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Back pain and exposure to whole body vibration in helicopter pilots a
a
a
a
P. M. BONGERS , C. T. J. HULSHOF , L. DlJKSTRA , H. C. BOSHUIZEN , H. J. M. b
GROENHOUT & E. VALKEN
b
a
Coronel Laboratory, University of Amsterdam , Meibergdreer 15, 1105, AZ, Amsterdam, The Netherlands b
Occupational Health Service of the Royal Airforce , The Netherlands Published online: 24 Oct 2007.
To cite this article: P. M. BONGERS , C. T. J. HULSHOF , L. DlJKSTRA , H. C. BOSHUIZEN , H. J. M. GROENHOUT & E. VALKEN (1990) Back pain and exposure to whole body vibration in helicopter pilots, Ergonomics, 33:8, 1007-1026, DOI: 10.1080/00140139008925309 To link to this article: http://dx.doi.org/10.1080/00140139008925309
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ERGONOMICS, 1990, VOL. 33, No.8, 1007-1026
Back pain and exposure to whole body vibration in helicopter pilots P. M. BONGERS, C. T. J. HULSHOF, L. DUKSTRA, H. C. BOSHUIZEN Coronel Laboratory, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands
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H. 1. M. GROENHOUT and E. VALKEN Occupational Health Service of the Royal Airforce, The Netherlands Keywords: Backache; Vibration; Posture; Helicopter.
In a questionnaire survey the prevalence of back pain in 163 helicopter pilots was compared to that in a control group of 297 non-flying air force officers who underwent the same pre-employment medical examination. Since pilots document their hours of flight in a personal flightlog, an accurate estimate of the duration of exposure could be made. In addition, vibration levels of the helicopters were measured and an accumulative vibration dose was calculated for each pilot. 'Transient' back pain of a short duration was more frequent amongst the pilots compared to the control group, and the prevalence of 'chronic' back pain of a persistent nature was also higher amongst the helicopter pilots.Transient back pain seemed to be most strongly related to the average hours of flight per day, whereas chronic back pain was more closely related to total hours of flight or the accumulative vibration dose. A significant higher prevalence of this chronic back pain was observed only after 2000 hours of flightor a vibration dose of 400 m 2h/s·. The observed health effects may be due to vibration or constrained posture but are most likely due to concomitant exposure to both factors.
1. Introduction As part of an extensive research project on adverse health effects of long term exposure to whole body vibration, a questionnaire survey was conducted among almost all helicopter pilots in The Netherlands; both military and civilian flight crews. Helicopter pilots form a unique group for studying the relationship between exposure to whole body vibration and the adverse effects on the spinal system. Many studies have reported high prevalences of back pain in helicopter flight crews, but due to high motivation, the number of medical drop-outs is small. Helicopter pilots also have little prior exposure to vibration or other physically demanding work environments. In addition, the duration of exposure is systematically documented in a flight log and measurements of vibration levels of currently in use helicopters can be used to estimate an accumulative vibration dose, since most pilots spend the majority of their flying career in these helicopters. Further helicoper design has changed little over the last decades. Therefore, in contrast to most other occupations which involve exposure to whole body vibration, a relatively accurate estimate of the accumulative exposure dose can be made. The assessment of a dose-response relationship between exposure to vibration and long-term health effects is complicated by the poor posture of pilots during flight. For the majority of the time, helicopters require active input from all four extremities of the 00!4-{)139/90 $3·00
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pilot in order to maintain full control of the aircraft. The seat often lacks sufficient lumbar support and the control configuration often forces the pilots to assume a slightly asymmetrical position: they have to bend forward and lean slightly to the left. This typical posture, 'the helicopter hunch' and exposure to vibration are the factors most widely implicated in the etiology of back pain in helicopter pilots. However, most modern helicopters are equipped with auto-pilots which improve posture while cruising.
2. Literature 2.1. Epidemiological studies In several epidemiological studies on back pain in helicopter pilots a distinction has been made between transient back pain and chronic back pain (Shanahan et al. 1986, Froom et al. 1986). These terms are used to discriminate between temporary back pain associated with actual flight and back pain not directly associated with actual flight. The transient pain is generally described as a typically dull pain without radiation and is confined to the lower back, whereas chronic back pain is described in several ways, e.g.,as sciatic pain that radiates to a leg,as pain that leads to bedrest or the incapacity to fly, or as pain lasting several days or longer. According to Bowden (1987) this 'chronic' back pain might be compared to idiopathic low back pain in the general population. Back pain prevalences in helicopter pilots reported by several authors vary from 21 to 95% (Delahay et al. 1982, Schulte- Wintrop and Knoche 1978, Fischer et al. 1980, Braithwaite and Vyrnwy Jones 1986, Shanahan et al. 1986, Froom et al. 1986). The prevalence of back pain during or shortly after flight ranges from 34 to 64%. Back pain not primarily associated with flight is reported in II to 27% of the respondents (Shanahan et al. 1986, Froom et al. 1986). Two studies show an increase of lumbar X-ray abnormalities in helicopter pilots compared to jet pilots (Fischer et al. 1980, Froom et al. 1984). 2.2. Experimental studies In a mock-up of a UH-I H helicopter seat and control configurations II subjects were asked to indicate when they experienced back pain and with what intensity (visual analogue scale) (Shanahan and Reading 1984). Each pilot was asked to adopt the slumped and slightly asymmetrical posture which he assumed in normal flight and was tested for two separate periods of 120min, one with simulated helicopter vibration and one without. All subjects reported pain identical to the discomfort experienced when actually flying a UH-IH helicopter for similar periods. No significant difference was found between the vibration and non-vibration conditions for either time of onset or intensity of pain. The pain was uniformly described as a dull ache or numbness confined to the lower back and/or buttocks without radiation into the legs. The authors concluded that for the transient back pain experienced by these aviators, posture is the most important etiologic factor. They hypothesized that the slumped asymmetrical posture assumed for extended periods of time leads to spasm of the paraspinous musculature and increases the pressure sensitivity of the buttocks. The authors do not extend this conclusion to more chronic back pain. Pope et al. (1986) tested the change in muscle response (measured as a shift in the centre frequency of the EMG spectrum) due to sustained posture and vibration in a simulated UH-I H cockpit. Marginally significant fatigue occurred only as a result of
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Back pain in helicopter pilots
the sustained static posture. In contrast, all tests both with and without vibration produced subjective discomfort both in the· buttocks and the lower back. Froom et al. (1987)compared the onset and intensity of back pain between aviators occupying the pilots seat and the gunner's seat of a helicopter. In the gunner's seat the aviators maintain an upright position whereas in the pilot's seat they have to lean forward and to the left in order to operate the controls. Vibration levels were similar in both positions. Most pilots experienced pain or discomfort in both positions. However, the intensity was greater and the onset of the pain quicker in the pilot's seat. Voss and Krogh-Lund (1986) compared the occurrence of a shift towards a lower frequency EMG signal between a group often civilian helicopter pilots and a group of ten office workers. They concluded that the helicopter pilots working conditions caused significantly higher localized muscular fatigue in the lumbar compartment of the erector spinae muscles compared to the control group.
3. Materials and methods 3.1. The population A questionnaire was submitted to 100 army helicopter pilots, 51 civilian helicopter pilots and 12 army observers. The control group comprised 297 non-flying air force officers. The response rate was 83% for the helicopter aircrew and 78% for the control group (table I). Eight pilots, observers or controls, who had been exposed to vibration prior to their current job or during their current job (controls), were excluded from the research population. Air force officers with a physically demanding job, e.g., sports instructors, were not included in the control group. The major part of the job of the controls consisted of office work comparable to the activities of the pilots between flights. Both the army pilots and the control group officers underwent the same medical examination of the vertebral column upon entry to the military service and were recruited from the same age categories. The majority of civilian helicopter pilots were trained and formerly employed by the army. They are currently in charge of transport to and from the off shore oil platforms. 3.2. The questionnaire The questionnaire was similar to those used in two other studies on the health effectsof occupational vibration in The Netherlands (M usson et al. 1987, Boshuizen et al. 1989). It included items on accumulative exposure, potential confounding factors and health. Data collected were hours of flight for each type of helicopter ever flown and the average daily and weekly hours of flight. The potential confounding factors investigated were age, height, weight, climatic conditions and experienced tension during work. The respondents were also asked to assess the time spent in a bent
Table 1. The response.
Index group civilian pilots air force pilots observers Control group
Research population
Returned questionnaires
163
133
12
136 (83%) 39 (76%) 89 (89%) 8 (67%)
297
233 (78%)
228
51 100
Analysis 39
87 7
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Table 2. Questionnaire items on back pain. Questionnaire item
label
• Do you regularly experience pain or stiffness in the back? • Where do you experience pain or stiffness in the back? - the upper part of the back - the middle part of the back - the lower part of the back - the entire back • How long does the back pain usually last? - several hours - several days - several weeks - several months • How often do you experience back pain? .. per year • Do you have or have you ever had lumbago? • Do you have or have you ever had a prolapsed disc? • Did you receive medical treatment for your back pain? • Does the back pain radiate to one of your legs? • Have you experienced back pain for a continuous period of two weeks or longer in the last year? • What is the pattern of your back pain? - back pain that varies from day to day - back pain for long unbroken periods alternated with periods without back pain - back pain that is almost always present
back pain
The following back pain parameters combine frequency and duration of back pain: • Back pain lasting several hours for less than 50 times a year or back pain lasting several days for less than ten times a year • Back pain lasting several hours for more than 50 times a year or back pain lasting several days for more than ten times a year or back pain lasting several weeks or longer I
2
low back pain hours' days? »wecks!
lumbago hnp treatment? sciatica!
>2 weeks"
alternating' periodical! always!
mild long or frequent
'Transient' back pain 'Chronic' back pain.
forward or twisted seating position (5 points scale). Table 2 presents the questionnaire items on back pain. The last column gives the labels used in the tables with the results. As indicated in table 2 transient back pain is defined as back pain lasting several hours (hours) with a varying pattern (alternating). In agreement with the literature on back pain in helicopter pilots, chronic back pain is defined as all back pain lasting several days or longer (days, > weeks, > 2 weeks) or back pain that radiates to a leg (sciatica) or that needed medical treatment (treatment). In addition questions were asked to establish the prevalence of back pain directly associated with flight (table 3).
3.3 Vibration measurements Three-axial vibration measurements were conducted in agreement with the ISO 2631/1 guidelines (1985). The air force pilots flew in the Alouette III and the Bolkow 105, the civilian pilots in the Sikorsky S61N and the Sikorsky S76A. The vibration levels of two helicopters of each type were measured under representative flight conditions (van Wijk and van der Weiden 1989). The acceleration levels of the vibration depended on the type of flight, e.g., hovering, starting and cruising. These flights were measured
Back pain in helicopter pilots
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Table 3. Questions on back pain directly associated with flight. Do you experience back pain in one of the following situations: -
during or shortly after every flight (independent of flight time)? during or shortly after a flight of less than 2 h? during or shortly after a flight of more than 2 h? during or shortly after a flight with high mental concentration? during or shortly after a period of intensive flying (e.g., more than 20 h of flight per week)
Table 4. Vibration measurements.
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Rating method
Alouette III
X Y Z vectorsum average Bolkow 105 X Y Z vectorsum average X Sikorsky 61 y Z vectorsum average X Sikorsky 76 y Z vectorsum average
Frequency' (Hz)
(m/s 2)
arms
Exposure time" (h)
16 16 16
0·06-1·21 0,91-1'31 0'33-1'08
16->24 14-21 4-18
25 6 25
0·06--D·90 0·06--D·21 0·52--D·93
>24 >24 9-20
16 16 8
0·04--D·39 0,08-1,01 0'08--D'68
>24 19->24 7->24
20 20 8
0·04-1·07 0,08-1,05 0·09--D·24
23->24 23->24 9->24
Weighting method arms
(m/s 2)
0·12--D·17 0·17--D·25 0·44--D·67 0·56--D·75 0·60 0·09--D·13 O' t3--D. I8 0·29--D·49 0·36--D·58 0·45 0·06--D·11 0·1O--D·21 0·17--D·44 0·24--D·55 0·36 0·07--D·14 0·1O--D·19 0·17--D·36 0·28--D·45 0·38
Exposure time' (h)
13-20 7-20 3-5 2-4 18->24 12-21 4-9 3-7 23-> 24 9->24 5-20 4-13 17->24 10-->24 7-20 5-10
'Centre frequency of the one-third octave band with the highest acceleration level in relation to the ISO fatigue/decreased proficiency limit. 2Duration of exposure before reaching the fatigue decreased proficiency limit of ISO 2631/1.
separately and the range ofthese data is presented in table 4. The peaks in thefrequency spectrum are due to the frequencies of the main rotor system and the rotor blades and to multiples of these frequencies. The majority of these predominant frequencies are higher than the first resonant frequency of the spinal system of a seated subject (4 to 5 Hz in the vertical direction (Panjabi et al. 1986). The vibration signal was evaluated in relation to the international standard for evaluation of human exposure to whole-body vibration, ISO 2631/1 (ISO 1985). In this standard two methods are indicated to evaluate broad-band distributed vibration. With the rating method the rms acceleration level of each one-third octave frequency band is evaluated separately in respect to the fatigue/decreased proficiency boundary at that frequency. With the weighting method the overall vibration signal for the frequency range I to 80 Hz is
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weighted according the frequency dependence curves of the standard and expressed in one value. In the present study evaluation using the weighting method gave quite different results from that using the rating method. In order to calculate the accumulative vibration dose the average of the vector sums of the weighted acceleration levels was used. The acceleration level of the UH-ID helicopter flown in the past was derived from literature data (Theiler 1986). If no literature data were available the average value of all other levels (0'5 m/s 2 ) was substituted. Few data are a vailable to decide upon how to calculate a vibration dose value for more than 24 h. Two vibration dose values were calculated Dose!(La i ' t, m 2h/s4 ) and Doseltat· t i m 4h/s 8 ). In which ai is the estimated level of acceleration (m/s") in helicopter i and t i is the.total flight time (h) in helicopter i. Dose! approximates the time dependency relationship proposed in ISO 2631/1 for daily exposures, following the energy equivalence principle. In Dose, the more recent suggestion ofa time dependence of daily vibration exposure as a fourth power relationship has been followed (ISO 1988, Griffin 1982). Since neither of these dose values are applicable to exposures of more than 24 h, their choice is somewhat arbitrary. 3.4. Statistical analysis A multivariate logistic regression analysis (Breslow and Day 1980) was conducted to investigate the relationship between the exposure variables and the health effect parameter. In this analysis odds-ratios were computed for back pain parameters and exposure variables adjusted for several potential confounding factors, with the control group as reference. In addition, pooled maximum likelihood odds-ratios after stratification for age in ten year categories were computed (Rothman and Boice 1979). The Mantel Haenszel Chi-square test was used to determine whether these odds-ratios significantly exceeded 1. Since the pooled odds-ratios did not differ from the oddsratios obtained by the multivariate analysis these odds-ratios are not presented. The logistic model was fitted with all potential confounders unless the number of cases was less than four times the number of variables in the model. In this case only the variables that considerably changed the odds ratio of the exposure variable were included in the model. Multiplicative interactions between the exposure variables and potential effect modifiers were studied. In order to clarify the interrelationship between the various back pain variables a factor analysis of these variables was also conducted.
4. Results Table 5 presents the characteristics for the for the observers are officers in the control
arithmetic mean and standard deviation of several personal different categories. Due to the small numbers, no separate data presented. The age of the pilots ranged from 21 to 56 yr. The group were on average slightly older with an age range of 20 to
61yr. The civilian pilots had experienced significantly more flight hours both in total flight and in hours of flight per day than the army pilots (table 6). Figures I and 2 show that the pilots indicated spending significantly more time in a twisted seating posture than did the controls. The time spent bending forward did not differ significantly. The pilots also reported more draught and cold (77% vs 33%). Almost half (45%) of both army and civilian pilots and 34% of the control group indicated feeling tense during their work, whereas 63% of the army pilots, 80% of the civilian pilots and 72% of the control workers experienced mental stress during their work.
Back pain in helicopter pilots
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Table 5. Personal characteristics. Army pilots (n= 87)
Age (yrs) Height (em) Weight (kg)
* p';; 0·05 T-test, ** p';;O·Ol.
Civilian pilots (n= 39)
Control group (n=228)
Index group (n= 133)'
mean
s.d.
mean
s.d.
31*2 182 77
8 7 8
39 182 78
4 7 9
mean
s.d.
mean
s.d.
8 7 8
35 181
to
33 182*3
77
77
two sided.
, observers included. army pilots tested against civilian pilots. 3 index group tested against control group.
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2
Table 6.
Duration of exposure and exposure dose. Army pilots (n= 87)
Total flight time (yr) Total flight time (h) Hours of flight per day Dose, (m 2h/s4 ) Dose, (m4 h/s8 )
Civilian pilots (n=39)
All pilots (n= 126)
mean
s.d.
mean
s.d.
mean
s.d.
7.9**' 1986**1 2'4**' 652**' 228
5·8 1324 0·7 468 181
14·6 5473 5-3 1081 179
4·0 1594 1·0 314 120
9·9 2974
6·2 192 1·5 471 166
3-2 774 213
** p,;;O'OI T-test
two sided. , Army pilots tested against civilian pilots.
Table 7 presents the prevalence of back pain and the adjusted odds-ratios for the back pain parameters in the index and control group. Although the odds-ratios for the transient pain parameters were particularly high, also the odds-ratios for the more chronic pain parameters were significantly greater than I. From figure 3 it can be concluded that, particularly in the younger age group, back pain prevalence for the pilots were relatively high. An illustration of the duration of the back pain is given in figure 4. The factor analysis showed four different back pain dimensions in the pilots: (I) Low back pain that lasted several hours with symptoms varying from day to day; (2) back pain lasting several days with extended periods with symptoms and pain free periods sometimes with radiation to a leg; (3) back pain lasting several weeks; and (4) lumbago or back pain that needed medical treatment. In addition to the impact of flying helicopters, also age, experienced mental stress and reported time sitting in a twisted posture were of significant influence on the prevalence of back pain after allowing for exposure variables and other potential confounders. The pilots with back pain had significantly more total hours of flight, hours of flight per day, experienced mental stress and tension and were sitting more often in a bent forward and twisted posture than those without back pain. On the other hand personal characteristics such as age, height and weight were not significantly different between the two groups.
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P. M. Bongers et al.
1014 11111
•
911 811 711
III always
01/2
611 (x)
511
• never
48 311
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211 111 II
Index
contr
Figure 1. Part of the time of work spent in a forward bent posture. 11111 911 811
(x)
I-
•
always
l!l 3 ....4
711
01/2
611
~
511
1....4
• never
411 311 28 111 8 index
contr
Figure 2. Part of the time of work spent in a twisted posture. Table 8 presents the prevalence of back pain directly associated with flight. From table 8 it is obvious that particularly high back pain prevalences are reported after a flight of more than 2 h duration or after a period of intensive flying. Table 9 presents the odds-ratios for back pain of pilots with increasing hours of flight compared to the control group. From table 9 it is evident that transient back pain, e.g., pain lasting only several hours, often started before 1000 h of total flight time. Additional analyses also showed a significantly higher prevalence of transient back pain compared to the controlgroup before 500 h of total flight time. However, the prevalence of back pain lasting more than several.days only increased after 2000 h of total flight time. As expected, the prevalence of back pain of short duration did not increase with increasing exposure time.
Back pain in helicopter pilots
pi
1015 tots
88
•
78
~ contro Is
68 58 (Yo)
48 38 28
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18 8
28-38
31-48
41-58
51-&8
age('jears)
Figure 3. Prevalence of back pain stratified for age. 78
&8
I:B
58
o weeks
Months
-. ~
48 (Yo)
38 28
da!js
18 8 index
contr
Figure 4. Duration of back pain.
Due to the similar flight career of most pilots, exposure time in hours of flight and accumulative dose (Dose.) are highly correlated (R = 0·87 within the group of pilots). Therefore, the relationship between categories of accumulative dose and the prevalence of back pain is similar to the relationship between total hours offlight and prevalence of back pain (table 10). The threshold of 2000 h of flight time corresponds with a dose of ca400m 2h/s4 • The correlation between Dose! and Dose, is also very high (R=0·81 within the group of pilots). No clear relationship between Dose, and the back pain prevalences was observed (not shown). The data suggest that back pain of short duration develops into more serious back pain with increasing hours of total flight time or vibration dose. Comparison of the odds-ratios for 'mild' and 'long or frequent' back pain gives some support for this hypothesis. 'Mild' back pain decreased with increasing exposure time, whereas 'long or frequent' back pain increased as total flight time increased. This is also illustrated in
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P. M. Bongers et at.
Table 7. Back pain prevalences, and odds-ratios adjusted for age, height, weight, climate, bending forward, twisted posture and feeling tense during work. Index group (n=133)1
Control group (n=228)1
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(90% confidence
Back pain parameter
%
abs
%
abs
DR
interval)
Back pain Low back pain > two weeks last year Lumbago Sciatica hnp' Treatment Duration Hours Days >weeks J Pattern Alternating Periodical Mild Long or frequent
68 55 21 13 12 5 17
(91) (69) (26) (16)
(39) (24) (16) (20) (14)
(6) (22)
17 II 7 9 6 5 16
(37)
8·0" 9·0" 3·0·· 2·6· 3·3· 1·9 1-3
(4'5-14'3) (4'9-16,4) (1-4--6'4) (1,1-6,0) (1'3-8'5) (0'6-5,9) (06-2'6)
36 21 9
(45) (26) (11)
4 9 4
(9) (20) (9)
11-3··· 2·4· 1'4··
(5'2-24'8) (1'1-5'0) (0'5-3-8)
41 20 44 22
(52) (25) (55) (27)
6 8 II 6
(13) (17) (25)
9·5·· 2·5· 5·6··· 3·4··
(4'8-18,9) (1'2-5'4) (3'1-10'2) (1'5-7-6)
(C)2 (C) (C) (T)
(C) (C) (T)
(C)
(IS)
(10)
(13)
• p 2 weeks Lumbago Sciatica hnp3 Treatment
4000 (n=29)'
(2,4-11,4) (2,9-14,6) (1'8-12'1) (0'6-6'2) (0'7-8'1) (0'5-9'9) (1'0-5'6)
11·1"" (4,4-28,9) IN"" (5'7-32'0) 3·7" (1'4-9'2) H" (1'2-8'4) 303" (1'0-10'4) 304 (1'0-11'8) 1·7 (0'7-H)
5·8"" (2'1-15'9) 2-4 (0'9-6'4) 2·1 (0'6-7'0)
11·6"" (3'8-34'2) 304" (1'4-8'2) 1·5 (0'5-4'7)
Duration
Hours Days »weeks' Pattern
Alternating Periodical Mild Long or frequent
(T)
(C) (C) (T)
(C)
12·2"" (4'1-36'1) 15·3"" (5'6-41'9) 3-6 (0'9-13-7) 0·6 (0'I-H) 1·1 (0'1-7'7) 19·2"" 1-4 17'1"" 0·6
(6'3-58-4) 15·8"" (4'7-29'7) (0'3-5'8) 1·4 (0'5-6'3) (6'3-33'2) 1l-8"" (4'7-29'7) (0,1-4,1) 1·0 (0,2-4'5)
4·2"" 2'8"" 6·7"" 5·0"
(1'6-10'6) (1'2-6'6) (2'2-20'1) (1'8-13-6)
9·3"" 3·0" 2·4 5·2""
(3'8-23'5) (1'1-6'7) (0'9-7-3) (2'0-1303)
"e< 0·05 one side test. "" p';; 0·0 I.
, Due to missing valuesfor either the back pain parameters or the confoundersthe numbers in the four categories do not add up to 133. 2 Chronic back pain (C), transient back pain (T). 3 Only adjusted for age, height,and twistedposture - estimate very unstable due to small numbers in that category.
prevalence of transient back pain increases with daily exposure time and more chronic back pain increases mainly with total flight time and total vibration dose. Some support for this hypothesis was found when the daily exposure time and the total flight time were both included in the model. Due to the rather high correlation between these variables the point estimates of the odds-ratios become more imprecise, but the oddsratios for 'transient' back pain increase significantly with hours of flight per day also after adjustment for total hours of flight. The elevated odds-ratios for categories of more than 2000 h of flight for 'chronic' back pain become smaller, but are still greater than unity after adjustment for current daily exposure time. Since the civilian pilots have flown for significantly more hours in total and fly more hours per day than the army pilots and since they also experience more back pain, the observed positive trend of back pain with increasing exposure time might be due to the difference in prevalence of back pain between the army pilots and the civilian pilots. Therefore, the relationship between the prevalence of back pain in different categories of exposure (total dose and hours of flight) in each subgroup was compared to the control group separately. Analysis of the data of army pilots showed an increase in the prevalence of 'chronic' back pain parameters after 2000 h of total flight time. Due to the small numbers the confidence intervals are rather wide and sometimes include I. The prevalence of 'transient' back pain in the army pilots decreased with increasing total
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P. M. Bongers et al.
Table 10. Odds-ratios and 90% confidence intervals for back pain for categories of total vibration dose adjusted for age, height, weight, climate, bending forward, twisted posture and feeling tense during work compared to the control group. Vibration dose' (m 2h/s4)
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Back pain parameters Back pain Low back pain >2 weeks Lumbago Sciatica hnp' Treatment Duration Hours Days > weeks' Pattern Alternating Periodical Mild Long or frequent
1200 (n= 15)'
5·2·· 6·6·· 2·7· 2·5
(B-1 \-4) 17-8·· (4'6-68'4) (2'9-15,1) 39·5·· (10'8-15'6) (1'0-7-3) 4·9·· (1'6-15'1) (0'9-7-40) 2'7 ((}8-9'1) B· (1'1-10'0) 5·6· (1'5-21'2) 6·1· (1'6-22'8) 1·3 (0,2-8,3) ),7 (0'7-4'2) 1·6 (0'5-4'5)
11·2·· (309-31'5) 13·5" (3-4-5'3) 1·8 (0'7-4'2) 4·2· (1'4-11'9) 0·8 (0'2-3-2) 3-2 (1'0-10'5) 7·1·· 1·9 4·5·· 2·5·
(2-9-17-4) 12·5·· (309-40'0) 3·1· (1'2-8'1) (0'8-4'6) (0,7-7,0) (2,0-10'1) B (1,0-6'1) )2·5·· (309-40'0)
• p :0;; 0·05 one sided test. •• p:O;;O·OJ. I Due to missing values for either the back pain parameters or the confounders the numbers in the four categories do not add up to 133. 2 Chronic back pain (C), transient back pain (T). 'Only adjusted for age, height, twisted posture - point estimate very unstable due to small numbers in that category.
exposure time compared to the control group and increased with daily exposure time. Although the group of civilian pilots was rather small for separate analysis the prevalence of 'transient' back pain appeared to increase with both daily and total exposure time. The odds-ratios for the 'chronic' back pain parameters were well above I for all exposure categories. All civilian pilots have more than 2000 h of total flight time. The multiplicative interactions between exposure time or accumulative dose and subjectively assessed posture of experienced tension were not significant; with oddsratios very close to I. No relationship was found between the current acceleration levels of the vibration exposure (m/s 2 ) and back pain prevalence 5. Discussion 5.1. Levels of vibration The results of the two measurements of vibration levels during starting, cruising and landing for each type of helicopter were very similar. However, the average level of exposure during a specific mission depended on the relative contribution of each of these types of flight. Because no measurements or literature data were available for several helicopters flown in the past, some inaccuracy of the estimate of vibration magnitude might have been introduced. The levels of vibration measured were on
Back pain in helicopter pilots
* 100
control group (N-223) ._- vibration dose Q)
...... vibration dose >800 m2h/s· (N-491
~
0..
60
.................
40
20
0 20
30
40
50 60 age (in years)
Figure 6. Prevalence of back pain lasting several days or longer in relation to age in the control group and three vibration dose categories.
average below the exposure limit and also in some cases below the fatigue/decreased proficiency boundary ofISO 2631/1. In the new draft proposal ofISO 2631 it is argued that the vibration levels of frequencies from 8 to 16 Hz are of more relevance to the adverse health effects than is expressed in the current weighting procedure of the ISO 2631/1. The weighted acceleration levels according to this new proposal would therefore have been much higher. This new document also provides guidelines for the measurement of vibration transmitted through the backrest of the seat. Griffin (1982)
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Table t 1. Odds-ratios and 90% confidence intervals for back pain for categories of hours of flight per day, adjusted for age, height, weight,climate, bending forward, twisted posture and feeling tense during work compared to the control group. Hours of flight per day Back pain parameter Back pain Low back pain >2 weeks lumbago Sciatica
(C)2 (C)
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hnp?
Treatment Duration Hours Days c-weeks'
Pattern Alternating Periodical Mild Long or frequent
(C)
(T) (C) (C)
(T) (C)
Ergonomics Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/terg20
Back pain and exposure to whole body vibration in helicopter pilots a
a
a
a
P. M. BONGERS , C. T. J. HULSHOF , L. DlJKSTRA , H. C. BOSHUIZEN , H. J. M. b
GROENHOUT & E. VALKEN
b
a
Coronel Laboratory, University of Amsterdam , Meibergdreer 15, 1105, AZ, Amsterdam, The Netherlands b
Occupational Health Service of the Royal Airforce , The Netherlands Published online: 24 Oct 2007.
To cite this article: P. M. BONGERS , C. T. J. HULSHOF , L. DlJKSTRA , H. C. BOSHUIZEN , H. J. M. GROENHOUT & E. VALKEN (1990) Back pain and exposure to whole body vibration in helicopter pilots, Ergonomics, 33:8, 1007-1026, DOI: 10.1080/00140139008925309 To link to this article: http://dx.doi.org/10.1080/00140139008925309
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ERGONOMICS, 1990, VOL. 33, No.8, 1007-1026
Back pain and exposure to whole body vibration in helicopter pilots P. M. BONGERS, C. T. J. HULSHOF, L. DUKSTRA, H. C. BOSHUIZEN Coronel Laboratory, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands
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H. 1. M. GROENHOUT and E. VALKEN Occupational Health Service of the Royal Airforce, The Netherlands Keywords: Backache; Vibration; Posture; Helicopter.
In a questionnaire survey the prevalence of back pain in 163 helicopter pilots was compared to that in a control group of 297 non-flying air force officers who underwent the same pre-employment medical examination. Since pilots document their hours of flight in a personal flightlog, an accurate estimate of the duration of exposure could be made. In addition, vibration levels of the helicopters were measured and an accumulative vibration dose was calculated for each pilot. 'Transient' back pain of a short duration was more frequent amongst the pilots compared to the control group, and the prevalence of 'chronic' back pain of a persistent nature was also higher amongst the helicopter pilots.Transient back pain seemed to be most strongly related to the average hours of flight per day, whereas chronic back pain was more closely related to total hours of flight or the accumulative vibration dose. A significant higher prevalence of this chronic back pain was observed only after 2000 hours of flightor a vibration dose of 400 m 2h/s·. The observed health effects may be due to vibration or constrained posture but are most likely due to concomitant exposure to both factors.
1. Introduction As part of an extensive research project on adverse health effects of long term exposure to whole body vibration, a questionnaire survey was conducted among almost all helicopter pilots in The Netherlands; both military and civilian flight crews. Helicopter pilots form a unique group for studying the relationship between exposure to whole body vibration and the adverse effects on the spinal system. Many studies have reported high prevalences of back pain in helicopter flight crews, but due to high motivation, the number of medical drop-outs is small. Helicopter pilots also have little prior exposure to vibration or other physically demanding work environments. In addition, the duration of exposure is systematically documented in a flight log and measurements of vibration levels of currently in use helicopters can be used to estimate an accumulative vibration dose, since most pilots spend the majority of their flying career in these helicopters. Further helicoper design has changed little over the last decades. Therefore, in contrast to most other occupations which involve exposure to whole body vibration, a relatively accurate estimate of the accumulative exposure dose can be made. The assessment of a dose-response relationship between exposure to vibration and long-term health effects is complicated by the poor posture of pilots during flight. For the majority of the time, helicopters require active input from all four extremities of the 00!4-{)139/90 $3·00
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pilot in order to maintain full control of the aircraft. The seat often lacks sufficient lumbar support and the control configuration often forces the pilots to assume a slightly asymmetrical position: they have to bend forward and lean slightly to the left. This typical posture, 'the helicopter hunch' and exposure to vibration are the factors most widely implicated in the etiology of back pain in helicopter pilots. However, most modern helicopters are equipped with auto-pilots which improve posture while cruising.
2. Literature 2.1. Epidemiological studies In several epidemiological studies on back pain in helicopter pilots a distinction has been made between transient back pain and chronic back pain (Shanahan et al. 1986, Froom et al. 1986). These terms are used to discriminate between temporary back pain associated with actual flight and back pain not directly associated with actual flight. The transient pain is generally described as a typically dull pain without radiation and is confined to the lower back, whereas chronic back pain is described in several ways, e.g.,as sciatic pain that radiates to a leg,as pain that leads to bedrest or the incapacity to fly, or as pain lasting several days or longer. According to Bowden (1987) this 'chronic' back pain might be compared to idiopathic low back pain in the general population. Back pain prevalences in helicopter pilots reported by several authors vary from 21 to 95% (Delahay et al. 1982, Schulte- Wintrop and Knoche 1978, Fischer et al. 1980, Braithwaite and Vyrnwy Jones 1986, Shanahan et al. 1986, Froom et al. 1986). The prevalence of back pain during or shortly after flight ranges from 34 to 64%. Back pain not primarily associated with flight is reported in II to 27% of the respondents (Shanahan et al. 1986, Froom et al. 1986). Two studies show an increase of lumbar X-ray abnormalities in helicopter pilots compared to jet pilots (Fischer et al. 1980, Froom et al. 1984). 2.2. Experimental studies In a mock-up of a UH-I H helicopter seat and control configurations II subjects were asked to indicate when they experienced back pain and with what intensity (visual analogue scale) (Shanahan and Reading 1984). Each pilot was asked to adopt the slumped and slightly asymmetrical posture which he assumed in normal flight and was tested for two separate periods of 120min, one with simulated helicopter vibration and one without. All subjects reported pain identical to the discomfort experienced when actually flying a UH-IH helicopter for similar periods. No significant difference was found between the vibration and non-vibration conditions for either time of onset or intensity of pain. The pain was uniformly described as a dull ache or numbness confined to the lower back and/or buttocks without radiation into the legs. The authors concluded that for the transient back pain experienced by these aviators, posture is the most important etiologic factor. They hypothesized that the slumped asymmetrical posture assumed for extended periods of time leads to spasm of the paraspinous musculature and increases the pressure sensitivity of the buttocks. The authors do not extend this conclusion to more chronic back pain. Pope et al. (1986) tested the change in muscle response (measured as a shift in the centre frequency of the EMG spectrum) due to sustained posture and vibration in a simulated UH-I H cockpit. Marginally significant fatigue occurred only as a result of
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Back pain in helicopter pilots
the sustained static posture. In contrast, all tests both with and without vibration produced subjective discomfort both in the· buttocks and the lower back. Froom et al. (1987)compared the onset and intensity of back pain between aviators occupying the pilots seat and the gunner's seat of a helicopter. In the gunner's seat the aviators maintain an upright position whereas in the pilot's seat they have to lean forward and to the left in order to operate the controls. Vibration levels were similar in both positions. Most pilots experienced pain or discomfort in both positions. However, the intensity was greater and the onset of the pain quicker in the pilot's seat. Voss and Krogh-Lund (1986) compared the occurrence of a shift towards a lower frequency EMG signal between a group often civilian helicopter pilots and a group of ten office workers. They concluded that the helicopter pilots working conditions caused significantly higher localized muscular fatigue in the lumbar compartment of the erector spinae muscles compared to the control group.
3. Materials and methods 3.1. The population A questionnaire was submitted to 100 army helicopter pilots, 51 civilian helicopter pilots and 12 army observers. The control group comprised 297 non-flying air force officers. The response rate was 83% for the helicopter aircrew and 78% for the control group (table I). Eight pilots, observers or controls, who had been exposed to vibration prior to their current job or during their current job (controls), were excluded from the research population. Air force officers with a physically demanding job, e.g., sports instructors, were not included in the control group. The major part of the job of the controls consisted of office work comparable to the activities of the pilots between flights. Both the army pilots and the control group officers underwent the same medical examination of the vertebral column upon entry to the military service and were recruited from the same age categories. The majority of civilian helicopter pilots were trained and formerly employed by the army. They are currently in charge of transport to and from the off shore oil platforms. 3.2. The questionnaire The questionnaire was similar to those used in two other studies on the health effectsof occupational vibration in The Netherlands (M usson et al. 1987, Boshuizen et al. 1989). It included items on accumulative exposure, potential confounding factors and health. Data collected were hours of flight for each type of helicopter ever flown and the average daily and weekly hours of flight. The potential confounding factors investigated were age, height, weight, climatic conditions and experienced tension during work. The respondents were also asked to assess the time spent in a bent
Table 1. The response.
Index group civilian pilots air force pilots observers Control group
Research population
Returned questionnaires
163
133
12
136 (83%) 39 (76%) 89 (89%) 8 (67%)
297
233 (78%)
228
51 100
Analysis 39
87 7
P. M. Bongers et at.
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Table 2. Questionnaire items on back pain. Questionnaire item
label
• Do you regularly experience pain or stiffness in the back? • Where do you experience pain or stiffness in the back? - the upper part of the back - the middle part of the back - the lower part of the back - the entire back • How long does the back pain usually last? - several hours - several days - several weeks - several months • How often do you experience back pain? .. per year • Do you have or have you ever had lumbago? • Do you have or have you ever had a prolapsed disc? • Did you receive medical treatment for your back pain? • Does the back pain radiate to one of your legs? • Have you experienced back pain for a continuous period of two weeks or longer in the last year? • What is the pattern of your back pain? - back pain that varies from day to day - back pain for long unbroken periods alternated with periods without back pain - back pain that is almost always present
back pain
The following back pain parameters combine frequency and duration of back pain: • Back pain lasting several hours for less than 50 times a year or back pain lasting several days for less than ten times a year • Back pain lasting several hours for more than 50 times a year or back pain lasting several days for more than ten times a year or back pain lasting several weeks or longer I
2
low back pain hours' days? »wecks!
lumbago hnp treatment? sciatica!
>2 weeks"
alternating' periodical! always!
mild long or frequent
'Transient' back pain 'Chronic' back pain.
forward or twisted seating position (5 points scale). Table 2 presents the questionnaire items on back pain. The last column gives the labels used in the tables with the results. As indicated in table 2 transient back pain is defined as back pain lasting several hours (hours) with a varying pattern (alternating). In agreement with the literature on back pain in helicopter pilots, chronic back pain is defined as all back pain lasting several days or longer (days, > weeks, > 2 weeks) or back pain that radiates to a leg (sciatica) or that needed medical treatment (treatment). In addition questions were asked to establish the prevalence of back pain directly associated with flight (table 3).
3.3 Vibration measurements Three-axial vibration measurements were conducted in agreement with the ISO 2631/1 guidelines (1985). The air force pilots flew in the Alouette III and the Bolkow 105, the civilian pilots in the Sikorsky S61N and the Sikorsky S76A. The vibration levels of two helicopters of each type were measured under representative flight conditions (van Wijk and van der Weiden 1989). The acceleration levels of the vibration depended on the type of flight, e.g., hovering, starting and cruising. These flights were measured
Back pain in helicopter pilots
1011
Table 3. Questions on back pain directly associated with flight. Do you experience back pain in one of the following situations: -
during or shortly after every flight (independent of flight time)? during or shortly after a flight of less than 2 h? during or shortly after a flight of more than 2 h? during or shortly after a flight with high mental concentration? during or shortly after a period of intensive flying (e.g., more than 20 h of flight per week)
Table 4. Vibration measurements.
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Rating method
Alouette III
X Y Z vectorsum average Bolkow 105 X Y Z vectorsum average X Sikorsky 61 y Z vectorsum average X Sikorsky 76 y Z vectorsum average
Frequency' (Hz)
(m/s 2)
arms
Exposure time" (h)
16 16 16
0·06-1·21 0,91-1'31 0'33-1'08
16->24 14-21 4-18
25 6 25
0·06--D·90 0·06--D·21 0·52--D·93
>24 >24 9-20
16 16 8
0·04--D·39 0,08-1,01 0'08--D'68
>24 19->24 7->24
20 20 8
0·04-1·07 0,08-1,05 0·09--D·24
23->24 23->24 9->24
Weighting method arms
(m/s 2)
0·12--D·17 0·17--D·25 0·44--D·67 0·56--D·75 0·60 0·09--D·13 O' t3--D. I8 0·29--D·49 0·36--D·58 0·45 0·06--D·11 0·1O--D·21 0·17--D·44 0·24--D·55 0·36 0·07--D·14 0·1O--D·19 0·17--D·36 0·28--D·45 0·38
Exposure time' (h)
13-20 7-20 3-5 2-4 18->24 12-21 4-9 3-7 23-> 24 9->24 5-20 4-13 17->24 10-->24 7-20 5-10
'Centre frequency of the one-third octave band with the highest acceleration level in relation to the ISO fatigue/decreased proficiency limit. 2Duration of exposure before reaching the fatigue decreased proficiency limit of ISO 2631/1.
separately and the range ofthese data is presented in table 4. The peaks in thefrequency spectrum are due to the frequencies of the main rotor system and the rotor blades and to multiples of these frequencies. The majority of these predominant frequencies are higher than the first resonant frequency of the spinal system of a seated subject (4 to 5 Hz in the vertical direction (Panjabi et al. 1986). The vibration signal was evaluated in relation to the international standard for evaluation of human exposure to whole-body vibration, ISO 2631/1 (ISO 1985). In this standard two methods are indicated to evaluate broad-band distributed vibration. With the rating method the rms acceleration level of each one-third octave frequency band is evaluated separately in respect to the fatigue/decreased proficiency boundary at that frequency. With the weighting method the overall vibration signal for the frequency range I to 80 Hz is
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P. M. Bongers et al.
weighted according the frequency dependence curves of the standard and expressed in one value. In the present study evaluation using the weighting method gave quite different results from that using the rating method. In order to calculate the accumulative vibration dose the average of the vector sums of the weighted acceleration levels was used. The acceleration level of the UH-ID helicopter flown in the past was derived from literature data (Theiler 1986). If no literature data were available the average value of all other levels (0'5 m/s 2 ) was substituted. Few data are a vailable to decide upon how to calculate a vibration dose value for more than 24 h. Two vibration dose values were calculated Dose!(La i ' t, m 2h/s4 ) and Doseltat· t i m 4h/s 8 ). In which ai is the estimated level of acceleration (m/s") in helicopter i and t i is the.total flight time (h) in helicopter i. Dose! approximates the time dependency relationship proposed in ISO 2631/1 for daily exposures, following the energy equivalence principle. In Dose, the more recent suggestion ofa time dependence of daily vibration exposure as a fourth power relationship has been followed (ISO 1988, Griffin 1982). Since neither of these dose values are applicable to exposures of more than 24 h, their choice is somewhat arbitrary. 3.4. Statistical analysis A multivariate logistic regression analysis (Breslow and Day 1980) was conducted to investigate the relationship between the exposure variables and the health effect parameter. In this analysis odds-ratios were computed for back pain parameters and exposure variables adjusted for several potential confounding factors, with the control group as reference. In addition, pooled maximum likelihood odds-ratios after stratification for age in ten year categories were computed (Rothman and Boice 1979). The Mantel Haenszel Chi-square test was used to determine whether these odds-ratios significantly exceeded 1. Since the pooled odds-ratios did not differ from the oddsratios obtained by the multivariate analysis these odds-ratios are not presented. The logistic model was fitted with all potential confounders unless the number of cases was less than four times the number of variables in the model. In this case only the variables that considerably changed the odds ratio of the exposure variable were included in the model. Multiplicative interactions between the exposure variables and potential effect modifiers were studied. In order to clarify the interrelationship between the various back pain variables a factor analysis of these variables was also conducted.
4. Results Table 5 presents the characteristics for the for the observers are officers in the control
arithmetic mean and standard deviation of several personal different categories. Due to the small numbers, no separate data presented. The age of the pilots ranged from 21 to 56 yr. The group were on average slightly older with an age range of 20 to
61yr. The civilian pilots had experienced significantly more flight hours both in total flight and in hours of flight per day than the army pilots (table 6). Figures I and 2 show that the pilots indicated spending significantly more time in a twisted seating posture than did the controls. The time spent bending forward did not differ significantly. The pilots also reported more draught and cold (77% vs 33%). Almost half (45%) of both army and civilian pilots and 34% of the control group indicated feeling tense during their work, whereas 63% of the army pilots, 80% of the civilian pilots and 72% of the control workers experienced mental stress during their work.
Back pain in helicopter pilots
1013
Table 5. Personal characteristics. Army pilots (n= 87)
Age (yrs) Height (em) Weight (kg)
* p';; 0·05 T-test, ** p';;O·Ol.
Civilian pilots (n= 39)
Control group (n=228)
Index group (n= 133)'
mean
s.d.
mean
s.d.
31*2 182 77
8 7 8
39 182 78
4 7 9
mean
s.d.
mean
s.d.
8 7 8
35 181
to
33 182*3
77
77
two sided.
, observers included. army pilots tested against civilian pilots. 3 index group tested against control group.
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2
Table 6.
Duration of exposure and exposure dose. Army pilots (n= 87)
Total flight time (yr) Total flight time (h) Hours of flight per day Dose, (m 2h/s4 ) Dose, (m4 h/s8 )
Civilian pilots (n=39)
All pilots (n= 126)
mean
s.d.
mean
s.d.
mean
s.d.
7.9**' 1986**1 2'4**' 652**' 228
5·8 1324 0·7 468 181
14·6 5473 5-3 1081 179
4·0 1594 1·0 314 120
9·9 2974
6·2 192 1·5 471 166
3-2 774 213
** p,;;O'OI T-test
two sided. , Army pilots tested against civilian pilots.
Table 7 presents the prevalence of back pain and the adjusted odds-ratios for the back pain parameters in the index and control group. Although the odds-ratios for the transient pain parameters were particularly high, also the odds-ratios for the more chronic pain parameters were significantly greater than I. From figure 3 it can be concluded that, particularly in the younger age group, back pain prevalence for the pilots were relatively high. An illustration of the duration of the back pain is given in figure 4. The factor analysis showed four different back pain dimensions in the pilots: (I) Low back pain that lasted several hours with symptoms varying from day to day; (2) back pain lasting several days with extended periods with symptoms and pain free periods sometimes with radiation to a leg; (3) back pain lasting several weeks; and (4) lumbago or back pain that needed medical treatment. In addition to the impact of flying helicopters, also age, experienced mental stress and reported time sitting in a twisted posture were of significant influence on the prevalence of back pain after allowing for exposure variables and other potential confounders. The pilots with back pain had significantly more total hours of flight, hours of flight per day, experienced mental stress and tension and were sitting more often in a bent forward and twisted posture than those without back pain. On the other hand personal characteristics such as age, height and weight were not significantly different between the two groups.
7 10
P. M. Bongers et al.
1014 11111
•
911 811 711
III always
01/2
611 (x)
511
• never
48 311
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211 111 II
Index
contr
Figure 1. Part of the time of work spent in a forward bent posture. 11111 911 811
(x)
I-
•
always
l!l 3 ....4
711
01/2
611
~
511
1....4
• never
411 311 28 111 8 index
contr
Figure 2. Part of the time of work spent in a twisted posture. Table 8 presents the prevalence of back pain directly associated with flight. From table 8 it is obvious that particularly high back pain prevalences are reported after a flight of more than 2 h duration or after a period of intensive flying. Table 9 presents the odds-ratios for back pain of pilots with increasing hours of flight compared to the control group. From table 9 it is evident that transient back pain, e.g., pain lasting only several hours, often started before 1000 h of total flight time. Additional analyses also showed a significantly higher prevalence of transient back pain compared to the controlgroup before 500 h of total flight time. However, the prevalence of back pain lasting more than several.days only increased after 2000 h of total flight time. As expected, the prevalence of back pain of short duration did not increase with increasing exposure time.
Back pain in helicopter pilots
pi
1015 tots
88
•
78
~ contro Is
68 58 (Yo)
48 38 28
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18 8
28-38
31-48
41-58
51-&8
age('jears)
Figure 3. Prevalence of back pain stratified for age. 78
&8
I:B
58
o weeks
Months
-. ~
48 (Yo)
38 28
da!js
18 8 index
contr
Figure 4. Duration of back pain.
Due to the similar flight career of most pilots, exposure time in hours of flight and accumulative dose (Dose.) are highly correlated (R = 0·87 within the group of pilots). Therefore, the relationship between categories of accumulative dose and the prevalence of back pain is similar to the relationship between total hours offlight and prevalence of back pain (table 10). The threshold of 2000 h of flight time corresponds with a dose of ca400m 2h/s4 • The correlation between Dose! and Dose, is also very high (R=0·81 within the group of pilots). No clear relationship between Dose, and the back pain prevalences was observed (not shown). The data suggest that back pain of short duration develops into more serious back pain with increasing hours of total flight time or vibration dose. Comparison of the odds-ratios for 'mild' and 'long or frequent' back pain gives some support for this hypothesis. 'Mild' back pain decreased with increasing exposure time, whereas 'long or frequent' back pain increased as total flight time increased. This is also illustrated in
1016
P. M. Bongers et at.
Table 7. Back pain prevalences, and odds-ratios adjusted for age, height, weight, climate, bending forward, twisted posture and feeling tense during work. Index group (n=133)1
Control group (n=228)1
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(90% confidence
Back pain parameter
%
abs
%
abs
DR
interval)
Back pain Low back pain > two weeks last year Lumbago Sciatica hnp' Treatment Duration Hours Days >weeks J Pattern Alternating Periodical Mild Long or frequent
68 55 21 13 12 5 17
(91) (69) (26) (16)
(39) (24) (16) (20) (14)
(6) (22)
17 II 7 9 6 5 16
(37)
8·0" 9·0" 3·0·· 2·6· 3·3· 1·9 1-3
(4'5-14'3) (4'9-16,4) (1-4--6'4) (1,1-6,0) (1'3-8'5) (0'6-5,9) (06-2'6)
36 21 9
(45) (26) (11)
4 9 4
(9) (20) (9)
11-3··· 2·4· 1'4··
(5'2-24'8) (1'1-5'0) (0'5-3-8)
41 20 44 22
(52) (25) (55) (27)
6 8 II 6
(13) (17) (25)
9·5·· 2·5· 5·6··· 3·4··
(4'8-18,9) (1'2-5'4) (3'1-10'2) (1'5-7-6)
(C)2 (C) (C) (T)
(C) (C) (T)
(C)
(IS)
(10)
(13)
• p 2 weeks Lumbago Sciatica hnp3 Treatment
4000 (n=29)'
(2,4-11,4) (2,9-14,6) (1'8-12'1) (0'6-6'2) (0'7-8'1) (0'5-9'9) (1'0-5'6)
11·1"" (4,4-28,9) IN"" (5'7-32'0) 3·7" (1'4-9'2) H" (1'2-8'4) 303" (1'0-10'4) 304 (1'0-11'8) 1·7 (0'7-H)
5·8"" (2'1-15'9) 2-4 (0'9-6'4) 2·1 (0'6-7'0)
11·6"" (3'8-34'2) 304" (1'4-8'2) 1·5 (0'5-4'7)
Duration
Hours Days »weeks' Pattern
Alternating Periodical Mild Long or frequent
(T)
(C) (C) (T)
(C)
12·2"" (4'1-36'1) 15·3"" (5'6-41'9) 3-6 (0'9-13-7) 0·6 (0'I-H) 1·1 (0'1-7'7) 19·2"" 1-4 17'1"" 0·6
(6'3-58-4) 15·8"" (4'7-29'7) (0'3-5'8) 1·4 (0'5-6'3) (6'3-33'2) 1l-8"" (4'7-29'7) (0,1-4,1) 1·0 (0,2-4'5)
4·2"" 2'8"" 6·7"" 5·0"
(1'6-10'6) (1'2-6'6) (2'2-20'1) (1'8-13-6)
9·3"" 3·0" 2·4 5·2""
(3'8-23'5) (1'1-6'7) (0'9-7-3) (2'0-1303)
"e< 0·05 one side test. "" p';; 0·0 I.
, Due to missing valuesfor either the back pain parameters or the confoundersthe numbers in the four categories do not add up to 133. 2 Chronic back pain (C), transient back pain (T). 3 Only adjusted for age, height,and twistedposture - estimate very unstable due to small numbers in that category.
prevalence of transient back pain increases with daily exposure time and more chronic back pain increases mainly with total flight time and total vibration dose. Some support for this hypothesis was found when the daily exposure time and the total flight time were both included in the model. Due to the rather high correlation between these variables the point estimates of the odds-ratios become more imprecise, but the oddsratios for 'transient' back pain increase significantly with hours of flight per day also after adjustment for total hours of flight. The elevated odds-ratios for categories of more than 2000 h of flight for 'chronic' back pain become smaller, but are still greater than unity after adjustment for current daily exposure time. Since the civilian pilots have flown for significantly more hours in total and fly more hours per day than the army pilots and since they also experience more back pain, the observed positive trend of back pain with increasing exposure time might be due to the difference in prevalence of back pain between the army pilots and the civilian pilots. Therefore, the relationship between the prevalence of back pain in different categories of exposure (total dose and hours of flight) in each subgroup was compared to the control group separately. Analysis of the data of army pilots showed an increase in the prevalence of 'chronic' back pain parameters after 2000 h of total flight time. Due to the small numbers the confidence intervals are rather wide and sometimes include I. The prevalence of 'transient' back pain in the army pilots decreased with increasing total
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Table 10. Odds-ratios and 90% confidence intervals for back pain for categories of total vibration dose adjusted for age, height, weight, climate, bending forward, twisted posture and feeling tense during work compared to the control group. Vibration dose' (m 2h/s4)
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Back pain parameters Back pain Low back pain >2 weeks Lumbago Sciatica hnp' Treatment Duration Hours Days > weeks' Pattern Alternating Periodical Mild Long or frequent
1200 (n= 15)'
5·2·· 6·6·· 2·7· 2·5
(B-1 \-4) 17-8·· (4'6-68'4) (2'9-15,1) 39·5·· (10'8-15'6) (1'0-7-3) 4·9·· (1'6-15'1) (0'9-7-40) 2'7 ((}8-9'1) B· (1'1-10'0) 5·6· (1'5-21'2) 6·1· (1'6-22'8) 1·3 (0,2-8,3) ),7 (0'7-4'2) 1·6 (0'5-4'5)
11·2·· (309-31'5) 13·5" (3-4-5'3) 1·8 (0'7-4'2) 4·2· (1'4-11'9) 0·8 (0'2-3-2) 3-2 (1'0-10'5) 7·1·· 1·9 4·5·· 2·5·
(2-9-17-4) 12·5·· (309-40'0) 3·1· (1'2-8'1) (0'8-4'6) (0,7-7,0) (2,0-10'1) B (1,0-6'1) )2·5·· (309-40'0)
• p :0;; 0·05 one sided test. •• p:O;;O·OJ. I Due to missing values for either the back pain parameters or the confounders the numbers in the four categories do not add up to 133. 2 Chronic back pain (C), transient back pain (T). 'Only adjusted for age, height, twisted posture - point estimate very unstable due to small numbers in that category.
exposure time compared to the control group and increased with daily exposure time. Although the group of civilian pilots was rather small for separate analysis the prevalence of 'transient' back pain appeared to increase with both daily and total exposure time. The odds-ratios for the 'chronic' back pain parameters were well above I for all exposure categories. All civilian pilots have more than 2000 h of total flight time. The multiplicative interactions between exposure time or accumulative dose and subjectively assessed posture of experienced tension were not significant; with oddsratios very close to I. No relationship was found between the current acceleration levels of the vibration exposure (m/s 2 ) and back pain prevalence 5. Discussion 5.1. Levels of vibration The results of the two measurements of vibration levels during starting, cruising and landing for each type of helicopter were very similar. However, the average level of exposure during a specific mission depended on the relative contribution of each of these types of flight. Because no measurements or literature data were available for several helicopters flown in the past, some inaccuracy of the estimate of vibration magnitude might have been introduced. The levels of vibration measured were on
Back pain in helicopter pilots
* 100
control group (N-223) ._- vibration dose Q)
...... vibration dose >800 m2h/s· (N-491
~
0..
60
.................
40
20
0 20
30
40
50 60 age (in years)
Figure 6. Prevalence of back pain lasting several days or longer in relation to age in the control group and three vibration dose categories.
average below the exposure limit and also in some cases below the fatigue/decreased proficiency boundary ofISO 2631/1. In the new draft proposal ofISO 2631 it is argued that the vibration levels of frequencies from 8 to 16 Hz are of more relevance to the adverse health effects than is expressed in the current weighting procedure of the ISO 2631/1. The weighted acceleration levels according to this new proposal would therefore have been much higher. This new document also provides guidelines for the measurement of vibration transmitted through the backrest of the seat. Griffin (1982)
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Table t 1. Odds-ratios and 90% confidence intervals for back pain for categories of hours of flight per day, adjusted for age, height, weight,climate, bending forward, twisted posture and feeling tense during work compared to the control group. Hours of flight per day Back pain parameter Back pain Low back pain >2 weeks lumbago Sciatica
(C)2 (C)
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hnp?
Treatment Duration Hours Days c-weeks'
Pattern Alternating Periodical Mild Long or frequent
(C)
(T) (C) (C)
(T) (C)
