RESEARCH ARTICLE
Neck Muscle Strength and Endurance in Fighter
Pilots: Effects of a Supervised Training Program
Marie Alricsson, Karin Harms-Ringdahl,
Börje Larsson, Jan Linder, and Suzanne Werner
ALRICSSON M, HARMS-RINGDAHL K, LARSSON B, LINDER J, WERNER
S. Neck muscle strength and endurance in fighter pilots: effects of a
supervised training program. Aviat Space Environ Med 2004; 75:
23– 8.
Introduction: Cervical discomfort is common among pilots of high
performance aircraft. An exercise program was introduced to increase
the strength and endurance of the neck muscles. The purpose of this
study was to analyze whether coaching or reinforcement strategies by a
physical therapist was associated with improvement in neck muscle
strength, endurance, and neck complaints in two cohorts of fighter pilots
performing regular neck muscle exercises. Methods: A reinforced group
(RG) of 20 pilots (24 – 40 yr) at an Air Force base received weekly
encouragement to perform their standardized exercise program three
times per week. A non-reinforced reference group (NRG) of 20 pilots
(23–37 yr) from another Air Force base carried out the same program
without any supervision. Both groups performed the training program for
6 – 8 mo. Before and after the training period, isometric measurements of
the neck flexors and neck extensors were performed in both groups.
Results: After the completion of the 6 – 8 mo training period, the RG
pilots significantly increased their neck muscle strength (flexors: M ⫽
3.9 nm, p ⫽ 0.000 and extensors: M ⫽ 5.0 nm, p ⫽ 0.001) as well as
endurance in their neck extensors (M ⫽ 53 s, p ⫽ 0.000). The NRG
pilots significantly decreased both strength (M ⫽ 11.5 nm, p ⫽ 0.0001)
and endurance (M ⫽ 33 s, p ⫽ 0.003) of their neck extensors. Furthermore, the NRG pilots did not show any significant change of their neck
flexor strength. No significant changes in the frequency of neck complaints were reported in either group throughout the entire study period.
Conclusion: The reinforced training program increased the strength and
endurance of the appropriate muscle groups. In order to draw any
further conclusions concerning reducing neck complaints, a longer
observation period with a larger group of pilots might be needed.
However, it is likely that there is an individual correlation between
strength and endurance of the neck muscles and neck pain, which
means that any rehabilitation program should be tailored for each
individual.
Keywords: endurance tests, exertion, neck pain, strength tests, training
effects.
P
AIN AND DISCOMFORT in the neck are common
among pilots of high-performance aircraft. Several
investigations have shown a high frequency of acute
cervical discomfort in pilots exposed to high G-forces
(1,2,7,14,15,19). The largest number of acute cervical
discomfort cases was reported in a Japanese study,
where 89% of Japanese Air Self-Defense Force F-15
pilots reported muscle pain related to flying (14). The
frequency of neck complaints among Swedish highperformance fighter pilots has been reported to vary
between 29 –37% (1).
The cervical spine is extra sensitive when exposed to
high G-forces. During flying the cervical spine is often
exposed to high G-forces in a combination of rotation,
Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004
lateral flexion, and extension (7,9,14,15) of the neck.
Since the compressive force induced by the weight of
the head, neck, and helmet is increased by the G-force,
the load on the cervical spine should be reduced (3–5,7).
The most common neck complaints have been of less
severe muscular pain and strain. More severe injuries
like fractures, ligament ruptures, nerve compression,
and intervertebral disk injuries have also been reported
(12,16 –18,23). Magnetic resonance imaging (MRI) studies of high-performance aircraft pilots show more frequent degenerative cervical disk changes than in controls (9,22). Electromyography (EMG) investigations
show that pilots exposed to high G-forces over repeated, short periods use close to 100% of their neck
extensor muscle strength (5,21).
EMG recordings of cervical muscle strength and endurance while flying high-performance aircraft indicate
higher demands on strength and endurance in pilots
than in average persons (21). In a Swedish study on
conscripts who had had no exposure to high G-forces
while wearing a helmet and Air Force pilots, we found
that the Air Force pilots were superior to the conscripts
regarding strength of the neck flexors and extensors,
and in sub-maximal endurance of the neck extensors
(1). However, the conscripts were superior to the Air
Force pilots in sub-maximal endurance of the neck flexors (1).
Whether high muscle strength and endurance can
prevent acute cervical spine discomfort in conjunction
with high G-forces is not yet shown (8). Many authors
have suggested studying whether strength and endurance exercises could be a preventive measure for people
From the Swedish Air Force Base F4, Östersund, Sweden (M. Alricsson); the Division of Physiotherapy, Department of Neurotec,
Karolinska Institutet, Stockholm, Sweden (K. Harms-Ringdahl, S.
Werner); the Swedish Air Force Base F21, Luleå, Sweden (B. Larsson);
the Swedish Armed Forces Aeromedical Center, Stockholm, Sweden
(J. Linder); and the Division of Sports Medicine, Department of Surgical Sciences, Karolinska Institutet, Stockholm, Sweden (S. Werner).
This manuscript was received for review in October 2001. It was
revised in April, August, and September 2003. It was accepted for
publication in September 2003.
Address reprint requests to: Marie Alricsson, Ph.D, R.P.T, Swedish
Air Force Base, F4, Östersund Garnison, S-831 85 Östersund, Sweden;
[email protected].
Reprint & Copyright © by Aerospace Medical Association, Alexandria, VA.
23
NECK STRENGTH & TRAINING EFFECTS—ALRICSSON ET AL.
TABLE I. AGE, HEIGHT, WEIGHT, TOTAL FLYING HOURS, AND FLYING HOURS/MONTH (H/MO)
OF THE GROUPS EXAMINED. N ⫽ 54.
Age (years)
Height (m)
Weight (kg)
Total flying
time (h)
Flying h/mo
Group*
n
M
SD
M
SD
M
SD
M
SD
M
SD
RG
NRG
RG dr-o
NRG dr-o
20
20
10
4
29.4
29.4
31.7
26.0
4.5
3.1
4.2
0.9
1.82
1.80
1.83
1.82
5.2
5.1
4.1
2.7
81.1
80.0
79.0
75.5
8.0
6.8
6.0
4.9
1125
1153
1210
686
615
420
617
272
14.2
12.6
14.2
11.1
4.0
2.2
2.6
1.3
Reinforced training group (RG), reference group (NRG), Rg drop-outs (dr-o) and NRG drop-outs (dr-o). n ⫽ 54
*RG ⫽ reinforced training group; NRG ⫽ reference group; RG dr-o ⫽ RG drop-outs; and NRG dr-o ⫽ NRG drop-outs.
with cervical complaints (5,14,23,24). Good strength
and endurance of the neck muscles might contribute to
preventing neck complaints in Air Force pilots. Therefore, for some years, a standardized muscle training
program has been developed and is available together
with photos and written instructions for exercises at all
Swedish Air Force bases.
The present objective was to assess cervical muscle
strength, endurance, and complaints as an indication of
whether the exercises carried out really did give increased strength and endurance. The first study group
was located at an Air Force base where the physical
therapist took an active, supporting role in encouraging
the pilots to perform the training program. The other
study group was located at another Air Force base
where only the training equipment, including written
instructions, was provided. Our aim was to study
whether active reinforcement in a group of fighter pilots improved neck muscular strength, endurance, and
a reduction in neck complaints compared with a group
of fighter pilots who were merely instructed to perform
neck muscle exercises on a regular basis.
age, height, and/or weight at the beginning of the
study.
Prior to the start of the present investigation, 10 out of
20 RG pilots and 7 out of 20 NRG pilots reported having
had neck complaints within the past 3 mo. Among the
drop-outs, 6 out of 10 pilots among the RG drop-outs
and 1 out of 4 pilots among the NRG drop-outs complained of neck pain before the study started. An independent Chi squared test showed no significant difference regarding neck complaints among the four groups
(RG, NRG, RG drop-outs, NRG drop-outs).
Questionnaire
The measurements used were approved by the Ethical Research Committee at the Karolinska Institutet,
Stockholm, and the training program was regarded as a
quality evaluation. The development stage was approved by the Regional Ethics Committee before routine testing among Air Force pilots. All subjects volunteered and gave their informed consent.
In addition to the demographic data, the pilots were
asked on a questionnaire whether they perceived discomfort/pain in the neck/cervical, thoracic/spine,
and/or shoulder/shoulder joint. The frequency of complaints during the 3 mo prior to the start of the study
was reported on a scale of 0 –3, where 0 ⫽ never, 1 ⫽
once/a few times over the previous 3 mo, 2 ⫽ once/a
few times per month, and 3 ⫽ once/a few times per
week. The questionnaire was answered on two occasions, before the investigation and 6 – 8 mo later. The
analysis of the frequency of complaints was dichotomized to either never (i.e., “0”), or once/a few times
over the previous 3 mo, the past month, or per week
(i.e.,“1” ⫹ “2” ⫹ “3” combined). Both groups of pilots,
RG and NRG, were physically active and were training
for an average of 6.5 hours per week all year. However,
very few pilots in either group performed any specific
exercise for the neck muscles.
Subjects
Measurements of Muscle Strength and Endurance
There were 54 fighter pilots who were divided geographically into a reinforced training group (RG) from
one Air Force base and a non-reinforced reference
group (NRG) from another Air Force base, both with
similar environments. However, before the first test
occasion, 10 pilots in the RG dropped out due to aircraft
accidents, change of duty, or absence from training, and
4 pilots in the NRG dropped out for similar reasons.
This left 40 male fighter pilots from the 2 bases to
participate in the study. Each group consisted of 20
pilots. Age, height, weight, and hours of flying are
presented in Table I, including the study groups as well
as the drop-outs. Analysis of variance (ANOVA) revealed no significant group differences with regards to
Isometric strength measurements of the flexors and
extensors of the neck muscles and sub-maximum measurement of the endurance of the neck extensors were
evaluated. These tests were carried out in the order
described, and are reported in detail elsewhere (1).
Testing muscular strength and endurance in this way is
part of a larger Swedish Air Force project (5) and is
performed regularly, blinded to previous results. A
test-retest in order to evaluate the instruments used for
measuring strength and endurance was performed earlier, showing very good reliability (1).
Cervical spine isometric flexor muscle strength: The subject sat on a bench with a firm foam pillow (Scandinavian Mobility Physiotherapy, Spånga, Sweden) be-
METHODS
24
Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004
NECK STRENGTH & TRAINING EFFECTS—ALRICSSON ET AL.
tween his back and the gymnasium wall-bars (Fig. 1).
The arms hung loosely. The knees were bent so that only
the tip of the toes touched the floor. The chin was slightly
retracted and vision fixed forward and downward. A
dynamometer (Salter 235, PIAB, Täby, Sweden) was secured to the wall-bars and fastened to a horizontal sling
around the subject’s helmet. The length of the moment
arm from the bilateral motion axis of cervico-thoracic motion segment C7-T1 was measured from a point on the
neck at the level of C7 to the middle of the sling (3). The
subject performed two maximum trials, of which the best
result was used in the analyses (13).
Cervical spine isometric extensor muscle strength: The
subject sat in the same way as for the neck flexors but
now facing the gymnasium wall-bars. The contraction
was performed as a neck backward extension (Fig. 2).
Cervical spine isometric extensor endurance: Cervical
spine extensor isometric endurance was also tested in a
sitting position (Fig. 2). With the neck extensors, the
subject pulled a load equivalent to 196 Newton (N) on
the dynamometer for as long as he possibly could. The
level of exertion was assessed every 15 s with the Borg
CR-10 scale. The test was discontinued when the subject
could no longer maintain the stipulated force. Both
groups carried out the strength and endurance measurements at the start of the study and after the training
period was completed 6 – 8 mo later. When recording
the measurements, the test leader did not have access to
previous results.
Fig. 2. Test position for measuring cervical spine isometric extensor
muscle strength.
Design of the Training Program
Fig. 1. Test position for measuring cervical spine isometric flexor
muscle strength.
Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004
The program was comprised of strength and endurance exercises for the neck and shoulder muscles using
weights attached directly on the head or on a training
helmet in order to give an increased resistance. The
weights consisted of 1, 2, and 4 k, and could be combined for additional training resistance. Based on improvements of strength, the weights were increased
progressively during the entire training period using
4 ⫻ 10 repetitions each training session. Since many
neck muscles are attached to the upper thoracic spine,
this region was also exercised for strength and endurance using a rubber tube as resistance. Mobilizing exercises (stretching) for the neck and the thoracic spine
region were also included in the program.
The training program for the RG was performed with
the help of a pilot who was familiar with the exercises.
Thus, this group was coached by him and the local physiotherapist together. They coached the pilots every week
via discussions, presence in the training room, and checks
of the exercise diaries. The pilots were strongly encouraged to perform the exercises three times per week, and
were followed during the whole study period of 6– 8 mo.
The NRG was left without such coaching support. The
training equipment with written instructions was available, but personal training instructions, encouragement to
25
NECK STRENGTH & TRAINING EFFECTS—ALRICSSON ET AL.
strength (M ⫽ ⫺11.5 nm, p ⫽ 0.0001) after 6 – 8 mo
(Table III).
Isometric neck extensor endurance: The pilots of the
combined RG (RG1 and RG2) increased the endurance
of their neck extensors to 53 s (p ⫽ 0.000) after 6 – 8 mo.
The pilots of the NRG decreased the endurance of their
neck extensors to 33 s (p ⫽ 0.003) when the training
period was completed (Table III).
Frequency of neck complaints: The number of pilots
with neck complaints during the last 3 mo of the investigation was unchanged in both groups (Table IV, Table V). There was no statistically significant difference
between the groups in this respect. There was no statistically significant difference either in neck muscle
strength or in endurance of the neck extensors between
those pilots who seldom or never reported neck complaints and those who frequently reported neck complaints.
TABLE II. MUSCLE STRENGTH OF BOTH NECK FLEXORS AND
EXTENSORS AND ENDURANCE OF NECK EXTENSORS AT THE
FIRST TEST OCCASIONS IN BOTH THE REINFORCED
TRAINING GROUP (RG) AND THE NON-REINFORCED
TRAINING GROUP (NRG). N ⫽ 40.
Strength (Nm)
Flexors
Endurance (s)
Extensors
Group
n
M
SD
M
RG
NRG
Sign.
20
20
38
45
7.6
10.6
56
13.9
70
14.3
p ⫽ 0.001
n.s
SD
Extensors
M
SD
68
36.5
118
49.6
p ⫽ 0.000
perform the program, and exercise diary checks by a
physical therapist were not available for the NRG unless
requested, i.e., “as usual.”
Statistics
DISCUSSION
ANOVA was used to study whether the groups investigated differed in personal data, number of flying hours,
or strength and endurance of the neck muscles prior to the
start of the investigation. Differences between the results
of the first and the last test occasion were analyzed using
the paired t-test. Possible group differences with regards
to frequency of complaints during the study period were
analyzed with an independent Chi-squared test. The significance level was set at 5%.
Good neck muscle strength and endurance in Air
Force pilots might act as a preventive measure against
perceived neck discomfort. Previous authors maintain
the importance of neck muscle training in order to
prevent neck injuries (11,14,20). The present study
shows that completing an assigned training program of
three times a week over a 6 – 8 mo period to improve
neck muscle strength and endurance is consistent with
physiological training principles (6,25).
Both the RG and NRG pilots were physically active,
i.e., exercised on average 6.5 hwk⫺1 throughout the
whole year. However, only a few pilots performed specific exercises for the neck muscles. Other similar studies have usually involved patients with neck pain, but
with little or no training experience. Thus, comparing
the results of strength and endurance training in fit
pilots to non-fit patients with pain is not appropriate.
Untrained patients probably have a larger potential for
improvements of both muscle strength and endurance
and also for a reduction in the frequency of complaints.
(6,14,25).
In the present study it was not possible to evaluate
whether reducing or increasing the number of flying
hours would have influenced the results during the
training period. However, earlier EMG studies have
shown that the neck extensors are activated to nearly
their maximum capacity for repeated short periods during high-performance flight (5,21). The present investi-
RESULTS
Baseline data with regards to neck muscle strength
(Nm) and endurance (s) from the first test occasion,
prior to the start of the study, is presented in Table II.
Group differences were found regarding both isometric
neck extensor strength (p ⫽ 0.001) and isometric endurance of the neck extensors (p ⫽ 0.000), the NRG being
somewhat more fit than the RG.
Among the 20 RG pilots (reinforced training), 11 pilots carried out the exercises less than twice per week
(RG1), while 9 carried out the exercises twice per week
or more (RG2) (Table III).
Isometric muscle strength of neck flexors and extensors:
The pilots of the combined RG (RG1 and RG2) showed
increases in muscle strength of the neck flexors (M ⫽ 3.9
nm, p ⫽ 0.000) and extensors (M ⫽ 5.0 nm, p ⫽ 0.001)
when the training period was completed. The pilots of
the NRG showed no significant difference regarding
neck flexor strength, but a decrease of neck extensor
TABLE III. MUSCLE STRENGTH OF BOTH NECK FLEXORS AND EXTENSORS AND ENDURANCE OF NECK EXTENSORS
PRIOR TO THE START OF THE STUDY AND AFTER 6 – 8 MONTHS IN THE REINFORCED TRAINING GROUPS (RG)
AND THE REFERENCE GROUP (NRG), RESPECTIVELY. N ⫽ 40.
Strength (Nm)
Flexors
Endurance (s)
Extensors
Extensors
Group
n
M
SD
p
M
SD
p
M
SD
p
RG1
RG2
RG1 ⫹ 2
NRG
11
9
20
20
4.0
3.8
3.9
⫺0.6
3.4
3.2
3.3
6.1
0.003
0.008
0.000
0.7
4.4
5.7
5.0
⫺11.5
6.7
4.8
5.8
10.9
0.05
0.007
0.001
0.0001
55
52
53
⫺33
39.0
27.7
33.6
43.5
0.001
0.000
0.000
0.003
26
Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004
NECK STRENGTH & TRAINING EFFECTS—ALRICSSON ET AL.
TABLE IV. FREQUENCY OF COMPLAINTS IN THE STUDIED GROUPS, WITH REGARDS TO MUSCLE STRENGTH OF NECK FLEXORS
AND EXTENSORS BEFORE AND DIRECTLY AFTER THE STUDY PERIOD. N ⫽ 40.
Flexors (Nm)
Before
Extensors (Nm)
After
Before
After
Group
n
Md
range
Md
range
Md
range
Md
range
RG c 0*
RG c1–3
NRG c 0
NRG c1–3
10
10
13
7
36
39
45
40
35
14
38
35
41
41
48
38
36
9
39
33
53
53
74
71
62
18
44
45
55
59
58
55
53
17
25
30
*C 0 ⫽ No complaints; C 1–3 ⫽ frequent complaints at start and during last 3 mo of the study.
gation showed that the training program was possible
to accomplish. Pilots who exercised their neck muscles
regularly for 6 – 8 mo under supervision increased their
strength and endurance compared with those pilots
who were instructed to perform these exercises on a
regular basis without supervision. Statistically significant differences were found between the RG and the
NRG in maximum strength and sub-maximal isometric
endurance of the neck extensors in favor of the RG
pilots, who showed improvements to a higher extent.
Due to communication between the pilots, we could
not randomize the pilots in each base to two different
exercise strategies. For this reason, stronger reinforcement and individual discussions were given to all the
subjects at one Air Force base rather than to some pilots
and not others at each Air Force base. One weakness of
the present investigation was that the NRG was, for
some reason, stronger and showed higher endurance
levels at the beginning of the study. However, the two
Air Force bases involved were almost 1000 km apart
and to be able to perform this kind of investigation, we
had to have the RG at one Air Force base and the NRG
at the other. Furthermore, prior to the study, we did not
know about any group differences. This was revealed
when we made the statistical calculations and analyses
after the study period was completed. An earlier study
at an Air Force base in the same geographical area as
the RG showed strength and endurance at the same
level as in the present NRG at the beginning of the
study (1). The present RG had less strength than the
NRG, but on the other hand, this group was more
homogeneous. The standard deviation in the present
study was less than in the earlier study, which means
that pilots with extreme values were, for various reaTABLE V. FREQUENCY OF COMPLAINTS IN THE STUDIED
GROUPS, WITH REGARDS TO ENDURANCE OF NECK
EXTENSORS BEFORE AND DIRECTLY AFTER
THE STUDY PERIOD. N ⫽ 40.
‘
Extensor endurance (s)
Before
After
Group
n
Md
range
Md
range
RG c 0
RG c1–3
NRG c 0
NRG c1–3
10
10
13
7
64
67
123
99
139
98
148
115
108
116
74
94
165
164
121
70
See tables I and III for legend.
Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004
sons, absent from the base during the study. During the
training period, the strength and endurance increased
in the RG by almost 10%, but despite this, the average
values were not equivalent to the pilots from the NRG.
The RG pilots were instructed to carry out the program three times a week, but this became much less,
due to Air Force drills and in-service training. At the
end of the study, the groups were classified regarding
training frequency. However, it was difficult to establish an exact group classification since some weeks the
pilots exercised more regularly than other weeks.
Therefore, the frequency of performing the training
program was estimated as a mean value per week over
the entire study period. The test occasions at 6 – 8 mo
were adjusted according to the availability of the pilots.
The results of the muscle strength of both neck flexors
and extensors after 6 – 8 mo of training showed statistically significant improvements in all supervised training groups irrespective of low training frequency. The
NRG had a statistically significant reduction in muscle
strength and endurance of their neck extensors after
6 – 8 mo which, to some extent, might be explained by
one pilot experiencing pain after the first test. This
information spread among all the subjects in the NRG,
and they may have avoided exerting their maximum
force in the second strength and endurance test.
Another issue to consider is that the standardized
measuring methods are limited regarding the test positions, which makes it difficult to compare different
studies of strength and endurance (4,6,25). Our strongest pilots were not the ones with the longest endurance
time, and there was a high individual variation. No
significant correlation between muscle strength and endurance was found, which might have been expected,
as the endurance test was performed at an absolute
resistance force level and not related to individual extensor strength. However, the results from the 6 – 8 mo
of endurance training showed a clear improvement in
neck extensor endurance in all pilots of the RG.
The neck extensors show the greatest isometric endurance of the major neck muscle groups. However, we
have not found any study on prevention of neck pain by
performing endurance training of the neck extensors.
During the 3 mo immediately prior to the start of the
study, 10 pilots in the RG and 7 pilots in the NRG
reported complaints in the neck region. The same number of pilots had neck complaints at the end of the
investigation. This number may have been too small
and the observation period too short to show any pos27
NECK STRENGTH & TRAINING EFFECTS—ALRICSSON ET AL.
sible effects with regards to the frequency of complaints
due to increased exercise. Another reason could be that
all pilots in the present investigation were well-trained
individuals exercising, on average, 6.5 hwk⫺1. Their
general good physical condition might mean that they
already had relatively good neck muscle strength at the
start of the study despite not having carried out any
specific neck muscle exercises.
It has been previously reported that neck complaints
can be reduced after training periods with intensive
strength and endurance exercises between 3– 8 wk
(6,16,25). However, it should be pointed out that these
three studies did not include any control groups. Furthermore, these subjects might have had less neck muscle strength than the pilots in the present study. It is
also most likely that the correlation between neck muscle strength and neck pain is individual, which means
that any rehabilitation program should be tailored to
each individual.
Whether maximum muscular contractions were truly
“maximum” may be questioned, at least if some of our
symptomatic pilots had neck muscle pain. However, it
cannot be excluded that RG pilots carried out the training by using their neck muscles more appropriately, i.e.,
in a way tailored for pilots. This might, to some extent,
explain why they improved when performing the measurements. Whether this helps when flying, however,
remains to be studied.
We conclude that the continuous contact between the
physiotherapist and the pilots throughout the entire
study, with repeated reinforcement of the neck muscle
training program, encouraged the pilots to carry out the
exercises, which led to increased neck muscle strength
and endurance. In order to find out whether a reduction
of complaints is possible, the frequency of complaints
may need to be followed in a larger group of pilots over
a longer period with, possibly, more intensive and more
specific training. However, the correlation between
neck muscle strength and endurance, respectively, and
neck complaints might be individual, meaning that any
rehabilitation program for neck complaints should be
tailored for each individual.
ACKNOWLEDGMENTS
The authors would like to thank Ulla Romild for help with the
statistics. Financial support was given by the Swedish Defense Research Establishment (FOI).
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
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Aviation, Space, and Environmental Medicine • Vol. 75, No. 1 • January 2004