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Journal of Exercise Physiologyonline
April 2013
Volume 16 Number 2
Editor-in-Chief
Official Research Journal
Tommy
of the Boone,
American
PhD,
Society
MBA of
Review
Exercise
Board
Physiologists
Todd Astorino, PhD
ISSN 1097-9751
Julien Baker,
PhD
Steve Brock, PhD
Lance Dalleck, PhD
Eric Goulet, PhD
Robert Gotshall, PhD
Alexander Hutchison, PhD
M. Knight-Maloney, PhD
Len Kravitz, PhD
James Laskin, PhD
Yit Aun Lim, PhD
Lonnie Lowery, PhD
Derek Marks, PhD
Cristine Mermier, PhD
Robert Robergs, PhD
Chantal Vella, PhD
Dale Wagner, PhD
Frank Wyatt, PhD
Ben Zhou, PhD
Official Research Journal
of the American Society of
Exercise Physiologists
ISSN 1097-9751
JEPonline
Physiological Response to Static Muscle Contractions
in Standing and Supine Positions
Jens Melgaard Pedersen1, Thomas Bull Andersen1
1
Department of Sport Science, Aarhus University, Denmark
ABSTRACT
Pedersen JM, Andersen TB. Physiological Response to Static
Muscle Contractions in Standing and Supine Positions. JEPonline
2013;16(2):24-31. The purpose of this study was to investigate the
physiological responses to static muscle contractions in the standing
position and the supine position. Eight subjects performed static
contractions of the ankle extensors in both positions. Blood pressure
(SBP and DBP), heart rate (HR), electromyography (EMG), and nearinfrared spectroscopy (NIRS) were determined during the static
contractions. The results of main interest were a significantly larger
initial decline of muscle oxygen concentration and a steeper negative
slope of the frequency content in the EMG signals in the supine
position. These results indicate that the fatigue development is more
pronounced in a supine position, which is most likely due to changes
in muscle blood flow.
Key Words: Blood Pressure, Heart Rate, EMG, NIRS
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INTRODUCTION
The human circulatory system delivers nutrients and oxygen to the cells. The pumping function of the
heart promotes the blood flow through the arteries to the capillaries where it flows past each cell (3).
During the contraction of the ventricles, the systolic blood pressure (SBP) rises and then decreases
(i.e., diastolic blood pressure, DBP) until the next contraction. The filling of the heart with blood
depends on the venous return. The blood pressure in the veins is close to zero, which means that
the return of mixed venous blood to the heart is not directly affected by the pumping function of the
heart (4).
Due to the elasticity of the arteries and their branching to provide blood to every part of the body, the
blood pressure decreases towards the capillaries. However, a change in body position changes the
hydrostatic pressure. This change in pressure must be considered when discussing or measuring
blood pressure changes caused by the pumping function of the heart (5). For example, while in a
standing position, blood flows easily to the legs but venous blood return is negatively affected. The
decrease in venous return is offset by the increase in heart rate (HR). Yet, in the supine position,
venous return is increased and, therefore, the end-diastolic volume is increased. For a given stroke
volume (SV) at an increase in ventricular contractility, the increase in venous return is matched by the
decrease in HR.
When muscles contract the vessels decrease in diameter and the capillary blood flow is decreased.
At contraction levels above approximately 30% of maximum, the blood flow is completely obstructed
(2). When performing dynamic exercises, the contraction level of each muscle typically varies to the
extent that the blood is still delivered to the muscle cells at reasonable intervals. This has the effect of
putting pressure on the venous blood, which then returns more easily to the heart. However, when
performing static contractions, the blood-flow through the muscle is decreased or even obstructed.
The muscles receive less oxygen and, therefore, must sustain contraction with increasing anaerobic
resources. This leads to muscle fatigue and failure to maintain a certain force level.
Muscle fatigue in static contractions is typically analyzed by the use of electromyography (EMG) (1).
The frequency content of the signal decreases linearly and the amplitude increases just before
exhaustion. Through the development of reliable near-infrared spectroscopy (NIRS) measurements, it
has been possible to investigate the local oxygen level in and around the muscles during static
contractions. These measurements have revealed that the oxygen level decreases rapidly during the
first 10 sec and, then, either remains constant or decreases slowly (6). When the muscle contraction
is stopped, the oxygen level rapidly returns to base level. The rapid decrease at the beginning is most
likely that the oxygen is used in maintaining the muscle contraction.
We hypothesize that the standing position versus the supine position influences the blood flow
through the muscles as well as the venous return. This change in blood flow could cause differences
in the NIRS-measured oxygen level as well as differences in fatigue development. Accordingly, the
purpose of the present study was to investigate the physiological responses to a static contraction in
the standing position and the supine position.
METHODS
Subjects
Eight male subjects agreed to participate in this study. Their mean age was 24.75 yrs (range 21-27
yrs), mean height was 1.83 m (1.77-1.94 m), and mean mass was 80.7 kg (72.9-92.2 kg). The
subjects were reasonably well trained in different sports. None of the subjects had any leg or back
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injury at the time of the study. All subjects gave their informed consent to participate in the study prior
to the study getting underway.
Procedures
The weight of the subjects was measured on a force platform (AMTI, OR6-6) during static stance.
Surface electrodes (Blue Sensor N, Ambu A/S Denmark), were placed on the medial head of the
gastrocnemius of the right leg in accordance with the recommendations of SENIAM (with an interelectrode distance of 20 mm). The skin was shaved and cleaned with alcohol swabs prior to electrode
placement. The EMG signals were recorded with a wireless EMG system (TeleMyoSystem DTS,
Noraxon U.S.A. Inc., AZ).
Distal to the EMG electrodes a Near Inferred Spectroscopy (NIRS, Ohmatex, Denmark) sensor was
placed to measure blood oxygenation level. The sensor consisted of two diode light sources with
wavelengths of 660 nm and 940 nm, respectively and a detector receiving the reflected signal. The
sensor had a 1 Hz sampling frequency and the data was sampled in a for the purpose produced
program (NIRS, Ohmatex, Denmark). The subjects were equipped with a blood pressure monitor (M4,
Omron Matsusaka Co, Japan) on their left arm to measure blood pressure and HR. The subjects
performed two exercises of which the order was randomized between subjects.
Figure 1a. Standing position.
Figure 1b. Supine position.
1a. Each subject stood with straight legs and upright body on a force platform in a
Smith Machine (Figure 1a). Twenty-five percent of the subject’s body mass was
placed on the shoulders. Data from the force platform was sampled to ensure a
correct mass. The subject plantar flexed the ankle joint until there was an angle of
135° between the instep and the shank. The position was maintained for 90 sec.
Then, the subject lowered the heels and stood with straight legs again.
1b. Each subject was also placed in a diagonal sled leg press machine (Figure 1b)
with straight legs. The feet were placed on a mounted force platform. Mass was
placed on the sled until data from the force platform showed a force of 125% of the
subject’s body weight. The subject plantar flexed the ankle joint until there was an
angle of 135° between the instep and the shank. The position was maintained for 90
sec. The subject then assumed the straight leg position.
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During both exercises, blood pressure was measured after 2 min of weight bearing, before ankle
plantar flexion, during the contraction and again after the contraction. Data from NIRS sensor and
EMG sensors were sampled continuously during the contraction.
Data Analysis
The median frequency of the raw EMG was found for each of the 90 sec in each contraction, and
then a linear regression of the frequency shift was determined (Figure 2). The slope of the regression
was exported for statistical analysis. The EMG data were used to find the mean of 100 ms Root Mean
Square (RMS) during each contraction. All EMG analyses were made in Myo Research XP
MasterEdition 1.07.01, Noraxon U.S.A. Inc., AZ.
The recorded NIRS signal was inverted, as the stored data were expressed as r=HbO 2/Hb, and the
initial value was defined as 100%. The average slope of the NIRS signal was calculated during the
first 10 sec and the minimum value in the time between 10 sec and contraction end at 90 sec was
used in the statistical analysis.
Figure 1. The median frequency calculated in 1 sec intervals during a contraction for one
subject. The straight line shows the linear regression that was used to express the
decline in frequency during the contraction.
Statistical Analyses
A two-way repeated measures analysis of variance (ANOVA) was used for blood pressure and heart
rate to examine differences between standing and supine position, and between measurements taken
at different times. The level of significance was set at 0.05. If any significance was found, a post-hoc
analysis (Holm-Sidak method) was used to determine which factors differed significantly. A one-way
repeated measures ANOVA was used to examine differences between standing and supine position
for EMG and NIRS data. The level of significance was set at 0.05. If any significance was found, a
post-hoc analysis (Holm-Sidak method) was used to determine which factors differed significantly.
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RESULTS
Heart rate was 73.0 ± 2.0 beats·min-1 (mean ± SEM) before the contraction in the standing position,
78.9 ± 2.6 beats·min-1 during the contraction, and 77.6 ± 2.6 beats·min-1 after the contraction. In the
supine position HR was 56.5 ± 2.0 beats·min-1 before the contraction in the standing position, 72.0 ±
2.0 beats·min-1 during the contraction, and 57.3 ± 2.4 beats·min-1 after the contraction. There was a
significant difference between HR in supine and standing position both before and after the
contraction (P<0.001 and P<0.001), but not during the contraction (P=0.108). Furthermore, there was
a significant difference in HR in the supine position between HR during the contraction and both HR
before and after the contraction (P<0.001 and P<0.001) (see Figure 3).
Systolic blood pressure was 133.1 ± 1.7 mmHg before the contraction in the standing position, 139.3
± 2.1 mmHg during the contraction, and 133.5 ± 2.1 mmHg after the contraction. In the supine
position SBP was 139.2 ± 1.7 mmHg before the contraction in the standing position, 145.1 ± 1.7
mmHg during the contraction, and 140.5 ± 2.0 mmHg after the contraction. There were no significant
effects of time or position (P>0.05) (see Figure 3).
Contraction start
Contraction end
Diastolic BP (mmHg)
Systolic BP (mmHg)
-1
Heart rate (s )
**
80
**
70
*
60
150
145
140
135
130
**
95
**
90
85
*
80
75
-20
0
20
40
60
80
100
120
140
160
Time (s)
Standing
Supine
Figure 2. Heart rate (HR), Systolic blood pressure (SBP) and Diastolic blood
pressure (DBP) before, during and after the static contraction. Full circles
indicate the standing position and open circles the supine position and are the
mean of all 8 subjects. The error bars indicate the standard error of the mean
(SEM). One star (*) indicates a significant difference between measurements
during the contraction and both before and after the contraction. Two stars (**)
indicate a significant difference between standing and supine positions.
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Diastolic blood pressure was 87.4 ± 1.1 mmHg before the contraction in the standing position, 91.6 ±
1.4 mmHg during the contraction, and 90.7 ± 1.4 mmHg after the contraction. In the supine position
DBP was 80.4 ± 1.1 mmHg before the contraction in the standing position, 88.6 ± 1.1 mmHg during
the contraction, and 78.8 ± 1.3 mmHg after the contraction. There was a significant difference
between DBP in supine and standing position both before and after the contraction (P=0.002 and
P=0.003), but not during the contraction (P=0.575). Furthermore, there was a significant difference in
DBP in the supine position between DBP during the contraction and both DBP before and after the
contraction (P=0.003 and P<0.001) (see Figure 3).
The EMG amplitude was 116.0 ± 20.0 µV in the standing position and 102.1 ± 11.7 µV in the supine
position; these were not significantly different (P=0.341). The slope of the linear regression on the
frequency content was -0.205 ± 0.025 Hz s-1 in the standing position and -0.275 ± 0.017 Hz s-1 in the
supine position. The slope in the supine position was significantly lower than in the standing position
(P=0.013).
r
The slope of the relative oxygen saturation during the first 10 sec of the contraction was steeper in
the supine position (P=0.016). In the standing position it was -0.00042 ± 0.00023 s-1 and in the supine
position it was -0.00096 ± 0.0245 s-1. The minimal value was 0.985 ± 0.003 in the standing position
and 0.981 ± 0.003 in the supine position; these were not significantly different (P=0.333) (Figure 4).
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140 150 160
Time (s)
Standing
Supine
Figure 3. Development of the relative oxygen-level during and after the static
contraction. The dotted line indicates the supine position and the black line the
standing position. The error bars indicate the standard deviation (SD).
DISCUSSION
Based on the results from the present study, it can be seen that the fatigue development is different
between a standing position and a supine position. Muscle force cannot explain the differences since
it was ensured that the muscle force produced in both situations was equal. This was ensured by
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controlling the force level during the experiments by using force platforms, and also by investigating
the mean EMG-amplitude, which correlates with force production (1).
The parameters that were calculated based on the EMG-measurements resemble those found in
other studies (6). Also, the NIRS-data correspond to what is previously published (6). We saw a
decline in frequency content of the EMG-signal, which was more pronounced when placing the
subjects in the supine position. Furthermore, the initial decline of the NIRS-estimated oxygen content
was more pronounced in the supine position. We believe that these differences are caused by
changes of blood flow through the muscles during the contraction. This assumption is supported by
the measurement of blood pressure at the heart level. In the standing position, the blood pressure in
the distal part of the leg is approximately 100 mmHg higher than in the supine position, since during
the contraction, we found no significant differences in blood pressure. A difference of 100 mmHg can
possibly increase the flow of the oxygenated hemoglobin through the muscle. The muscle will then be
able to work more aerobically and develop signs of fatigue less rapidly.
CONCLUSIONS
Based on the present study, it can be concluded that in a static contraction the local blood pressure
response that result from changes in body position influences fatigue development. Furthermore, it
can be concluded that during static contractions the NIRS-sensors can reveal information on the local
blood oxygenation but more information is needed on blood flow to estimate metabolism.
ACKNOWLEDGMENTS
This work was supported by the European Space Agency, ESA Contract No. 22861.
Address for correspondence: Andersen TB, Department of Sport Science, Aarhus University,
Denmark , Dalgas Avenue 4, DK-8000. Phone (+45) 86150000; Email. [email protected].
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