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Functional test measures as risk indicators for low back pain among fixed-wing military pilots
  1. Tuomas Honkanen1,
  2. H Kyröläinen2,
  3. J Avela2 and
  4. M Mäntysaari1
  1. 1Aeromedical Centre, Centre for Military Medicine, The Finnish Defence Forces, Helsinki, Finland
  2. 2Department of Biology of Physical Activity, University of Jyväskylä, Jyväskylä, Finland
  1. Correspondence to Tuomas Honkanen, Aeromedical Centre, Centre for Military Medicine, The Finnish Defence Forces, P.O. Box 50, Helsinki FI-00301, Finland; tuomas.honkanen{at}mil.fi

Abstract

Purpose The purpose of this study was to find out the risk value of functional fitness test (FFT) results for low back pain (LBP) among fixed-wing military pilots.

Methods A total of 104 male military pilots were recruited for this study. The study was conducted with a self-administered questionnaire and FFT. The functional tests were performed in the beginning of study (baseline). The questionnaire was carried out at the baseline and 5 years later.

Results The isometric low back endurance test result was associated with physical activity-related LBP experienced 5 years later. Demographic information was not associated with LBP. The prevalence of overall LBP was 71% and the flight-related LBP prevalence was 31% at the baseline.

Discussion Our findings show that LBP among military pilots is a common problem but it is also associated with tasks other than flying. The functional test results were not associated with flight-related LBP but adequate isometric back endurance may have protective role in LBP caused in physical activities. When trying to find the pilots with increased risk of flight-related LBP, a more sensitive set of tests should be considered.

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Key messages

  • The isometric low back endurance test result predicted leisure time sport activity-related low back pain (LBP) in 5-year follow-up.

  • The functional tests did not associate with flight-related LBP experienced 5 years after examination.

  • LBP is a common problem among Finnish Air Force military aviators and the cause of the pain is multifactorial.

  • High-performance aircraft pilots suffer more from flight-related LBP than other fixed-wing carrier pilots.

Introduction

Exposure to high +Gz force (headward acceleration resulting in a downward force) in high-performance aircraft (HPA) has been associated with an increased risk of spinal disorders in military aviation.1 Long-term studies suggest that flying HPA has an adverse effect on the cervical2 and lumbar spine,3 with evidence that flying with HPA subjects the spinal muscles to high loads,4 and one out of three HPA pilots report low back pain (LBP).3 LBP is problem in a high +Gz environment, and is also a major concern among other fixed-wing carrier (FWC) pilots5 as over half of commercial airline pilots have experienced LBP during the past year.6 A review by Lis et al7 suggested that sitting for more than half a work day in combination with whole body vibration or awkward postures increases the likelihood of having LBP. Even though FWC pilots are known to suffer from LBP their risk factors and preventative strategies are not described in literature unlike for their counterparts in rotary-wing aviation8 and there are no studies of LBP follow-up among FWC military aviators.

Many studies have investigated the relationship between functional measures of the trunk muscles and the presence of low back disorders.9 Weak trunk muscles appear to be associated with persistent LBP, while good isometric endurance of the back muscles seems to prevent the occurrence of LBP among the general working-age population.10 ,11 Alaranta et al12 reported that the risk (OR) of LBP is three times greater in subjects with poor performance in the isometric back endurance test as compared with subjects with medium or good performance in a blue-collar and white-collar working populations. A reduced flexibility of the hamstring muscles and/or reduced mobility of the spine have also been related to LBP in the general population.10 ,11 ,13 The predictors and risk factors of LBP, such as physical inactivity or poor muscle endurance, have been reported in many populations,14 but there are no relevant clinical studies that discuss risk factors or prevention of LBP among military pilots.

The aim of the present study was to find out the correlation between functional fitness test results and LBP by investigating the association between the test results and LBP reported 5 years later. The hypothesis was that reduced lower limb flexibility, spinal mobility and strength of the trunk muscles are risk factors for future LBP. The secondary aims were to investigate the association between the demographic information collected at the baseline and LBP reported 5 years later and to find the cause of the LBP.

Methods

Subjects and design

The data for this cohort study were collected from the database of the Aero Medical Centre (AeMC) of The Finnish Defence Force's Centre for Military Medicine. All Finnish Air Force (FINAF) pilots undergoing their obligatory annual aeromedical examination in the AeMC from September to October were chosen for this study. The selection criteria required that the pilots must stay on active duty during the 5-year period of the study, and that they gave permission to use their records in the study.

The test battery, including seven functional tests, was conducted only at the baseline for each pilot. The test battery is standard at the annual aeromedical examination for each FINAF FWC military pilot. The results of these functional tests were used as independent variables of the study. A self-administrated questionnaire was carried out at the baseline and 5 years after the functional tests. The initial questionnaire was used as an independent variable and the second questionnaire was used as a dependent variable in this study. Pilots in the non-HPA (NHPA) group were not exposed to acceleration levels higher than +2.5 Gz. Pilots who were flying fighter and/or jet trainers were allocated to the HPA group where the maximum Gz exposure was +8 Gz. The Ethical Committee of the Central Finland Health Care District approved the study.

Functional tests

Seven well-described functional tests (three spinal mobility tests, two lower limb flexibility tests and two core strength tests) were measured during the clinical examination. Schober's and Stibor test15 measured flexion of the lumbar and thoracolumbar spine as well as lateral flexion of the spine.16 The straight leg raise (SLR) test was performed to measure flexibility of the hamstring muscles (semitendinosus, semimembranosus, biceps femoris) and the Thomas test (TT) measured flexibility of the iliopsoas muscles (psoas major and iliacus).15 The modified isometric back endurance test (Sorensen test) was used to measure fatigue of the back muscles10 while the modified isometric abdominal test measured fatigability of the abdominal muscles.16

Self-administered questionnaire

All FINAF pilots answered the self-administered questionnaire eliciting information about previous (in the past year) and present symptoms of LBP as well as pain of the neck and thoracic back. The questionnaire is in standard use in FINAF pilots’ annual aeromedical examination and has been used in previous studies.17 The cause of LBP was classified as either leisure time or work-related LBP in the questionnaire. The work-related origin of LBP was further classified whether it was flight or non-flight duty-related pain; the leisure time-related LBP was further classified into sport or other task-related pain. In the current FINAF pain questionnaire the incidence of LBP is asked as: “Have you had LBP during last year? If ‘yes’, the pilot is asked to name the cause of the pain. The intensity of pain was assessed using the visual analogue scale (VAS), where 0 is no pain and 10 is the maximum imaginable pain according to his/her pain on the examination day. In addition to information about LBP data on age, weight, height and use of tobacco products were collected. The baseline questionnaire was completed at the same time as the functional tests were completed and the second questionnaire was completed 5 years after the functional tests.

Statistical analyses

Data were analysed using SPSS Statistics for Windows V.21.0 software. Logistic regression and χ2 tests were used to study the association between functional test results and LBP and the association between demographic factors and LBP. The subgroups of HPA and NHPA pilots were compared with Student’s t test. The change of LBP intensity over time was analysed with non-parametric McNemar test. The significance threshold in each test was 0.05.

Results

In total, 131 consecutive pilots underwent examination in the 2-month period; 13 pilots who due to retire within 5 years and 11 pilots who did not give the permission to use their medical records were excluded from the study. In addition, the two female pilots were excluded because of their small representation in the population and one HPA pilot was excluded because he had a severe back injury due to ejection seat jump. The final study group consisted of 104 male FINAF FWCs (70 HPA and 34 NHPA) military pilots.

The whole study group remained fit to fly, and there were no permanent grounding or any non-flying tours without withdrawal of flying category due to medical or any other reasons. There were six pilots who moved from HPA to NHPA during the follow-up due to normal work rotations. The HPA pilots were exposed to high +Gz force during their flight missions with F-18 Hornet fighters or Hawk Mk 51 jet trainers. The NHPA pilots flew transport aircraft (EADS CASA C-295M and Fokker F27) or liaison aircraft (Gates Learjet 35A and Pilatus PC-NG 12). The mean age of the study population was 31 years (SD±4 years) at baseline with mean height, mass and body mass indices (BMIs) of 179 cm (SD±5 cm), 78.8 kg (SD±7.4 kg) and 24.6 (SD±2.0), respectively. There was no statistical difference between the functional test results or the demographic factors of the HPA and NHPA pilots at baseline (Table 1).

Table 1

Comparison between demographic information and functional test measures among pilots flying HPA and NHPA at baseline

Association between functional tests and LBP

The hypothesis that reduced isometric back muscle strength would be a risk factor for LBP was accepted when the test results were compared with future leisure time sports-related LBP. The isometric low back endurance test result was associated (p=0.029) with leisure time sport-related LBP, but it was not associated with future desk job (p=0.625) or flight-related (p=0.267) LBP experienced 5 years after examination.

We had to reject the hypothesis that reduced leg flexibility, spinal mobility or isometric abdominal muscle endurance test results would be risk indicators for LBP. Abdominal muscle endurance (p=0.255), Stibor (p=0.132) and Schober's (p=0.808) test as well as lateral flexion to left (p=0.116) and right (p=0.157) did not have statistically significant associations with LBP experienced 5 years after examination; neither there was any association between lower leg flexibility tests (SLR or TT) and future LBP.

Association between demographic information and LBP

The hypothesis that HPA pilots would experience more LBP was accepted when the association with aircraft flown was compared with flight-related LBP (experienced inflight or directly after flight). HPA pilots experienced more flight-related LBP than the NHPA pilots (p=0.013) (Table 2). The aircraft type flown was not associated with any other type of incidents of LBP other than flight-related pain. The demographic information (age, weight, height and BMI or tobacco usage) collected were not associated with LBP in the baseline questionnaire or in the questionnaire conducted 5 years later.

Table 2

Flight-related LBP among HPA and NHPA pilots

LBP (in any situation) reported in the baseline questionnaire was associated with LBP in the questionnaire conducted after 5 years. Neck pain and pain in the level of the thoracic spine at baseline was not associated with future LBP. The intensity of LBP described with the VAS at baseline was not associated with LBP after 5 years.

The most common causes of LBP were flight and leisure time sports-related pain. Both flight and sports-related LBP decreased significantly in 5-year follow-up (Table 3).

Table 3

Prevalence and the cause of LBP among all fixed-wing aircraft pilots, at baseline and after 5 years

Discussion

In the present study, the functional tests did not associate with flight-related LBP experienced 5 years after examination. However, there was an association between isometric back endurance and leisure time sport-related LBP after 5 years. This finding is in line with previous studies,10–12 which suggest that decreased back muscle endurance is a risk factor for LBP; other functional test results were not associated with LBP 5 years later. Although there is evidence that limited mobility of the spine is a risk factor for future loss of functional capacity11 and LBP,18 we could not find associations between LBP and any of the spinal mobility tests. The evidence for flexibility of the lower leg muscles being a cause of LBP is conflicting,13 and our findings suggest that there is no relationship between reduced flexibility of the hamstring or thigh muscles and LBP.

There was no relationship between LBP and height, weight, BMI or using tobacco products in the present study. Previously published data demonstrate, however, that rotary-wing pilots with height >180 cm were at risk of LBP.19 Smoking is also considered as a risk factor of LBP and known to promote degeneration of the lumbar discs.20 We did not find any association between pilots’ LBP and smoking or other tobacco product use. In the present study, 25% of pilots reported smoking with 27% of them smoking everyday which is not higher than in the general population of Finland, but it is substantially higher than the 3% that has been reported among US military aviators.21 The daily use of snuff products was 10%, which is higher than in the general Finnish population (5%).22

Even though the prevalence of overall LBP was high in this military pilot cohort, the prevalence of occupation-related pain was lower than in many earlier studied occupation-related populations.23–25 Overall, 31% of the FINAF pilots reported flight-related LBP at baseline and 16% of them reported flight-related LBP after 5 years. The prevalence is low compared with other occupations with sitting in awkward postures, as work-related LBP prevalence reported among rotary-wing military pilots is as high as 80%,23 and even taxi and truck drivers seem to have higher prevalence (50%–51%) of LBP.24 ,25 This study does not explain why pilots with cumulative low back loading are experiencing less occupation-related LBP than the general population, but one considered factor might be a high motivation to work, neither are we able to explain why the reported LBP rate drops in our follow-up but it might be due to increased muscle strength caused by regular exposure to G force.

This study showed that the cause of pain is not related to HPA flying alone; LBP related to leisure time sports was as common as LBP related to flying itself. Interestingly, there were no extreme or out of ordinary-related activities causing LBP, as the most common activities causing LBP were running and weight lifting. We also found that non-flight duty sitting tasks related to LBP were as common as flight-related LBP. On the other hand, we have proven that HPA pilots suffer from more back pain than the NHPA pilots. This finding supports the idea that cumulative low back loading at work has causal effect on LBP.26 Conclusions from these findings must be made with a caution, because military aviation is not easily compared with traditional physically demanding jobs.

A limitation of this study was that the authors were not able to use flight hours as independent variable and the classification for HPA and NHPA does not take into account the different +Gz exposure between the HPA pilots; however, the flight hours are not accurate measurement either, because the exposure between different missions among FINAF pilots varies a lot. The strength of this study is that the study cohort can be generalised to FINAF pilot population.

Conclusions

None of the seven functional tests were able to identify the military pilots having flight-related LBP at baseline or 5 years later. The results of this study do not support the original hypothesis that functional tests could be used to evaluate the risk of future flight-related LBP. Adequate isometric back endurance may have protective role in LBP caused in leisure time physical activities.

  • Received July 2, 2015.
  • Revision received December 11, 2015.
  • Accepted January 2, 2016.

References

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Footnotes

  • Contributors All authors have contributed in planning and reporting in this study. TH has been responsible for data collection and statistical analyses with the help of statistician (from Jyväskylä University). TH is responsible for the overall content as guarantor.

  • Competing interests None declared.

  • Patient consent Obtained.

  • Ethics approval The Ethical Committee of the Central Finland Health Care District.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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