20-l
LOW BACK PAIN IN HELICOPTER PILOTS.
Capt. Med. P. Vallejo, Capt. Med. J. Lopez, Maj. Med. F. Rios-Tejada Maj. Med. J. Azofia, Maj. Med. J. Del
Valle, Maj. Med. C. Velasco, Maj. Med. L. Garcia-Mora.*
C.I.M.A., Arturo Soria 82, Madrid, SPAIN.
*F.A.M.E.T. Colmenar A.B. Colmenar Viejo. Madrid. SPAIN
INTRODUCTION. deformations and displacements to which organs or
tissues are subjected at certain frequencies (16). The
Back pain is a widespread problem among tolerance of standing or sitting subject is at a minimum
industrialized countries Incidence of back pain is between 4 and 8 Hz. Unfortunately it is at these
between 60 and 80%. Prevalence rate does not exceed frequencies that the majority of vibrations in flight occur
35% of the general population (1). (2). But there is no good evidence that vibration forces
A&et and Villefond determined that back pain are responsible directly for low back pain, since neither
is twice more frequent in helicopter pilots than in the study could separate them from other associated stresses.
general population (19). Shanahan and Reading (3) studied 11 pilots in a
Many papers reported prevalence rates in excess mock-up of UH-IH helicopter. Each pilot was subjected
of 50%, many reported prevalence exceeding 75% (3), to two hours test periods; one with simulated helicopter
and pain in the lumbar area is the most common vibration and one without. The experimental conditions
experienced by the pilots. produced back pain in all subjects, which they described
Most of these studies have been made through as identical to the pain they typically experience while
questionnaires purely subjective in regards of data flying helicopters Furthermore, there were no significant
collection, performed in the aircrews immediately after differences in the time of onset or the intensity of pain
flight (4,5,6,7,8.9,10, I 1). One of these papers, performed for the vibration and no vibration test condition.
in the Fuerzas Aeromoviles de1 Ejercito de Tierra However, even vibration plays a very small role
(F.A.M.E.T.) showed figures up to 78% of pain related in the etiology of the acute back pain, authors like Aufret
to helicopter flying activities and 60% of the total and Viellefond (2) speculated that repeated exposures to
number referred to lumbar area (7). such conditions cause microtrauma to the spine, the
Fitzgerald et al (12) stated that, among military exposure to vibration can lead over the long term to
helicopters aircrew world-wide, backache is perceived as fractures of the thoracic spine or to herniation of the
so common that the majority of sufferers accept it as an lumbar discs.
occupational nuisance an rarely seek medical advise by The second factor is consider the major agent
the flight surgeon. causing back pain, The seat and control configuration in
Pain appears during or immediately after the most helicopters, force the pilot to assume an
flight. It could be transient and last for less than 24 asymmetrical posture for extended periods of time
hours, moderate in intensity, dull, localized in the lumbar The right hand operates the cyclic pitch control,
area and buttocks and without irradiation situated between the legs, and the left hand operates the
(3,4,6,7,11,13,14). But there are many other aircrews collective pitch lever in the let? side of the seat (16). So,
who do not refer pain with such characteristics, while the pilot keeps his hands on the controls, the body
symptoms are more frequent, persist for more than 48 will be bend to the left, and the requirements of flight
hours, intensity is greater and it is associated to visibility will necessitate forward flexion of the trunk,
paresthesias of the lower extremities due to nerve the back will not be firmly held against the seat back
compression (1 1,14) (Figure 1). In addition, the trunk-thigh angle in this
From this perspective we can differentiate two posture is less than 105 degrees, Wisner established that
types of pain features in helicopter pilots: this angle, in a individual seat, might be 135 degrees for
a. Pain lasts for less than 24 hours: transient. a good relaxation of opposite muscle group, which is the
b. Pain lasts for more than 48 hours: persistent. physiological equivalent to the subjective notion of
Two main facts are directly responsible of back comfort (17)
pain in helicopter crewmembers, one is the exposure to Froom et al. (4) made an interesting work to
vibration and the other is poor posture (4,12,15.16). prove that poor posture in flight is an important factor in
Vibration consists of a series of displacements the etiology of low back pain.
of a mass in both directions from its equilibrium point. They studied 18 pilots of the AH-IS helicopter
These displacements are characterized by their flying alternately in the gunner’s seat, where they
frequency, amplitude and velocity (2). The frequencies maintain a vertical sitting and in the pilot’s seat, where
of vibrations recorded in helicopters are between 2 and they lean forward and to the let? in order to operate the
20 Hz (16). controls. The intensity of the back pain was great and the
These vibrations are transmitted to the pilot onset was quicker in the pilot’s seat than in the gunner’s
through an undampened seat, which amplifies them. position.
Physiological effects of vibration are caused by the
Paper presented at the RTO HFM Symposium on “Current Aeromedical Issues in Rotary Wing Operations”,
held in San Diego, USA, 19-21 October 1998, and published in RTO MP-19.
20-2
Figure I. Pilot’s position in the cockpit
muscleskeletal disorders, pain should be the answer to
poor posture and is related to muscle spasm or other
transient mechanical factor, since it disappears during the
flight or immediately atter the flight
On the other hand, the fact of more flight time
in the second group made the conclusion that repeated
esposures to such tlight conditions lead to pathological
changes, which would explain that particular
simptomatology Although the same authors mentioned
the fact that the symptoms could be related to previous or
underlying pathology of the spine which make those
pilots more prone to such helicopter stimuli f I 1,14).
Summing up, although the poor posture alone
can cause pain, it is important to consider that this
postural condition may be aggravated over the long term
by the concomitant exposure to vibration The
combination of these factors over time may act
synergistically to cause pathological changes in the
spinal system.
And in addition to that tie have to consider
other pain contributing factors such as:
a. Presence of underlying disease of the spine
(congenital disease or trauma).
b. Total flying time of the pilot in rotary wing
aircraft
c. Frequency of flights. Symptoms appear more
in advance in pilots who fly more than 4 hours/day and
more than 40 hours /week or more than 2 hours in a row
!I61
d. Muscle tension in order to keep the
appropriate handling and control of instruments,
The constantly maintained asymmetrical depending of the flight type (7).
position does not permit relaxation of the spinal e. Cool air during open door maneuvers. might
musculature (IS), this situation probably leads to spasm at‘fect the lumbar area (IO).
of paraspinous musculature, which become fatigued, and f Age. Hoiber et al (21) found a lineal relation
the resultant is a straightening of the normal lumbar between back pain and age, in addnion to the tlying time
lordosis ( 19). factor
The spine losses the normal curve. the vertebral g. Finally, some other variables related to the
bodies tend to be closer together in front, and an increase life style (physical training, sedentarism. .) (14).
in hydraulic pressure develops in the anterior part of the Objective data obtained by su r-face
intervertebral discs. The nucleus pulpous is displaced electromyography has been used by Anderson et al. (20)
towards the rear of the intervertebral space where it can and results showed very accurate data in relation to
irritate the nerve roots with which it comes into contact. muscular activity. In the same line we think that skin
Furthermore this flattens the lumbar curve leads to a eiectromyography can provide very reliable information
stretching to the posterior longitudinal spinal ligament related to muscle response to the stress above mentioned
which is very pain sensitive. This physiopathological caused by helicopter operations.
mechanism is the source of the acute back pain in pilots
of helicopters.
In this line is very interesting the objective data OBJETIVES
obtained by Anderson and coworkers (20). They made
quantitative measurements of intradiscal pressure in the Purpose ofthis study will be:
lumbar spine and the myoelectric activity of back . To compare the right and left side lumbar muscular
tnuscles of various postures activity in helicopter pilots under real flight
They found lumbar intradiscal pressure was conditions in order to objectively prove the effects
highest with spine flexed anteriorly and myoelectric of asymmetrical posture on the musculosketetal
activity increased with forward flexion of the spine and system by using surface electromyography
asymmetric loading for a constant degree of spinal . To correlate lumbar muscular activity with
flexion They concluded increased myoelectric activity environmental and other variables such as type of
was indicative of localized muscle fatigue. Bight, type of helicopter. flight time, age, physical
Some authors (11,14) refer than those pilots fitness and height.
who complain of transient pain do not have spine
20-3
MATERIAL AND METHODS The Muscle Tester ME300 (Mega Electronics Ltd.
the
Kuopio, Finland) is usedto measure muscular
We have included in the study 35 pilots of activity in the lumbar area. The ME300 is a surface
helicopter of the Spanish Armed Forced. They were electromiographthat registerssimultaneouslyby means
destinated or have passeda formation period in the of two channelsthe potentials of action of two muscles
Helicopter Baseof the FuerzasAeromoviles de1Ejercito beingthe sensibilityof + / - 1 microvolt.
de Tier-t-a (F.A.M.E.T.) in Colmenar Viejo, Madrid, The knowledge of the potentials of muscular
which consented carry out a helicopterpilot’s course.
to action is since of great interest and the techniquesof
It is considered exclusion approach those surfaceelectromyography provides objective data of the
subjects that were discoveredafter a brief clinical history electric activity in the tnuscularcells (22). The muscular
that they had some pathology, traumatism or activity is measured by monitoring electrods with
degenerativeillnessof the spine. support of Micropore and solid gel applied directly on
To all the subjectsincluded in the study were the skin, in both paravertebral areas on the lumbar
carried out a survey to determinethe following variables: musculature at level of 1~1-1-2.Three eiectrodes are
age, weight, height, body mass index, total hours of placed in each side, two actives and one ground
flight and hours of flight in specific helicopter. The to
separated about 3 cm and forming a triangle and like
physical activity data was picked out as a dicotomic oneobservesin the Figure 2.
variable: - No sportat all
- One sport is practisedat leastoncea week. Figure 2. Placeofelectrodes
The flights were carried out in two types of
helicopters (the HR-12 and the HU-IO) and the
helicopter cockpit mock-up Frasca model 3OOH The
HR-12 or Kyowa, it is an American helicopter of 1.360
kilograms, with a capacity of two pilots’ transport and
three passengers, speed of cruise of 156 Km./h and
autonomy of 2 hours 30 minutes. The HU- IO or Huey
also American of 4.3 18 kilograms, with a capacity of
transport of three crews and ten passengers, speed of
cruise of 166 Km./h and autonomy of 2 hours IO
minutes. To avoid errors in the registration, the test was
In the HR-I 2 two types of flights have been performed alternating the channels1 and 2 respectively
picked out. The “basic” or “accommodation“, in the right and left paravertebral muscles.In this way
to of
corresponds the first phase helicopter pilot’s course, we carry out a comparativeisometrictest of the right and
the flight is visual and the landings, taking-ot‘ts and letI lumbarmuscles
normal traffic are carried out. And the “tactical The registrations are carried out each one-
manoeuvres”is the secondphaseof pilot’s course and secondduring the wholetime of registration.
they are visual flights where besides the above- The information received by the surface
mentionedtactical manoeuvresand formations are also of
electrodesis picked out in a microprocessor smallsize
carried out. and reduced weight that the pilot put in one of the
In the H&IO they are also two types of flights pockets of his flight’ suit. The registration starts
selected The “instrumental” is the advancedphase the of immediately after placing the electrodesin the lumbar
pilot’s course, the flight is only carried out with the region of the pilots. An external marker cable connected
instruments,and therefore one doesnot have an external to the microprocessoris used to allow LIS to make
vision. And the “specialisation” or instructors’formation different marks it and oniotT selectsconcrete periods of
where besidesnormal and emergency manoeuvresare flying in the overall registration.
also made. The flights c.arried out in the helicopter Measurements:
cockpit mock-up were flights of “instrumental“ type. . Baseline The first period is selectedbefore entering
According to the helicopter and the flight type in the helicopter. The pilot remainsseateddown, in
we includethe subjects studiedin the following groups: a relaxed position and leaningthe back totally in the
. Basiccoursein HR- 12. back of the seat. This first phaseit is denominated
. Tactical manoeuvres coursein HR- 12. “basal period” At this time it is explained to the
. Instrumentalcoursein HI!-10 pilot that he should pressthe device of the marks
. Improvementcoursein HIJ- 10. during the flight when one will take or leave the
. Flying in the helicoptercockpit mock-up controls; this is the only way to selectthe periodsof
Sometimes helicopter is piloted on the right
the flight just when the pilot hasremainedin the posture
or left position in the cockpit with the control that we want to study. This phase accomplished only
instruments of in the left or right front respectively, in order to test the correct operation of the
although the cyclic pitch control and the collective pitch eletromiographin a relaxedposition
lever, manualcontrols, are in the sameposition in both . Flight According to how the tlight was done. we
seats. Thesedata are picked out in the survey. and
can obtain one or severalphases onceselected
20-4
we can study them as one. We called this phase “flight
period .‘. been “Instrumental”, 9 “Tactical Manoeuvre “, 8
The data accumulated in the memory of the “Accommodation” and 4 of “Improvement.” In Table 2 it
microprocessor are transferred to a Computer system shows the distribution of the types of flight according to
where they are analysed by a specific program of the the kind of helicopter. The pilot’s position in the cockpit
ME300 allowing the analysis of the data. The data of has been in the right side in 28 cases (80%) and in the
both channels are the following: time, minimum and left side in 7 cases (20%).
maximum potential, mean and area or the sum of all the The maximum duration of the flight was of 80
potentials of action. The work carried out by the right minutes being the minimum of 16 minutes, in 75% of the
and the left musculature is expressed in percentages, cases the duration oscillated between 30 and 60 minutes.
defining the variable “right prevalence” as the area under (mean-+/- DS; 43.8 15.9). Four (11%) of the thirty-five
the curve of the voltage obtained in the right lumbar area pilots they referred light lumbar nuisances after the test.
divided by the sum of the right and left areas. Only 3 subjects (9%) presented a larger activity
We used new marks on the record handling the in the left lumbar muscular side than in the right one,
ME300 software to annul the interferences produced by being for the 32 remaining (91%) predominate the
the VI-IF band waves of radio frequency of activity of the region lumbar right. The mean percentage
communication used by the pilots (30 -2OOMhz.) as can of “right prevalence” for the total of the cases is 63.1%.
be seen in Figure 3. In Figure 4 the distribution of the right prevalence is
picked up for the 35 pilots.
Figure 3. Normal record, with interferences and marks to
annul them. Figure 4. Distribution of right prevalence in studied
pilots.
-----------
18
__________-_ I
18
---------- ___________-
16
---------- ___________-
1-l
---------- ____________
12
___________-
E
a ________ ____
i)
_________-__
The resulting variables of the survey and the JO-SO% S-60% 61-70% 7140% al-90%
Muscle Tester ME300 are introduced in a database using % of light prPvalPnce
the Statistical Package for the Social Sciences (SPSS)
statistical computer program for Windows Release 6.0.
To study the association of the variable “right
.For the analysis it have been used the tests of prevalence” with the rest of the variables collections,
comparison of means (t of Student and ANOVA).
only included in the analysis 32 pilots with values of
Chance probability of “p” inferior or equal to 0,015 is
“right prevalence” superior to 50%. In Table 3 the
accepted as critical for statistical significance.
distribution of the value means of the characteristic
studied antropometric is shown for three levels of “right
prevalence.” As it is reflected in this table neither the
RESULTS.
weight, height neither the corporal mass index associated
statistically in a significant way to our variable in the
The study has been carried out with 35
study.
helicopter pilots that have accepted to participate
When increasing the level of “right prevalence”
voluntary in this study. All the pilots are male. The mean
so much the mean values of the age and total hours of
age is 28.2 years with a range 23-39 years. The
flight as in specific helicopter in with the test was carried
antroprometric characteristic and the age of the
out also increase, although this tendency is not
participant appears in the Table I The 80% of the pilots
statistically significant ( Table 4).
practise a sport at least once a week.
It is statistically significant the association
Regarding their experiences as pilots we find a
among the level of “right prevalence” and the duration of
stocking of 577.8 total hours of flight in helicopters
the flight, being increase in the mean duration from the
(range 56-3300 hours) and a stocking of 342.9 hours of
flight w-hen increasing the level of “right prevalence.”
flight in a specific type of helicopter that each of the
These results are picked up in Table 5.
pilots have carried out the study (range S-1900 hours).
In Figure 5 the regression straight line is shown
Of the 35 flights in those that measurements 17
for the relationship among the variables “right
were obtained with the helicopter HR- 12, 12 with HU- 10
prevalence” and “duration of the flight”
and 6 with the Trainer. As for the type of flight 16 have
20-5
Figure 5 Distribution of right prevalence in function of pilots causedby Gz forces and head movementson the
the durationof the flight. cervical erector spinaemusclesduring flight missions.
This surfaceelectromyographicrecordsaction potentials,
they calculatedthe relative strainon the cervical muscles
by comparing these potentials with those representing
the highest maximal voluntary contraction produced by
the same muscles in each subject. The mean data
obtained during flight missions were expressed in
percentof the maximal voluntary contraction (100%).
This method has its limitations, it records only
muscularactivity and does not show directly the stress
on intervertebral discs or ligaments, nevertheless these
authors assumedto exist some correlation between
4 -. * of
muscularactivity and compression the structuresof
50 . I
IO 33 30 40 50 60 70 80 90 the cervical spine.
The ME300 is a EMG surface similar to which
Duration of the fligh(minut.) Hamalaine the
used(23) in his work that measures action
potentials by meansof two channelsthat allow us to
carry out a comparative isometric test of the right and
left lumbar musculature.
When comparing the value means of “right The work carried out by the right and lefl
prevalence” according to the different types of used lumbar musculature is expressed in percentages
helicopterswe do not find significant differences,neither regardingthe total activity (100%) denominatingto the
not existing according to the type of carried out flight percentages right and left, respectively, right prevalence
(Table 6) nor according to the groups defined in the and left prevalence. According to that we can study the
material and methodssectionas a result of the crossing difference amongthe work carried out by the right and
of helicopter type and flight. Neither the subjectsthat do the letI lumbar musculatureand in this way we can
not practice sport nor thosethat sufferednuisances after the
demonstrate pilot’s asymmetricposture
the realisation of the study have demonstratedto have In the period of flight, that is to say, in the sum
figures stockings of “right prevalence” statistically of the phases during which the pilot meets with the hands
different from those that do not present these using the manualcontrols, the work carried out by the
characteristics. right lumbar musculaturewasbigger in 32 pilots out of
the 35 studied (91%) and the work of the left
musculaturewas lightly greater in the remaining three
DISCUSSION (9%).
In Figure 4 the distribution of the right
In order to study back pain in pilots, mostof the prevalencein the pilots studiedis represented:3 pilots
works have obtained their results by questionnaires or (9%) had a right prevalence below 50%, concretely
others methods which get data purely subjective. between the 40 and 50%; the largest group, 18 pilots
There are very few paperswhich have madean (51%) represented those whose right prevalence
objective study. Among them we would like to oscillated between 61 and 70%; 9 pilots (26%) were
emphasizein which was used surfaceelectromyography between 51 and 60%; 3 (9%) between71 and 80% and
of
to obtain an objective and fiable result by means a fast only two pilots (6%) they had a right prevalencebetween
and bloodlesstest of the muscleactivity of the muscular 81 and 90%.
activity (22). For being superiorthe numberof cases great of
For example, Le Menn et al. (8) studied the activity of the right musculaturewith respectto the left
myoelectric signals from the trapezius and the one, were only used in the study the 32 pilots whose
stemocleidomastoid muscleson tighter aircraft aircrew, right prevalence,in the period of flight, was greaterthan
to analyze the fatigue of theseneck muscles during head 50%.
movementsunderhigh-Gz loading in flight. Thesedata take usclearly to the conclusionthat,
Another reference work which used a surface the
in more or smaller measure, pilot during the flight,
electromyography was performed by Anderson et al. when he is using the manual controls, he does not
(20). They made quantitative measurementsof the maintain a symmetrical posture and that makes him
myoelectric activity of the back in various postures. contract a sidemorethan the other.
They concluded increased myoelectric activity was They are many authorsthat have referred to this
indicative of localized muscle fatigue, because the asymmetry in the posture of the flight in their
activity increased with asymmetricloading for a constant publications (3,4,6,7,10,11,13,14,15,16). They all
degreeof spinalflexion. coincide in that the helicopter pilot of while flying
Hamalainen and co-workers (23) used the maintain their trunks slightly forward and rotated to the
Muscle Tester ME3000 to investigatethe strain in fighter left, this would explain the biggest right contraction.
20-6
This great right contraction shows more the Shanahan et al. (11,14) suggested that, the fact
longer the flight. In Table 5 the mean duration of flights that the pilots with more flight experience had a back
are exposed in different levels of right prevalence, this pain with some specific characteristics (bigger duration,
way the pilots whose right prevalence oscillated between irradiation to inferior members, etc.) it makes one think
50 and 60%, carried out flights of mean 37.4 minutes; of some evolutionary muscleskeletal pathology like
between 61 and 70% the flights lasted 40.58 mean consequence from a chronic exhibition to some adverse
minutes; and lastly in the pilots whose mean flight was conditions.
65.6 minutes the right prevalence was great than 70%. Our opinion about the fact that there is more
This increase of the right prevalence in right prevalence on the left side in aircrew with a larger
connection with the flight duration was statistically amount of flight time, seems to be related with the
significant (p 70% 25,05 77,80 I,76
Statistic sigr@cation (p) 0.8 0.9 0. I
Table 4. Appearance of the age and the experience as pilots for different levels of right prevalence
Means
Level of “right prevalence” STOKING HOURS
50-60% 9 384
61-70% 657
>70% 693
Statistic signilfication (j$ 0.5
Table 5. Appearance of the duration of the flight for different levels of right prevalence
Table 6. Comparison of the percentages means of right prevalence according to type of helicopter and kind of flight.
“right prevalence” statistic
CATHEGORIAS
mean percentages signification
FIR-12 60.7
Kind of helicopter Hu-10 64.5 0. 7
COKF’IT MOCK-UP 66.7
INSTRUMENTAL 66.3
TACTICAL MANEUVERS 59.9
Kind of flight 0.2
ACOMODATION 61.4
lMFROVEMENT 60.3
20-S
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