Superficial_Lumbopelvic_Muscle_Overactiv

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SPINE Volume 32, Number 1, pp E23–E29
©2007, Lippincott Williams & Wilkins, Inc.
Superficial Lumbopelvic Muscle Overactivity and
Decreased Cocontraction After 8 Weeks of Bed Rest
Daniel L. Belavý, BPhty,*† Carolyn A. Richardson, PhD,† Stephen J. Wilson, PhD,*
Jörn Rittweger, PhD,‡ and Dieter Felsenberg, PhD§
Study Design. Longitudinal study.
Objective. To gain insight into the effects of inactivity
on lumbopelvic stabilization.
Summary of Background Data. Some authors have
suggested a link between inactivity and lumbopelvic (LP)
pain. Studies examining the superficial musculature in LP
pain have shown overactivity and increased cocontrac-
tion. Studies in inactivity (bed rest, microgravity) have
shown similar effects on the musculature of the legs. Here
we examine the effect of bed rest on superficial LP activity
and cocontraction.
Methods. Ten male subjects participated in the “Berlin
Bed Rest Study” and underwent 8 weeks of bed rest with a
1-year follow-up. A repetitive knee movement model at four
movement speeds in non–weightbearing was used to as-
sess, via electromyography, activity (amplitude-ratios) in
five superficial LP muscles and abdominal flexor-lumbar
extensor cocontraction for LP stabilization. Testing was con-
ducted at regular intervals during and after bed rest.
Results. Analysis of the amplitude-ratio data showed a
significant effect of testing date (F � 2.38, P � 0.005). This
effect was generalized across all muscles, however (F �
0.59, P � 0.989). During bed rest, subjects exhibited higher
levels of activity but lower levels of cocontraction (F � 8.84,
P � 0.001), and the changes were still present up to 1 year
after bed rest.
Conclusions. The bed rest protocol resulted in the de-
velopment of superficial muscle overactivity but decreased
cocontraction. These changes could reflect dysfunction of
central nervous system control of LP stabilization. These
changes were still apparent 1 year after bed rest, suggesting
a stable change in motor control.
Key words: inactivity, sedentary lifestyle, spine, stabi-
lization, unloading, spaceflight, motor control, abdomi-
nal, erector spinae, Berlin Bed Rest Study, low back pain.
Spine 2007;32:E23–E29
It is not new to suggest that sedentary western lifestyle
could be involved in lumbopelvic (LP) pain etiology.1
Dysfunction in the central nervous system (CNS) control
of the LP musculature for stabilization is considered to
be one risk factor for LP pain.2 Inactivity itself results in
a number of changes in multiple body systems,3 but its
association to change in LP motor control for stabiliza-
tion and any potential link to LP pain etiology is poorly
understood.
Changes in activity levels and cocontraction of the
superficial LP musculature have been implicated in LP
pain. In studies of activation levels, a common finding is
that of increased activity (overactivity) in superficial
muscles.4–11 Studies of LP antagonist cocontraction in
LP pain find that it is increased and results in increased
spinal loads.12–14 It has been argued that these changes in
motor control reflect “splinting” of the trunk are mal-
adaptive for LP stabilization.15
While it is difficult to study LP motor control changes
secondary to sedentary lifestyle, studies in “unloading”
(microgravity, bed rest, and related environments16,17)
do provide insight. Leg and trunk muscle antagonist co-
contraction has been observed to increase during18 and
after19–21 exposure. Although postural instability after
microgravity exposure has been documented,22,23 a
component of motor control change must be involved in
the increased cocontraction as the changes develop dur-
ing unloading, rather than solely afterwards.18,24
Activation levels of superficial muscles have also been
studied in unloading. Generally, superficial muscles are
either less active or show little change during unload-
ing.25–31 After unloading, however, studies in animals
have found superficial muscle activity to be in-
creased.21,24 Human postural studies show similar pat-
tern of increased superficial muscle activity after unload-
ing.20,32–36 All of these studies have, however, been
undertaken on the leg musculature, and no data are
available on the LP region.
We hypothesized that similar changes of increased co-
contraction and overactivity would occur in the superfi-
cial muscles of the LP region during inactivity (bed rest).
The European Space Agency’s “Berlin Bed Rest Study”
provided the opportunity to examine this hypothesis.
This has not been studied before and also begins to ad-
dress a large gap in the literature on the effects of unload-
ing on the LP region.
Materials and Methods
Bed Rest Protocol. The Berlin Bed Rest Study (BBR) was
undertaken at the Charité Benjamin Franklin Hospital in Ber-
From the *School of Information Technology and Electrical Engineer-
ing, University of Queensland, Brisbane, Australia; †School of Health
and Rehabilitation Sciences, University of Queensland, Brisbane, Aus-
tralia; ‡Institute for Biophysical and Clinical Research into Human
Movement, Manchester Metropolitan University, Cheshire, England;
and §Zentrum für Muskel- und Knochenforschung, Charité Campus
Benjamin Franklin, Berlin, Germany.
Acknowledgment date: August 2, 2006. First revision date: August 21,
2006. Acceptance date: August 21, 2006.
The Berlin Bed Rest Study was supported by Grant No. 14431/02/NL/
SH2 from the European Space Agency.
The manuscript submitted does not contain information about medical
device(s)/drug(s).
Institutional funds were received in support of this work. No benefits in
any form have been or will be received from a commercial party related
directly or indirectly to the subject of this manuscript.
Address correspondence and reprint requests to Daniel L. Belavý, BPhty,
Zentrum für Muskel- und Knochenforschung, Charité Campus Benjamin
Franklin, Hindenburgdamm 30, 12200 Berlin, Germany; E-mail:
belavy@gmail.com
E23
lin, Germany, from February 2003 to May 2005. Ten male
subjects underwent 8 weeks of bed rest with a 1-year follow-up.
The bed rest protocol as well as inclusion and exclusion criteria
are discussed in detail elsewhere.37 However, in brief, subjects
were required to remain in bed at all times and were required to
restrict activity in bed to the minimal required for hygiene and
other necessary daily tasks. Adherence to this protocol was
monitored by continuous video recordings and force transduc-
ers in the frame of the bed. The institutional ethics committee
approved this study and subjects gave their informed consent.
A history of LP pain was not a specific exclusion criterion
for the BBR. As LP suffers are known to exhibit differences in
motor control, a qualified physiotherapist attained subjects’
subjective history of LP pain at the beginning of the study.
During bed rest and the follow-up period also, subjects’ mus-
culoskeletal health was monitored at regular intervals with
pain questionnaires.37 Incidence of LP pain was defined as any
report of pain or discomfort between the first lumbar vertebrae
and the gluteal fold.
Repetitive-Movement Model and Testing Protocol. To
stimulate LP muscle activity for LP stabilization, a gravity-
eliminated repetitive knee movement model was used. Subjects
were positioned in prone lying with a spring attached to their
right ankle and to the apparatus and were able to view a feed-
back-monitor under the apparatus. The setup was optimized
such that, at rest (no activity in the hamstring musculature), the
knee was held at 60° of flexion by the spring. This biomechani-
cal configuration has been shown previously to eliminate the
gravitational weight of the lower-leg during movement.38,39
Right knee movement was conducted between 0° and 45° of
flexion at four movement speeds (50, 75, 100, and 125 cycles
per minute). Three repetitions were conducted at each move-
ment speed. Each movement repetition was performed for 11
seconds. Subjects paused their breathing during each move-ment repetition to remove the influence respiration on LP mus-
cle activity.40 An electrogoniometer was placed at the right
knee to monitor knee position.
Baseline data collection occurred on the first day of bed rest
(BR1). Further testing was conducted on the fourth, 13th, 27th,
41st, and 53rd days of bed rest (BR4, BR13, BR27, BR41, and
BR53) and 3rd, 7th, 14th, 28th, 90th, 180th, and 360th days of
recovery (R � 3, R � 7, R � 14, R � 28, R � 90, R � 180, and
R � 360).
Lumbopelvic Electromyography. Five superficial LP mus-
cles were monitored. Bipolar Ag/AgCl surface electrodes were
placed over the following right-sided muscles with an interelec-
trode distance of 35 mm: 1) lumbar erector spinae with multi-
fidus (LES) positioned at the level of the fifth lumbar vertebrae
between the spinous process and a line drawn from the poste-
rior superior iliac spine (PSIS) to the interspace between the first
and second lumbar vertebrae41,42; 2) thoracic erector spinae
(TES) at the level second and third lumbar interspace, 1 cm
medial to a line drawn from PSIS to the lateral border of the
erector spinae at the 12th rib41,42; 3) internal oblique (IO): the
superior electrode is placed 1 cm medial to the ASIS, the infe-
rior electrode is placed parallel to the inguinal ligament with
the standard interelectrode distance43; 4) external oblique (EO)
at the most inferior point of the costal margin orientated along
a line from that point to the contralateral pubic tubercle43; and
5) inferior gluteus maximus (IGM) placed inferior and medial
to a line drawn between the PSIS and posterior greater trochan-
ter.44 Electrodes were also placed over the biceps femoris mus-
cle to monitor leg muscle activity at rest. A ground electrode
was placed at the right elbow. Standardized skin preparation
was performed involving washing the skin, shaving and the
application of an abrasive-conductive gel.
Data Acquisition. EMG and goniometer data were sampled
simultaneously at 2000 Hz using a Powerlab system using
Chart software (version 4.2, AD Instruments, Sydney, Austra-
lia) and were stored for offline processing. A second computer
also sampled the goniometer signal and implemented custom
written software in the Labview environment (version 6.1, Na-
tional Instruments) to provide real-time feedback to the subject
on position and movement speed.
Goniometer Signal Processing. Offline signal processing first
examined the goniometer signal to find which “regions” of data
fulfilled the criteria: 1) three consecutive movement cycles, start-
ing at a minimum nearest 0°, 2) each cycle’s speed within �5
cyc/min of target, and 3) peak flexion and extension angles
within � 4° of their target (45° and 0°, respectively). This process
limited the effect of extremes of performance/movement on ob-
served EMG patterns and also provided data on movement accu-
racy: mean-squared-error (MSE) of movement speed (MSEspeed),
maxima positions (MSE45°), and minima positions (MSE0°). MSE
values were calculated both within each data “region” (to corre-
late with EMG variables) and also over the entire 11-second
movement repetition at each speed (to examine motor skill acqui-
sition over the course of the study).
EMG Signal Processing. Abdominal flexor-lumbar extensor
antagonistic cocontraction (CoCon) was quantified using an
“area-normalization” method adapted from prior work.45,46
The following algorithm was used: 1) generation of a “linear-
envelope” of the EMG signals of the IO, EO, LES, and TES
muscles (high-pass filtering at 100 Hz using a digital 10th order
quasi-Butterworth filter, rectification, and then low-pass filter-
ing using a 10 Hz digital 10th order Bessel filter) and partition-
ing into the appropriate data region; 2) normalization of the
area of each linear-envelope region to “1” by dividing each
value by the total area under the curve; 3) the greater value of
the LES and TES signals was taken at each data point and the
resulting signal renormalized as in step 2, which generates an
“extensor linear-envelope”; 4) abdominal IO and EO signals
are processed in the same fashion to yield a “flexor linear-
envelope”; 5) the area of overlap of the flexor and extensor
linear-envelopes is calculated; and 6), the overlap area ranges
from 0 to 1 and generates the flexor and extensor cocontraction
variable (CoCon).
To quantify EMG amplitude, activity-ratios (ratio) across
movement speeds were used. To obtain this data, root-mean-
square (RMS) values of the raw EMG signal in each data region
were first measured. Then, at each movement speed 75 cycles/
min and above, the RMS value in each data region was divided
by the median RMS value of the speed below. This resulted in
three activity-ratios for each muscle (ratio75/50, ratio100/75, ra-
tio125/100). Such relative measures of EMG amplitude have
been used previously40 and have been shown to reduce inter-
subject variability.47
Statistical Analyses. Linear mixed-effects models were used
in statistical analysis.48 Analysis of variance of the EMG data
E24 Spine • Volume 32 • Number 1 • 2007
examined fixed effects for muscle, testing date, movement
speed, interactions between each variable, as well as fixed ef-
fects for incidence of LP pain in the past or in the follow-up
period. The goniometer MSE data for each repetition
(MSEspeed, MSE45° , and MSE0°) were also analyzed to examine
improvements in movement performance. A natural-log trans-
formation was used for the ratio and MSE data to approximate
normality. Where appropriate, allowances were made for het-
erogeneity of variance across different grouping levels (such as
movement speed, muscle, or date). The “R” statistical environ-
ment (version 2.0.1, www.r-project.org) was used to imple-
ment these analyses. An � of 0.05 was taken for statistical
significance. As multiple measurement sessions were under-
taken on the same subjects, a Bonferroni adjustment was not
performed; rather, we looked for consistent significant differ-
ences across time points.
To consider the association between motor control and
movement accuracy changes, correlation analyses between
CoCon, ratio and the goniometer MSE data (MSEspeed, MSE45°,
and MSE0° values from each data “region”) were undertaken.
SPSS for Windows (version 10.0.1, www.SPSS.com) was used
to calculate Spearman’s rank correlation coefficient (�) between
each of the variables.
Results
Due, for example, to subject absence or technical difficul-
ties, data from each subject on every scheduled testing day
were not always available. Table 1 presents the number of
subjects able to be included in statistical analysis on each
testing day. Based on the classification used for low back
pain occurrence, 6 subjects had a prior history of LP pain
and 7 subjects experienced LP pain in the 1-year period
after bed rest. Two of these represented “new” incidents of
LP pain, the remaining being recurrences.
Abdominal and Lumbar Extensor Cocontraction
The measure of IO, EO, LES, and TES cocontraction
(CoCon) showed a significant main effect for movement
speed (F � 42.13, P � 0.001), with decreasing antago-
nistic cocontraction with increasing speed (50 cycles/
min: 0.909 � 0.012; 75 cycles/min: 0.858 � 0.012; 100
cycles/min: 0.815 � 0.012; 125 cycles/min: 0.801 �
0.013). History of LP pain and occurrence of LP pain
during follow-up showed no significant main effect in the
CoCon data set (F � 0.06, P � 0.820 and F � 0.20, P �
0.675, respectively). Significant effects over testing date
(F � 8.00, P � 0.001) and date � speed (F � 8.34, P �
0.001) existed for the CoCon data.
Changes of the CoCon variable over testing date and
each movement speed compared to baseline (BR1) data
are presented in Figure 1. Strong effects can be seen at the
125 and 100 cycles/min movement speeds for decreased
cocontraction over testing date. The effects appear to
develop quickly in bed rest (by BR4) and persist until 1
year after bed rest at the 125 cycles/min speed and up to
6 months (R � 180) atthe 100 cycles/min speed (with
trend to persistence at R � 360). Weak effects for de-
creased cocontraction are seen at the 75 cycles/min speed
Table 1. No. of Data Sets (Subjects) Available for
Inclusion in Analysis on Each Testing Day
Testing Day No. of Subjects
BR1 8
BR4 6
BR13 8
BR27 8
BR41 8
BR53 10
R � 3 10
R � 7 7
R � 14 9
R � 28 9
R � 90 8
R � 180 6
R � 360 4
BR, day of bed rest; R�, day of recovery.
Figure 1. Decreases in antago-
nist lumbopelvic cocontraction
during and after bed rest. Error
bars represent standard error of
the mean difference to baseline
(BR1) values. *P � 0.05; †P �
0.01; ‡P � 0.001. BR, bed rest;
R�, recovery.
E25Superficial Lumbopelvic Muscle Overactivity • Belavý et al
during early to mid recovery (R � 4 to R � 90). No
significant changes in cocontraction are seen over time at
the 50 cycles/min speed.
Activity Levels: Amplitude-Ratio Data
Significant effects existed for muscle and ratio (muscle:
F � 4.91, P � 0.004; ratio: F � 9.22, P � 0.001; mus-
cle � ratio: F � 3.09, P � 0.004) as well as testing date
(F � 2.37, P � 0.005). Further interactions between
testing date, ratio and muscle did not reach significance
(testing date � ratio: F � 1.22, P � 0.209; muscle �
testing date: F � 0.59, P � 0.989; testing date � ratio �
muscle: F � 1.01, P � 0.456), suggesting the effect was
generalized across muscles and movement speeds. His-
tory of LP pain was not significant in the ratio data set
(F � 0.12, P � 0.745), nor was LP pain incidence in the
follow-up period (F � 0.54, P � 0.493).
Table 2 gives the baseline values of ratio for each
muscle. Changes in amplitude-ratio values over testing
date are presented in Figure 2a. Increased activity levels
are seen across the study period, and this is significant
immediately after bed rest up to 3 months afterwards
(R � 90) with trend to significance 6 months afterwards
(R � 180, P � 0.08). Although the testing date � ratio
Table 2. First Day of Bed Rest (Baseline)
Amplitude-Ratio Values
Ratio75/50 Ratio100/75 Ratio125/100
External oblique 1.34 � 0.05 1.82 � 0.07 1.68 � 0.13
Internal oblique 1.67 � 0.09 2.24 � 0.13 1.53 � 0.08
Inferior gluteus maximus 1.43 � 0.08 2.18 � 0.15 2.18 � 0.15
Thoracic erector spinae 1.43 � 0.10 1.74 � 0.10 2.21 � 0.19
Lumbar erector spinae 2.64 � 0.31 2.49 � 0.21 1.96 � 0.12
Values are mean � SEM.
Figure 2. (A and B) Increases in
activation levels during and after
bed rest. Error bars represent
standard error of the mean dif-
ference to baseline (BR1) values.
*P � 0.05; †P � 0.01; ‡P � 0.001.
BR, bed rest; R�, recovery.
E26 Spine • Volume 32 • Number 1 • 2007
interaction did not reach significance, Figure 2b indi-
cates, similar to cocontraction, that the effect was pre-
dominantly localized to the highest movement speeds
and was persistent up to 1 year after bed rest (R � 360)
despite low subject numbers at this time point.
Movement Accuracy
Analysis of the goniometer MSE data from each speed-
repetition showed a significant effect for movement
speed (MSEspeed: F � 114.32, P � 0.001; MSE45°: F �
56.73, P � 0.001; MSE0° : F � 41.87, P � 0.001) and
testing date (MSEspeed: F � 3.45, P � 0.001; MSE45°: F �
2.91, P � 0.001; and MSE0°: F � 3.84, P � 0.001).
Interactions of movement speed over testing date were
nonsignificant (testing date � speed: MSEspeed: F � 0.70,
P � 0.937; MSE45°: F � 0.88, P � 0.692; MSE0°: F �
0.85, P � 0.753). At baseline (BR1) MSEspeed was
3.81 � 0.42 (cycles/min), MSE45° 2.15 � 0.20 and
MSE0° 0.51 � 0.09. Figure 3 shows the change these
variables over the study period. Accuracy increases (MSE
decreases) though the bed rest and early-to-mid recovery
testing dates. Later in the recovery period, with testing
dates occurring at greater time separations, movement
accuracy appears to in general decrease. At no time point
is movement accuracy significantly less than at baseline
testing.
Correlation Analyses
A significant negative correlation (P � 0.001) existed be-
tween the ratio and CoCon variables (� � �0.48); implying
that as muscle activity levels increase, cocontraction levels
decrease. Significant correlations (P � 0.001) existed be-
tween EMG and movement accuracy variables (Table 3).
These correlations imply that less accurate movement is
associated with higher activity levels (amplitude-ratio) and
lower antagonistic abdominal flexor-lumbar extensor an-
tagonistic cocontraction (CoCon).
Discussion
We found long-term increases in muscle activity and de-
creased cocontraction during a repetitive-movement task
in the superficial LP muscles after 8 weeks of bed rest.
The increased activity appears to develop during bed rest
at the highest movement speed, be generalized across all
five superficial LP muscles and is significant up to 1 year
after bed rest, despite low subject numbers at this time
point. While consistent with other studies’ findings of
increased superficial muscle activity after unload-
ing,20,21,24,34–36 the development of increased activity
during bed rest is contrary to prior research conducted
during unloading itself.25–31 They are, however, similar
Figure 3. Improvements in movement performance over the study period. Values represent mean difference to baseline (BR1) testing of
each MSE variable. Negative values imply greater accuracy. Error bars indicate the 95% confidence interval of the mean; hence, the
difference to baseline is significant when the error bars to not cross x-axis (zero). BR, bed rest; R�, recovery.
Table 3. Correlations Between Electromyographic and
Movement Accuracy Variables
EMG
Variable
Movement Accuracy Variables
MSEspeed MSE45° MSE0°
Activity-ratio 0.11 0.11 0.20
Cocontraction �0.16 �0.17 �0.27
Values represent Spearman’s rank correlation coefficient (�). All correlations
are significant (P � 0.001). MSEspeed, movement speed accuracy; MSE45°,
accuracy of peak knee flexion position; MSE0°, accuracy of terminal knee
extension position. A positive correlation coefficient implies less accurate
movement is associated with higher EMG variable values.
E27Superficial Lumbopelvic Muscle Overactivity • Belavý et al
to those in LP pain, which commonly observe superficial
LP muscle overactivity.4–11
Some authors have suggested that superficial LP mus-
cle overactivity in the pain population is a CNS strategy
to “splint” the trunk in response to deep muscle dysfunc-
tion.15 The repetitive-movement task employed in the
current study requires the development of muscle stiff-
ness at the LP region to allow efficient knee movement.
During bed rest, the same subjects show significant atro-
phy of the deep multifidus muscle, while this is not seen
in the lumbar erector spinae or abdominal muscles
(Hides JA, Belavý DL, Wilson SJ, Rittweger J, Felsenberg
D, Richardson CA, unpublished observations). We sug-
gest that due to this deep muscle atrophy, in order to
maintain the necessary amount of LP stiffness for the
knee movement task, the CNS increased the recruitment
of the superficial LP muscle systems.
Abdominal flexor and lumbar extensor cocontraction
decreases over the study period at the higher movement
speeds. These changes are significant early in bed rest and
persist up to 1 year afterwards. These findings of de-
creased cocontraction are contrary to findings of both
prior unloading studies18–21 and those in LP pain,12–14
where an increase in LP antagonist cocontraction are
observed. The prior research into cocontraction used
static tasks, such as load-release or postural perturba-
tion, but also active movements of the trunk. In such
tasks, antagonist cocontraction typically increases with
load, exertion, or speed of movement.49–52 Cocontrac-
tion is, however, both muscle and task specific. In the
current study, we used a repetitive limb movement task
requiring the antagonist LP muscles to stabilize the pelvis
for efficient knee movement. Although the findings of
overactivity suggest “splinting” of the trunk, reduction
in flexor-extensor cocontraction over time could suggest
reduced stabilization of theLP region. Further work is
necessary to resolve these differences.
Importantly, the observed changes in superficial LP mus-
cle motor control are unrelated to skill acquisition. The
subjects’ movement skill improves over the course of the
study, while activation levels increase and cocontraction
decreases. The correlation analyses show, however, that
greater movement accuracy correlates with lower ampli-
tude-ratios and higher levels of cocontraction. It is highly
unlikely, therefore, that improvement in accuracy of move-
ment is involved in the observed motor control changes.
In the current study, no link could be made between
incidence of LP pain and motor control changes during
the study. Studies in bed rest, such as the BBR, typically
comprise a limited number of subjects. Prospective stud-
ies of LP pain typically employ larger subject pools53–55
for sufficient statistical power. Furthermore, 6 of the 10
subjects in this study reported prior LP pain. Past LP pain
is a significant risk factor for future incidence.55,56 It
becomes more difficult, therefore, in a limited subject
pool to accurately test whether inactivity leads to a
greater risk of LP pain. Future bed rest studies attempt-
ing to address the same question should exclude subjects
with a prior history of LP pain. Also, statistical assess-
ment of whether risk of LP pain increases after bed rest
may require pooling of data from a series of studies.
Conclusion
The results suggest that long-term inactivity (bed rest)
results in motor control change in the superficial LP mus-
cles. In the current study, we observed this to be a gen-
eralized increase in superficial LP muscle activity and
decreased cocontraction for stabilization of the LP re-
gion during a repetitive-movement task. In light of
known motor control dysfunction in LP pain, these ad-
aptations could be maladaptive for normal LP function,
though further work in larger populations is required.
Key Points
● Superficial lumbopelvic muscle activity increases
during and after bed rest.
● Abdominal flexor and lumbar extensor cocon-
traction during a repetitive-movement task de-
creases during and after bed rest.
● These changes likely reflect alterations in lum-
bopelvic motor control for stabilization.
● Inactivity may result in maladaptive motor con-
trol patterns for lumbopelvic stabilization.
Acknowledgments
The authors thank the subjects who participated in the
study and the staff of ward 18A in the Charité Campus
Benjamin Franklin Hospital, Berlin, Germany for their
expert assistance, as well as Mr. Benny Elmann-Larsen
of the European Space Agency.
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