blood flow can be shown to be directly coupled to treadmill speed (contraction frequency), indicating that there is a direct mechanical coupling between contraction frequency and muscle blood flow (i.e., muscle pumping).
Rationale. The aim of imposing changes in grade alone (Figs. 2-4) was to evoke vasodilator responses in isolation (no alteration in muscle pump function) by increasing and decreasing the amount of work performed by the animal. The rationale was that changes in work rate imposed in this manner should only elicit changes in the concentration of vasodilator chemicals within the muscle. The justification for this view is as follows. At the onset of locomotion at various grades, differences in muscle blood flow and vascular conductance attributable to the different work rates arise only after a delay of ~8 s (12). This finding signifies that treadmill grade (muscle force production) does not alter muscle pump function, nor does it induce rapid vasodilation. If locomotion at different treadmill inclines altered either muscle pump function or elicited immediate vasodilation, then an effect on blood flow arising from the differences in grade would be expected to be observed very early after the onset of locomotion (no delay), and this is not seen (12). This view is reinforced by the findings of Sheriff and Zidon (16), which demonstrate that blood flow responses lag well behind sinusoidal changes in grade. The lag of vasodilation to changes in work rate imposed by sinusoidal changes in treadmill grade fits well with the known lag of metabolic vasodilation to twitch contractions (when vasdilation does in fact occur) (4) and to the direct application of vasodilator substances to isolated arterioles (when vasodilator signal production and transport delays are eliminated) (19). Finally, rats do not systematically alter their stride frequency during locomotion when the treadmill incline is altered cyclically (16), a finding in line with reports that other quadrupeds do not alter their gait characteristics even over extreme (0-100%) changes in grade (1), meaning that changes in grade do not influence muscle pump function via changes in stride frequency. Taken together, these findings indicate that changes in work rate imposed by changes in treadmill grade elicit a relatively "pure" metabolic vasodilation.
The aim of imposing changes in treadmill speed was to characterize the effects on muscle blood flow of relatively rapid changes in treadmill speed during locomotion. In contrast to the effects of changes in grade, changes in speed are expected to produce more complex effects on muscle blood flow and vascular conductance. This is because a change in speed is likely to have two effects, as opposed to only the single effect (vasodilation) of a change in grade. The rationale here is as follows. First, a change in speed will induce a relatively fast (immediate) alteration in the effectiveness of the muscle pump (12, 14). Second, the increase in work imposed by running at a faster speed is expected to lead to a slower (delayed) vasodilation in muscle, perhaps similar in its timing and magnitude to that produced by a change in grade. The justification for this view stems from the observation that the onset of locomotion is accompanied by an immediate rise in blood flow that is proportional to treadmill speed, which has been attributed to the muscle pump (12, 13). At locomotion onset at low speeds, this proposed immediate "speed" effect is followed a short time later by a second rise in blood flow, which has been attributable to metabolic vasodilation (12, 13). At locomotion onset at higher speeds, blood flow rises more continuously, suggesting that vasodilation begins somewhat earlier. In dogs, this early onset vasodilation can be blocked by inhibiting nitric oxide synthase (12), suggesting that it is not of metabolic origin. The aim of imposing simultaneous changes in speed and grade utilizes the information gained from the foregoing to develop a key test of the muscle pump hypothesis. The aim was to unmask the action of the muscle pump during relatively rapid, cyclic changes in treadmill speed. The approach was to simultaneously impose changes in speed and changes in grade out of phase with one another. That is, as treadmill speed was increased, the incline of the treadmill was lowered, and as the speed of the treadmill decreased, the incline of the treadmill was raised. In this way, the animal alternated between walking slowly uphill and walking more quickly downhill. The rationale was that the relatively slow vasodilator response to each of these factors (speed and grade) would be out of phase with one another and will thus tend to cancel each other out. This would leave only the effect of changes in speed, presumably acting solely by altering the effectiveness of the muscle pump, free to cause changes in flow. The alteration in blood flow resulting from the isolated cyclic alterations in grade shown in Figs. 2-4 corresponds closely to the changes in blood flow observed by Sheriff and Zidon (16) when treadmill grade was altered in a sinusoidal manner. The alterations in blood flow resulting from isolated cyclic changes in treadmill speed shown in Fig. 5 are characteristically different. First, the rise in blood flow after the rise in treadmill speed at second10 has less of a lag compared with the
corresponding change in flow seen when grade is increased. This finding
is consistent with the proposal that increases in speed lead to more
effective muscle pumping. A far less dramatic fall in blood flow occurs when treadmill speed is reduced at second 0 in Fig. 5. Given
that blood flow rises with rising treadmill speed, why doesn't blood flow fall with falling contraction frequency? A likely explanation stems from the proposal that a change in speed elicits both a muscle
pump-induced change in flow as well as a vasodilator-mediated change in
flow. Note in Fig. 4 that the vasodilator response to an increase in
grade is still ongoing when the treadmill grade is reduced at
second 0 in Fig. 5 and the vasodilator response in Fig. 4
does not peak until second 3 in the cycle. If one presumes that the vasodilator response to the change in speed in Fig. 5 is
similar in its timing and magnitude to the vasodilator response induced
by the change in grade in Fig. 4, then it follows that the delayed
vasodilator response to the earlier rise in speed in Fig. 5 likely
counteracts any immediate flow-reducing effect of the fall in speed at
second 0 in Fig. 5.
As expected, the largest changes in blood flow were seen when treadmill
speed and treadmill grade were altered in phase with on another. Also,
the amplitude of the cyclic changes in blood flow resulting from
isolated, cyclic changes in speed and grade was similar, suggesting the
magnitudes of vasodilator drives used to offset each other when these
two factors were altered out of phase with one another were roughly
similar. However, the amplitudes of the blood flow changes to isolated
changes in speed and in grade do not provide a fair comparison of the
actual changes in vasodilator drive in these two conditions. That is,
to the extent that the changes in stride frequency associated with
changes in treadmill speed altered blood flow via modulation of muscle
pump function, the actual changes in vasodilator drive resulting from changes in speed are reduced compared with the changes in vasodilator drive stemming from changes in treadmill grade. Thus, although the
overall amplitudes of changes in flow stemming from isolated changes in
speed and grade were similar, there may not have been enough change in
vasodilator drive stemming from the change in speed to completely
offset the changes in vasodilator drive stemming from the changes in
grade because the muscle pump appears to have accounted for some of the
change in flow stemming from the changes in speed.
The fall in blood flow from seconds 2 to 8 in
Fig. 7 adds credence to foregoing argument that the magnitudes of the
changes in vasodilator drive elicited by changes in speed and by
changes in grade were not identical. This fall corresponds directly to the 2 ml/min fall seen over the same period of time in Fig. 4, which
resulted from blood flow lagging the fall in grade. With respect to the
change in speed imposed in Fig. 7, the equivalent phase in Fig. 5
corresponds to the period from seconds 8 to
2, during which blood flow rose by ~1 ml/min (one-half
the change stemming from the change in grade). Given that blood flow
fell by 1 ml/min over the period of interest in Fig. 7, it appears that
the blood flow changes that occurred separately over the same periods
of interest in Fig. 4 (2 ml/min fall) and Fig. 5 (1 ml/min rise) summed
linearly (see below).
In the transition from rest to exercise at 10 m/min, terminal aortic
blood flow rises by 12 ml/min (12). With changes in treadmill speed between 5.5 and 9.5 m/min associated with changes in
stride frequency between 40 and 80 strides/min (Fig. 1), terminal aortic flow rose and fell by ~3 ml/min when vasomotor responses to
the change in speed were inhibited by opposing changes in treadmill grade (Table 1). If a rise in stride frequency from 0 to 40 strides/min induces a muscle pump effect of similar magnitude, then the total muscle pump effect (i.e., 6 ml/min) may account for 50% of the total
rise in blood flow associated with exercise at 10 m/min.
Summation of vasodilatory signals. The goal of the present study was to use opposing vasodilator signals to cancel the vasodilatory changes in blood flow associated with simultaneous alterations in treadmill speed and treadmill grade when these factors were imposed out of phase with one another. To best achieve this goal, relatively small alterations in treadmill speed were imposed at the low end of treadmill speeds for this species. As stated by Sagawa (10), it is extremely important to use small inputs so that when they are imposed simultaneously, the output is not saturated by any inherent limit of the effector responses of the system in responding to a large input. Accordingly, the cyclic alterations in treadmill speed employed elicited relatively small (e.g., 15% of the mean value) alterations in blood flow, and the same was true for the changes in treadmill grade employed.
As a direct test of the nature of the summation of the blood flow-altering effects of cyclic alterations in treadmill speed and grade, the blood flow responses measured when each of these two factors was imposed independently were summed digitally and compared with the measured response observed when these two factors were imposed simultaneously. Figures 8 and 9 demonstrate that the digital summation and the physiological summation of the blood flow-altering effects of changes in speed and grade are remarkably similar in both their timing and amplitude. This was true when these two factors are imposed in phase or out of phase with one another. Thus, for the magnitude of signals imposed, the summation appears to be linear (additive). For example, based on Fig. 8, there is no evidence for either inhibitory summation (measured response less than the additive summation of the individual responses) or for facilitatory summation (measured response greater than the additive summation of the individual responses). This finding of the desired linear (additive) summation provides a positive internal check on the rationale of the study, thereby bolstering the validity of the main finding. Comparison of the blood flow responses from seconds7 to
2 in Fig. 4, where blood flow rose, and Fig. 7, where
blood flow remained stable, provides further support that the delayed
vasoconstrictor effects stemming from the decrease in speed opposed the
delayed vasodilator effects stemming from the increase in grade. If it had not, blood flow should have risen during this time in Fig. 7 just
as it had in Fig. 4.
In summary, cyclic alterations in treadmill grade elicit cyclic
alterations in hindlimb blood flow. The digital summation of blood flow
responses to cyclic changes in treadmill speed and treadmill grade
imposed independently in separate exercise bouts corresponds directly
to the blood flow responses elicited by combined alterations in
treadmill speed and treadmill grade imposed simultaneously in a single
exercise bout. This finding holds whether the changes in treadmill
speed and treadmill grade are imposed in phase or out of phase with one
another and indicates that there is a linear summation of the
vasodilator signals. When treadmill speed and grade are altered
cyclically out of phase so that the resulting vasodilator signals
inhibit one another, muscle blood flow appears to be directly coupled
to treadmill speed (contraction frequency), indicating that there is a
direct mechanical coupling between contraction frequency and muscle
blood flow (i.e., muscle pumping).
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ACKNOWLEDGEMENTS |
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This work was presented in part in Skeletal Muscle Circulation: Neural and Mechanical Determinants: Wiggers Award featured topic symposium at the Experimental Biology annual meeting in April 2002 in New Orleans, LA and has been previously published in abstract form (FASEB J 16: A763, 2002).
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FOOTNOTES |
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This work supported by National Heart, Lung, and Blood Institute Grant HL-46314.
Address for reprint requests and other correspondence: D. D. Sheriff, Dept. of Exercise Science, 518 Field House, Iowa City, IA 52242.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpheart.01133.2002
Received 23 December 2002; accepted in final form 15 January 2003.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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,
Average of the four peaks and the four nadirs in blood flow observed in
Fig. 
