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Analysis of muscle fatigue in helicopter pilots
Applied Ergonomics 42 (2011) 913e918
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Analysis of muscle fatigue in helicopter pilots Venkatesh Balasubramanian a, *, Ashwani Dutt a, b, Shobhit Rai a, b a b
Rehabilitation Bioengineering Group, IIT Madras, Chennai 600036, India Corps of EME, Indian Army, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2010 Accepted 20 February 2011
Helicopter pilots espouse ergonomically unfavourable postures and endure vibration which result in low back pain. The objective of this study was to investigate the effects of a helicopter flight on pilots back and shoulder muscles using surface Electromyography (sEMG) analysis. This study also correlates low back pain symptoms from Rehabilitation Bioengineering Group Pain Scale (RBGPS) questionnaire with muscle fatigue rates obtained. RBGPS was administered on 20 Coast Guard helicopter pilots. sEMG was acquired before and after flight from erector spinae and trapezius muscles in 8 of these 20 pilots. Statistical analysis of time and frequency domain parameters indicated significant fatigue in right trapezius muscle due to flying. Muscle fatigue correlated with average duration of flight (r2 ¼ 0.913), total service as pilot (r2 ¼ 0.825), pain (r2 ¼ 0.463) and total flying hours (r2 ¼ 0.507). However, muscle fatigue weakly correlated with Body Mass Index (BMI) (r2 ¼ 0.000144) and age (r2 ¼ 0.033). Ó 2011 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Helicopter pilots Low back pain sEMG study RBGPS
1. Introduction Helicopter pilots are exposed to vibratory environments and ergonomically unfavourable working conditions (Fig. 1), which over a time period can cause back pain. Helicopters vibrate predominantly at frequencies related to the speed of revolution of its main rotor (4e11 Hz), tail rotor (30e60 Hz) and the engine (110 Hz). Aircrews in helicopter are therefore subjected to vibrations coinciding with the resonant frequencies of their body (4e8 Hz) (Griffin, 1990; Mansfield, 2005). The resonant frequencies for various parts of the human body are given in Table 1. As shown in Table 1, helicopters subject aircrew members to vibrations over a frequency range that coincides with the resonant frequencies of the body. Cockpit ergonomics is an area of concern for helicopter pilot fraternity which can be addressed by physiological and biomechanical analysis. One of the most important health problems is the onset of low back pain (LBP) due to straining of erector spinae muscle. Whole body vibration (Bonger et al., 1990; Srinivasan and Balasubramanian, 2007) and postural factors (Froom et al., 1984, 1986, 1987) are the main contributors to the occurrence of LBP. Studies relating to LBP aspects of helicopter pilots have relied
* Corresponding author. Rehabilitation Bioengineering Group, Department of Engineering Design, IIT Madras, Chennai 600036, India. Tel.: þ91 44 2257 4117; fax: þ91 44 2257 4732. E-mail addresses: [email protected], [email protected] (V. Balasubramanian).
on evaluating the same on the basis of questionnaire study (Balasubramanian and Prasad, 2006). While the results are very relevant, they have not been able to accurately point whether the problem exists entirely due to whole body vibration (Bonger et al., 1990; United States manual, 1991) or to posture (Froom et al., 1986, Hansen and Wagstaff, 2001) or a combination of both the factors (Froom et al., 1987; De Oliveira and Nadal, 2004). Whole body vibrations and postures lead to weakening of muscles, which result in higher fatigue rate than compared to a healthy muscle (De Oliveira et al., 2001; Toren, 2001; Wilder and Pope, 1996). Fatigue is defined as the time-dependent summation of internal and external influences, which adversely affects human performance, irrespective of any subjective awareness either of the influences or of the impairment (Bonger et al., 1990). sEMG parameters associated with either onset or occurrence of fatigue is signal amplitude, power, mean and median frequencies of the power density spectrum and number of zero crossings. It has been reported that, during fatigue, there is an increase in the sEMG signal amplitude and power (De Luca, 1993), and a decrease (or left shift) in the both mean and median frequencies (De Oliveira and Nadal, 2004; Balasubramanian and Srinivasan, 2009). This study evaluates the prevalence of low back pain in helicopter pilots flying a small helicopter (Chetak or Alouette III) by means of a questionnaire based study followed by determining rate of muscle fatigue in erector spinae and trapezius muscle group with help of sEMG analysis before and after flight. This study also correlates results of RBGPS with muscle fatigue in order to identify the parameters of RBGPS, which are responsible for onset of LBP.
0003-6870/$ e see front matter Ó 2011 Elsevier Ltd and The Ergonomics Society. All rights reserved. doi:10.1016/j.apergo.2011.02.008
914
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
Fig. 1. Cockpit of Alouette III (Chetak) used extensively by the Indian Coast Guard for maritime patrol.
2. Material and methods 2.1. Subjects Twenty helicopter pilots voluntarily participated in this study. The average age was 36.55 years (SD ¼ 3.53 years) and average BMI was 24.4 (SD ¼ 0.80). Eight male helicopter pilots are currently flying a small helicopter among the 20 pilots. Pilots had an average experience was 13.9 years (SD ¼ 3.48) as Chetak helicopter pilots and had average flying hours was 2370 (SD ¼ 760.96). Their average flying hours per month was 21.15 (SD ¼ 2.80) and the average duration of their routine sortie was 2.06 (SD ¼ 0.29) hours. None of the selected participants had a lower extremity injury, physical disability, or discomfort problem. Experiments were performed in the helicopter hangar from where the pilots used to embark for their flight and disembark. The experiment was performed as per the guidelines of the Institutional Ethics Committee of IIT Madras for human volunteer research. All volunteers also read and signed an informed consent before participating. 2.2. Questionnaire study Standardized questionnaires for the analysis of LBP symptoms in a helicopter environment (before, during and after flight) were administered on 20 helicopter pilots with the Indian Coast Guard. Questions presented were structured and were of the forced, binary or multiple choice types. Questions were specific and related to LBP caused due to flight induced activity of pilots. Pilots were instructed to grade the pain they experienced using the RBG pain scale presented in Table 2. 2.3. sEMG acquisition Among the twenty pilots, eight (8) had volunteered to be part of the sEMG study. The remaining pilots did not participate either due to administrative reasons (i.e., posting out, flying schedule being Table 1 Resonant frequency of various parts of human body. (Courtesy Field Manual of US Army No 3-04.301 Aeromedical training for flight personnel). Body part
Resonant frequency (Hz)
Whole body Shoulder girdle Head Eyes
4e8 4e8 25 30e90
out of the window of time allowed to visit the base, etc), technical reasons (i.e., flying condition, sortie nature, sortie duration, post sortie debriefing, etc) or choose not to participate. Muscles considered for this study were the trapezius (2 cm lateral to midpoint of C7 and acromium), erector spinae (slightly higher than the top of the iliac crest on the spine) bilaterally (Fig. 2(A)). Both these muscle groups are fairly large and easy to identify on test subjects. sEMG signals were recorded at a sampling frequency of 1000 Hz using EMG machine (Bagnoli-8TM, Delsys Inc, USA) with single differential electrode configuration and gain of 1000. EMG machine was powered by an isolated medical grade power supply with leakage current less than 10 mA and safely isolated to 3750 Vrms (Balasubramanian and Adalarasu, 2007). Dimension of the electrode bar was (19.8 5.4 35) mm with 10 mm contact spacing between bars. Electrode interfaces were made from a specifically designed medical grade adhesive. Before placing the measurement electrodes, placement site was identified, shaved, and cleaned with ethanol to avoid impedance mismatch and movement artifact. Disposable reference electrodes (Ag/Agcl 3 M e Red DotÔ model 2330 electrodes) were placed on the wrist bone. On each occasion, i.e., pre- and post-flight, pilots performed maximal voluntary contraction (MVC), which involved lifting of weight kept on the floor without causing any perceived discomfort to the body (Fig. 2(B)). Three trials of sEMG recordings of 10 s duration (pre- and post-flight) were acquired from each pilot. As this study was performed on Indian Coast Guard helicopter pilots while flying regular sorties, we had to abide by mission safety norms. Due to this, we had but no option to remove and re-site the electrodes between pre- and post-flight data collection. We had however taken extreme care by marking the electrode position and orientation. After pre-flight MVC tests, the electrode profile was marked on the skin using marker pens to identify the exact electrode location for post-flight MVC EMG data collection. In addition, electrodes were removed from the skin leaving behind the specially designed interface adhesive as an identification marker of the electrode location.
Table 2 RBG pain scale criteria used to grade LBP among Chetak (Alouette III) helicopter pilots during a routine flight. Grade
Criteria
0 1 2 3 4 5
No pain Very minor, barely felt Minor pain not interfering with the flight routine Discomforting pain not interfering with the flight routine Severe pain affecting routine flying Unbearable pain requiring medical attention on landing
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
915
Fig. 2. (A) The physical locations of the surface electrodes on a typical subject. Muscles that were examined included Right Trapezius (RT); Right Erector Spinae (RES). (B) Subject performing activity to elicit maximal voluntary contraction (MVC).
2.4. Signal processing and feature extraction sEMG signals were filtered using a 4th order Butterworth filter with a pass band range of 20e500 Hz. AC line interference was eliminated using a 2nd order notch filter with a stop band range of
47e51 Hz. Filtered sEMG signals were full wave rectified. Root mean square (rms) of sEMG signal is considered as the most reliable parameter in the time domain (Balasubramanian et al., 2009). A concomitant increase in the rms value with an advancing fatigue has been reported (Krogh-Lund and Jørgensen, 1993). Mean
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
amplitude and rms were extracted from time domain of the rectified signals. Median frequency (MF) changes during contraction period can be analyzed by linear regression (Merletti et al., 1991). The negative inclination or slope of the regression line will indicate fatigue rate (Gaudreault et al., 2005; Balasubramanian et al., 2009). Power spectrum was computed over the frequency range of 20e500 Hz by applying fast Fourier transformation on the filtered signal and median frequency (MF) was extracted. 2.5. Regression Muscle fatigue can be measured indirectly using the knowledge of direction and magnitude of the slope of line of best fit (Balasubramanian and Adalarasu, 2007). Hence in this study linear regression analysis was performed on mean amplitude, rms from time domain and MF values extracted from frequency domain of sEMG signals obtained from the muscles (trapezius and erector spinae) of each subject during pre- and post-flight condition.
2.4
(1 - 30) Min (30 - 60) Min (60 - 90) Min (90 - 120) Min
1.8
Scale of Pain
916
1.2
0.6
0.0 Time (Minutes) Fig. 3. Change of the pain grade with flying time.
2.6. Statistical test sEMG signals obtained during pre-flight and post-flight were not statistically independent. Owing to a smaller sample size, Wilcoxon signed rank test (non parametric test) was performed on mean amplitude, rms and MF to determine whether these features, which were extracted from sEMG signals, obtained during preflight and post-flight were significantly different. ManneWhitney U-test (non parametric test) was performed to determine whether the fatiguing rates of left and right trapezius and left and right erector spinae were significantly different. 3. Results 3.1. Questionnaire study Pain grades obtained using RBG pain scale were correlated (Spearmans correlation) with various attributes of subjects namely body mass index, age, average duration of flight, experience as Chetak helicopter pilots, average flying hours/month and total flying hours. Square of correlation r2 between the pain grade and the attributes is shown in Table 3. Total flying hours (r2 ¼ 0.86) and average flying hours/month (r2 ¼ 0.88) show excellent correlation with pain grade, where as age (r2 ¼ 0.066) and BMI (r2 ¼ 0.008) do not show any correlation. Pain grade was gradually increased from the beginning to the end of the flight as shown in Fig. 3. 3.2. sEMG study (LBP) Time domain parameter such as mean amplitude and rms features extracted bilaterally from trapezius and erector spinae muscles were significantly high in post-flight when compared to pre-flight condition (p < 0.05) as shown in Fig. 4(A) and (B).
Table 3 Correlation (r2) of Pain Grade with various attributes of the subjects. S No
Variables
Correlation with pain grade (r2)
1 2 3 4 5 6
Age (years) Body mass index Average duration of sortie (hours) Service as Chetak pilot (years) Average flying hours/month Total flying hours
0.066 0.008 0.264 0.384 0.875 0.856
Frequency domain parameter such as median frequency (MF) feature values extracted from the right trapezius muscle of significantly low (p < 0.05) in post-flight condition as compared to preflight condition as shown in Fig. 4(C). A concomitant increase in the MA, rms parameters value with time and MF value decrease, with advancing fatigue has been reported (Balasubramanian et al., 2009). Besides, it is also evident that the magnitude of fatiguing rate (mean amplitude, high rms values and low MF values) was higher in the right trapezius muscles during the post-flight as compared to its pre-flight. Significant difference (p < 0.04) in the fatiguing pattern was observed (in both time and frequency domain) only in right trapezius muscles. Correlation between various variables in the questionnaire study and fatigue rates of different muscle groups are shown in Table 4. Excellent correlation exists between the fatigue rate of the left erector spinae muscle extracted out from the mean amplitude feature with average duration of the sortie (r2 ¼ 0.913) while average correlation is obtained with average flying hours/ month (r2 ¼ 0.426) for the same feature. However in rms amplitude feature, the correlation is poor with average flying hours/month (r2 ¼ 0.289) and average with average duration of the sortie (r2 ¼ 0.567). In median frequency, a very good correlation is obtained between the fatigue rate of right trapezius muscle and service as a Chetak pilot (r2 ¼ 0.825). The correlation of the fatigue rate of the same muscle is average with scale of pain (r2 ¼ 0.463) and total flying hours (r2 ¼ 0.507). In MA and rms features there is weekly negative correction (r2 ¼ 0.000144) with BMI bilaterally erector spinae muscle. Muscle fatigue was weekly correlated (r2 ¼ 0.033) with age. 4. Discussion Objective of this study was to determine fatigue in erector spinae and trapezius muscle groups of a group of Chetak helicopter pilots by performing sEMG study. Questionnaire based on RBGPS was administered on a larger group of pilots to identify causes of LBP. It is evident from the results that an excellent correlation existed between total flying time and pain, which points that greater the time flown by the pilots, greater will be the prevalence of low back pain in them. This result agrees with previous studies (Bonger et al., 1990; Hansen and Wagstaff, 2001). Poor correlation was obtained between the LBP and the total service as Chetak helicopter pilot. This could be due to genetic constitution of subjects and also because pilots fly less after a certain number of
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
A *
0.12
*
*
* *
RMS (mV)
Mean Amplitude (mV)
600
B
Pre Flight Post Flight
*
917
400
*
Pre Flight Post Flight
*
0.06
200
0
0.00
L.T
R.T L.ES R.ES Muscle Group
C
L.T
R.T L.ES R.ES Muscle Group
Pre Flight Post Flight
400
*
MF (Hz)
300
200
100
0
L.T
R.T L.ES R.ES Muscle Group
Fig. 4. The average and standard deviation of (A) Mean Amplitude (MA) (B) rms from the time-series data and (C) Median Frequency (MF) from the power spectrum between preand post-flight condition is schematically depicted. Muscles that were examined included Right Trapezius (RT); Left Trapezius (LT); Right Erector Spinae (RES); Left Erector Spinae (LES). Note * significant difference (p < 0.05) between pre- and post-flight condition.
years of service. An excellent correlation was observed between the pain grade and average flying hours per month. This variable was not considered in any other study although it is an important factor which could lead to transient low back pain. Studies on fatigue measurements have established that fatigue occurs when time domain parameters such as mean amplitude and rms amplitude increase (Berzuini et al., 1985; Lariviere et al., 2003). Positive slopes signify the fatigue rates post-flight (De Luca, 1993; Hagg et al., 2000; Lariviere et al., 2003). In frequency domain, studies have hypothesized that decrease in median frequency signifies onset of muscle fatigue, which is associated with the negative slope of the line of regression (De Luca, 1993; Hagg et al.,
Table 4 Correlation (r2) between fatigue rates and the variables of questionnaire study. Acronyms used are MA e mean amplitude, RMS e root mean square, MF e median frequency. Variables/Features
Average flying h/month Average duration of the flight Pain grade Total flying hours Service as Chetak pilot BMI Age
MA
RMS
MF
Left erector spinae
Left erector spinae
Right trapezius
0.426 0.913 e e e 0.000144 0.033
0.289 0.567 e e e 0.000144 0.033
e e 0.463 0.507 0.825 0.000144 0.033
2000; Lariviere et al., 2003). Results of identification of fatigue in this study show that right trapezius muscles are conclusive while for other muscles it is inconclusive. These variations in median frequency can be attributed to other factors during flight like posture or the design of cockpit interiors. Results of this study imply that right trapezius muscle is more significantly recruited than the left while the recruitment in erector spinae is generally uniform. On observing the drills and flying posture adopted by the pilots, it was seen that the right arm is used to hold the stick for the entire duration of the flight. Thus the right gleno-femoral joint is in a state of flexion and the elbow joint is in the state of extension throughout the flight. Since the right trapezius is recruited for the movement of the gleno-femoral joint, these muscles are more likely to be additionally fatigued compared to left trapezius. The fatigue rate in left erector spinae has shown excellent correlation with the average duration of the flight in mean amplitude and an average correlation in rms amplitude. It can be concluded that the duration of a flight contributes to transient back pain. It shows similarities with the results reported by Bonger (Bonger et al., 1990) who stated that average hours of flight per day seemed most strongly related to transient back pain. In median frequency, an excellent correlation has been obtained between the fatigue rate of right trapezius muscle and the total service as a Chetak helicopter pilot. It implies that the fatiguing pattern of the trapezius muscles have excellent correlation with the service career of a pilot.
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5. Conclusion Various studies have covered the aspect of low back pain among helicopter pilots through questionnaire based studies. However, its relationship with flight induced muscle fatigue has not been explored. In this study, we have performed MVC study on select muscle groups on helicopters pilots flying real sorties. Pre- and postflight sEMG analysis indicates fatigue in both trapezius and erector spinae muscle groups. Statistical analysis showed that right trapezius muscle fatigues more than left trapezius muscle, which correlates with the right handed handling of the helicopter stick by the pilots. However there were lateral difference in fatigue of the back muscle (erector spinae), which indicates that the back muscle fatigue and hence the pain could result from the posture and whole body vibration exposure. Correlation studies between pain grades obtained using RBGPS and attributes of pilots evidently point out that total flying hours and average flying duration are two important basis factors for the occurrence of low back pain among helicopter pilots. References Balasubramanian, V., Adalarasu, K., 2007. sEMG-based analysis of change in muscle activity during simulated driving. Journal of Bodywork and Movement Therapies 11, 151e158. Balasubramanian, V., Prasad, G.S., 2006. Ergonomic assessment of bar cutting process in construction. International Journal of Systems and Industrial Engineering, 321e332. Balasubramanian, V., Srinivasan, J., 2009. Surface EMG based muscle activity analysis for aerobic cyclist. Journal of Bodywork and Movement Therapies 13, 34e42. Balasubramanian, V., Adalarasu, K., Regulapati, R., 2009. Comparing dynamic and stationary standing postures in an assembly task. International Journal of Industrial Ergonomics 39, 649e654. Berzuini, C., Figini, M.M., Bernardinelli, L., 1985. Evaluation of the effectiveness of sEMG parameters in the study of neurogenic diseases e a statistical approach using clinical and simulated data. IEEE Transactions on Biomedical Engineering 32, 15e27. Bonger, P.M., Hulshof, C.T.J., Dijkstra, L., Boshuizen, H.C., 1990. Back pain and exposure to whole-body vibration in helicopter pilots. Ergonomics 33, 1007e1026. De Luca, C.J., 1993. The Use of Surface Electromyography in Biomechanics, Wartenweiler Memorial Lecture. The International Society for Biomechanics.
De Oliveira, C.G., Nadal, J., 2004. Back muscle sEMG of helicopter pilots in flight: effects of fatigue, vibration and posture. Aviation, Space, and Environmental Medicine 75, 317e322. De Oliveira, C.G., Simpson, D.M., Nadal, J., 2001. Lumbar back muscle activity of helicopter pilots and whole-body vibration. Journal of Biomechanics 34, 1309e1315. Froom, P., Froom, J., Van Dyk, D., Caine, Y., Ribak, J., Margaliot, S., Floman, Y., 1984. Lytic spondylolisthesis in helicopter pilots. Aviation, Space, and Environmental Medicine 55, 556e557. Froom, P., Barzilay, J., Caine, Y., Margaliot, S., Forecast, D., Gross, M., 1986. Low back pain in pilots. Aviation, Space, and Environmental Medicine 57, 694e695. Froom, P., Hanegbi, R., Ribak, J., Gross, M., 1987. Low back pain in the AH-1 Cobra helicopter. Aviation, Space, and Environmental Medicine 58, 315e318. Gaudreault, N., Arsenault, A.B., Lariviere, C., DeSerres, S.J., Rivard, C.H., 2005. Assessment of the paraspinal muscles of subjects presenting an idiopathic scoliosis: an EMG pilot study. BMC Musculoskeletal Disorders 6, 14. Griffin, M.J., 1990. Handbook of Human Vibration. Academic Press Limited. ISBN 0123030404. Hagg, G.M., Luttmann, A., Jager, M., 2000. Methodologies for evaluating electromyographic field data in ergonomics. Journal of Electromyography and Kinesiology 10, 301e312. Hansen, O.B., Wagstaff, A.S., 2001. Low back pain in Norwegian helicopter aircrew. Aviation, Space, and Environmental Medicine 72, 161e164. Krogh-Lund, C., Jørgensen, K., 1993. Myo-electric fatigue manifestations revisited: power spectrum, conduction velocity, and amplitude of human elbow flexor muscles during isolated and repetitive endurance contractions at 30% maximal voluntary contraction. European Journal of Applied Physiology and Occupational Physiology 66 (2), 161e173. Lariviere, C., Arsenault, A.B., Gravel, D., Gagnon, D., Loisel, P., 2003. Surface electromyography assessment of back muscle intrinsic properties. Journal of Electromyography and Kinesiology 13, 305e318. Mansfield, N.J., 2005. Impedance methods (apparent mass, driving point mechanical impedance and absorbed power) for assessment of the biomechanical response of the seated person to whole-body vibration. Industrial Health 43 (3), 378e389. Merletti, R., Lo Conte, L.R., Orizo, C., 1991. Indices of muscle fatigue. Journal of Electromyography and Kinesiology 1, 20e33. Srinivasan, J., Balasubramanian, V., 2007. Low back pain and muscle fatigue due to road cycling - an sEMG study. Journal of Bodywork and Movement Therapies 11 (3), 260e266. Toren, A., 2001. Muscle activity and range of motion during active trunk rotation in a sitting posture. Applied Ergonomics 32, 583e591. Acceleration and vibration, United States Naval Flight Surgeons Manual, third ed., 1991 Chapter 2. Wilder, D.G., Pope, M.H., 1996. Epidemiological and aetiological aspects of low back pain in vibration environments e an update. Clinical Biomechanics 11 (2), 61e73.
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Analysis of muscle fatigue in helicopter pilots Venkatesh Balasubramanian a, *, Ashwani Dutt a, b, Shobhit Rai a, b a b
Rehabilitation Bioengineering Group, IIT Madras, Chennai 600036, India Corps of EME, Indian Army, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 June 2010 Accepted 20 February 2011
Helicopter pilots espouse ergonomically unfavourable postures and endure vibration which result in low back pain. The objective of this study was to investigate the effects of a helicopter flight on pilots back and shoulder muscles using surface Electromyography (sEMG) analysis. This study also correlates low back pain symptoms from Rehabilitation Bioengineering Group Pain Scale (RBGPS) questionnaire with muscle fatigue rates obtained. RBGPS was administered on 20 Coast Guard helicopter pilots. sEMG was acquired before and after flight from erector spinae and trapezius muscles in 8 of these 20 pilots. Statistical analysis of time and frequency domain parameters indicated significant fatigue in right trapezius muscle due to flying. Muscle fatigue correlated with average duration of flight (r2 ¼ 0.913), total service as pilot (r2 ¼ 0.825), pain (r2 ¼ 0.463) and total flying hours (r2 ¼ 0.507). However, muscle fatigue weakly correlated with Body Mass Index (BMI) (r2 ¼ 0.000144) and age (r2 ¼ 0.033). Ó 2011 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Helicopter pilots Low back pain sEMG study RBGPS
1. Introduction Helicopter pilots are exposed to vibratory environments and ergonomically unfavourable working conditions (Fig. 1), which over a time period can cause back pain. Helicopters vibrate predominantly at frequencies related to the speed of revolution of its main rotor (4e11 Hz), tail rotor (30e60 Hz) and the engine (110 Hz). Aircrews in helicopter are therefore subjected to vibrations coinciding with the resonant frequencies of their body (4e8 Hz) (Griffin, 1990; Mansfield, 2005). The resonant frequencies for various parts of the human body are given in Table 1. As shown in Table 1, helicopters subject aircrew members to vibrations over a frequency range that coincides with the resonant frequencies of the body. Cockpit ergonomics is an area of concern for helicopter pilot fraternity which can be addressed by physiological and biomechanical analysis. One of the most important health problems is the onset of low back pain (LBP) due to straining of erector spinae muscle. Whole body vibration (Bonger et al., 1990; Srinivasan and Balasubramanian, 2007) and postural factors (Froom et al., 1984, 1986, 1987) are the main contributors to the occurrence of LBP. Studies relating to LBP aspects of helicopter pilots have relied
* Corresponding author. Rehabilitation Bioengineering Group, Department of Engineering Design, IIT Madras, Chennai 600036, India. Tel.: þ91 44 2257 4117; fax: þ91 44 2257 4732. E-mail addresses: [email protected], [email protected] (V. Balasubramanian).
on evaluating the same on the basis of questionnaire study (Balasubramanian and Prasad, 2006). While the results are very relevant, they have not been able to accurately point whether the problem exists entirely due to whole body vibration (Bonger et al., 1990; United States manual, 1991) or to posture (Froom et al., 1986, Hansen and Wagstaff, 2001) or a combination of both the factors (Froom et al., 1987; De Oliveira and Nadal, 2004). Whole body vibrations and postures lead to weakening of muscles, which result in higher fatigue rate than compared to a healthy muscle (De Oliveira et al., 2001; Toren, 2001; Wilder and Pope, 1996). Fatigue is defined as the time-dependent summation of internal and external influences, which adversely affects human performance, irrespective of any subjective awareness either of the influences or of the impairment (Bonger et al., 1990). sEMG parameters associated with either onset or occurrence of fatigue is signal amplitude, power, mean and median frequencies of the power density spectrum and number of zero crossings. It has been reported that, during fatigue, there is an increase in the sEMG signal amplitude and power (De Luca, 1993), and a decrease (or left shift) in the both mean and median frequencies (De Oliveira and Nadal, 2004; Balasubramanian and Srinivasan, 2009). This study evaluates the prevalence of low back pain in helicopter pilots flying a small helicopter (Chetak or Alouette III) by means of a questionnaire based study followed by determining rate of muscle fatigue in erector spinae and trapezius muscle group with help of sEMG analysis before and after flight. This study also correlates results of RBGPS with muscle fatigue in order to identify the parameters of RBGPS, which are responsible for onset of LBP.
0003-6870/$ e see front matter Ó 2011 Elsevier Ltd and The Ergonomics Society. All rights reserved. doi:10.1016/j.apergo.2011.02.008
914
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
Fig. 1. Cockpit of Alouette III (Chetak) used extensively by the Indian Coast Guard for maritime patrol.
2. Material and methods 2.1. Subjects Twenty helicopter pilots voluntarily participated in this study. The average age was 36.55 years (SD ¼ 3.53 years) and average BMI was 24.4 (SD ¼ 0.80). Eight male helicopter pilots are currently flying a small helicopter among the 20 pilots. Pilots had an average experience was 13.9 years (SD ¼ 3.48) as Chetak helicopter pilots and had average flying hours was 2370 (SD ¼ 760.96). Their average flying hours per month was 21.15 (SD ¼ 2.80) and the average duration of their routine sortie was 2.06 (SD ¼ 0.29) hours. None of the selected participants had a lower extremity injury, physical disability, or discomfort problem. Experiments were performed in the helicopter hangar from where the pilots used to embark for their flight and disembark. The experiment was performed as per the guidelines of the Institutional Ethics Committee of IIT Madras for human volunteer research. All volunteers also read and signed an informed consent before participating. 2.2. Questionnaire study Standardized questionnaires for the analysis of LBP symptoms in a helicopter environment (before, during and after flight) were administered on 20 helicopter pilots with the Indian Coast Guard. Questions presented were structured and were of the forced, binary or multiple choice types. Questions were specific and related to LBP caused due to flight induced activity of pilots. Pilots were instructed to grade the pain they experienced using the RBG pain scale presented in Table 2. 2.3. sEMG acquisition Among the twenty pilots, eight (8) had volunteered to be part of the sEMG study. The remaining pilots did not participate either due to administrative reasons (i.e., posting out, flying schedule being Table 1 Resonant frequency of various parts of human body. (Courtesy Field Manual of US Army No 3-04.301 Aeromedical training for flight personnel). Body part
Resonant frequency (Hz)
Whole body Shoulder girdle Head Eyes
4e8 4e8 25 30e90
out of the window of time allowed to visit the base, etc), technical reasons (i.e., flying condition, sortie nature, sortie duration, post sortie debriefing, etc) or choose not to participate. Muscles considered for this study were the trapezius (2 cm lateral to midpoint of C7 and acromium), erector spinae (slightly higher than the top of the iliac crest on the spine) bilaterally (Fig. 2(A)). Both these muscle groups are fairly large and easy to identify on test subjects. sEMG signals were recorded at a sampling frequency of 1000 Hz using EMG machine (Bagnoli-8TM, Delsys Inc, USA) with single differential electrode configuration and gain of 1000. EMG machine was powered by an isolated medical grade power supply with leakage current less than 10 mA and safely isolated to 3750 Vrms (Balasubramanian and Adalarasu, 2007). Dimension of the electrode bar was (19.8 5.4 35) mm with 10 mm contact spacing between bars. Electrode interfaces were made from a specifically designed medical grade adhesive. Before placing the measurement electrodes, placement site was identified, shaved, and cleaned with ethanol to avoid impedance mismatch and movement artifact. Disposable reference electrodes (Ag/Agcl 3 M e Red DotÔ model 2330 electrodes) were placed on the wrist bone. On each occasion, i.e., pre- and post-flight, pilots performed maximal voluntary contraction (MVC), which involved lifting of weight kept on the floor without causing any perceived discomfort to the body (Fig. 2(B)). Three trials of sEMG recordings of 10 s duration (pre- and post-flight) were acquired from each pilot. As this study was performed on Indian Coast Guard helicopter pilots while flying regular sorties, we had to abide by mission safety norms. Due to this, we had but no option to remove and re-site the electrodes between pre- and post-flight data collection. We had however taken extreme care by marking the electrode position and orientation. After pre-flight MVC tests, the electrode profile was marked on the skin using marker pens to identify the exact electrode location for post-flight MVC EMG data collection. In addition, electrodes were removed from the skin leaving behind the specially designed interface adhesive as an identification marker of the electrode location.
Table 2 RBG pain scale criteria used to grade LBP among Chetak (Alouette III) helicopter pilots during a routine flight. Grade
Criteria
0 1 2 3 4 5
No pain Very minor, barely felt Minor pain not interfering with the flight routine Discomforting pain not interfering with the flight routine Severe pain affecting routine flying Unbearable pain requiring medical attention on landing
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915
Fig. 2. (A) The physical locations of the surface electrodes on a typical subject. Muscles that were examined included Right Trapezius (RT); Right Erector Spinae (RES). (B) Subject performing activity to elicit maximal voluntary contraction (MVC).
2.4. Signal processing and feature extraction sEMG signals were filtered using a 4th order Butterworth filter with a pass band range of 20e500 Hz. AC line interference was eliminated using a 2nd order notch filter with a stop band range of
47e51 Hz. Filtered sEMG signals were full wave rectified. Root mean square (rms) of sEMG signal is considered as the most reliable parameter in the time domain (Balasubramanian et al., 2009). A concomitant increase in the rms value with an advancing fatigue has been reported (Krogh-Lund and Jørgensen, 1993). Mean
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amplitude and rms were extracted from time domain of the rectified signals. Median frequency (MF) changes during contraction period can be analyzed by linear regression (Merletti et al., 1991). The negative inclination or slope of the regression line will indicate fatigue rate (Gaudreault et al., 2005; Balasubramanian et al., 2009). Power spectrum was computed over the frequency range of 20e500 Hz by applying fast Fourier transformation on the filtered signal and median frequency (MF) was extracted. 2.5. Regression Muscle fatigue can be measured indirectly using the knowledge of direction and magnitude of the slope of line of best fit (Balasubramanian and Adalarasu, 2007). Hence in this study linear regression analysis was performed on mean amplitude, rms from time domain and MF values extracted from frequency domain of sEMG signals obtained from the muscles (trapezius and erector spinae) of each subject during pre- and post-flight condition.
2.4
(1 - 30) Min (30 - 60) Min (60 - 90) Min (90 - 120) Min
1.8
Scale of Pain
916
1.2
0.6
0.0 Time (Minutes) Fig. 3. Change of the pain grade with flying time.
2.6. Statistical test sEMG signals obtained during pre-flight and post-flight were not statistically independent. Owing to a smaller sample size, Wilcoxon signed rank test (non parametric test) was performed on mean amplitude, rms and MF to determine whether these features, which were extracted from sEMG signals, obtained during preflight and post-flight were significantly different. ManneWhitney U-test (non parametric test) was performed to determine whether the fatiguing rates of left and right trapezius and left and right erector spinae were significantly different. 3. Results 3.1. Questionnaire study Pain grades obtained using RBG pain scale were correlated (Spearmans correlation) with various attributes of subjects namely body mass index, age, average duration of flight, experience as Chetak helicopter pilots, average flying hours/month and total flying hours. Square of correlation r2 between the pain grade and the attributes is shown in Table 3. Total flying hours (r2 ¼ 0.86) and average flying hours/month (r2 ¼ 0.88) show excellent correlation with pain grade, where as age (r2 ¼ 0.066) and BMI (r2 ¼ 0.008) do not show any correlation. Pain grade was gradually increased from the beginning to the end of the flight as shown in Fig. 3. 3.2. sEMG study (LBP) Time domain parameter such as mean amplitude and rms features extracted bilaterally from trapezius and erector spinae muscles were significantly high in post-flight when compared to pre-flight condition (p < 0.05) as shown in Fig. 4(A) and (B).
Table 3 Correlation (r2) of Pain Grade with various attributes of the subjects. S No
Variables
Correlation with pain grade (r2)
1 2 3 4 5 6
Age (years) Body mass index Average duration of sortie (hours) Service as Chetak pilot (years) Average flying hours/month Total flying hours
0.066 0.008 0.264 0.384 0.875 0.856
Frequency domain parameter such as median frequency (MF) feature values extracted from the right trapezius muscle of significantly low (p < 0.05) in post-flight condition as compared to preflight condition as shown in Fig. 4(C). A concomitant increase in the MA, rms parameters value with time and MF value decrease, with advancing fatigue has been reported (Balasubramanian et al., 2009). Besides, it is also evident that the magnitude of fatiguing rate (mean amplitude, high rms values and low MF values) was higher in the right trapezius muscles during the post-flight as compared to its pre-flight. Significant difference (p < 0.04) in the fatiguing pattern was observed (in both time and frequency domain) only in right trapezius muscles. Correlation between various variables in the questionnaire study and fatigue rates of different muscle groups are shown in Table 4. Excellent correlation exists between the fatigue rate of the left erector spinae muscle extracted out from the mean amplitude feature with average duration of the sortie (r2 ¼ 0.913) while average correlation is obtained with average flying hours/ month (r2 ¼ 0.426) for the same feature. However in rms amplitude feature, the correlation is poor with average flying hours/month (r2 ¼ 0.289) and average with average duration of the sortie (r2 ¼ 0.567). In median frequency, a very good correlation is obtained between the fatigue rate of right trapezius muscle and service as a Chetak pilot (r2 ¼ 0.825). The correlation of the fatigue rate of the same muscle is average with scale of pain (r2 ¼ 0.463) and total flying hours (r2 ¼ 0.507). In MA and rms features there is weekly negative correction (r2 ¼ 0.000144) with BMI bilaterally erector spinae muscle. Muscle fatigue was weekly correlated (r2 ¼ 0.033) with age. 4. Discussion Objective of this study was to determine fatigue in erector spinae and trapezius muscle groups of a group of Chetak helicopter pilots by performing sEMG study. Questionnaire based on RBGPS was administered on a larger group of pilots to identify causes of LBP. It is evident from the results that an excellent correlation existed between total flying time and pain, which points that greater the time flown by the pilots, greater will be the prevalence of low back pain in them. This result agrees with previous studies (Bonger et al., 1990; Hansen and Wagstaff, 2001). Poor correlation was obtained between the LBP and the total service as Chetak helicopter pilot. This could be due to genetic constitution of subjects and also because pilots fly less after a certain number of
V. Balasubramanian et al. / Applied Ergonomics 42 (2011) 913e918
A *
0.12
*
*
* *
RMS (mV)
Mean Amplitude (mV)
600
B
Pre Flight Post Flight
*
917
400
*
Pre Flight Post Flight
*
0.06
200
0
0.00
L.T
R.T L.ES R.ES Muscle Group
C
L.T
R.T L.ES R.ES Muscle Group
Pre Flight Post Flight
400
*
MF (Hz)
300
200
100
0
L.T
R.T L.ES R.ES Muscle Group
Fig. 4. The average and standard deviation of (A) Mean Amplitude (MA) (B) rms from the time-series data and (C) Median Frequency (MF) from the power spectrum between preand post-flight condition is schematically depicted. Muscles that were examined included Right Trapezius (RT); Left Trapezius (LT); Right Erector Spinae (RES); Left Erector Spinae (LES). Note * significant difference (p < 0.05) between pre- and post-flight condition.
years of service. An excellent correlation was observed between the pain grade and average flying hours per month. This variable was not considered in any other study although it is an important factor which could lead to transient low back pain. Studies on fatigue measurements have established that fatigue occurs when time domain parameters such as mean amplitude and rms amplitude increase (Berzuini et al., 1985; Lariviere et al., 2003). Positive slopes signify the fatigue rates post-flight (De Luca, 1993; Hagg et al., 2000; Lariviere et al., 2003). In frequency domain, studies have hypothesized that decrease in median frequency signifies onset of muscle fatigue, which is associated with the negative slope of the line of regression (De Luca, 1993; Hagg et al.,
Table 4 Correlation (r2) between fatigue rates and the variables of questionnaire study. Acronyms used are MA e mean amplitude, RMS e root mean square, MF e median frequency. Variables/Features
Average flying h/month Average duration of the flight Pain grade Total flying hours Service as Chetak pilot BMI Age
MA
RMS
MF
Left erector spinae
Left erector spinae
Right trapezius
0.426 0.913 e e e 0.000144 0.033
0.289 0.567 e e e 0.000144 0.033
e e 0.463 0.507 0.825 0.000144 0.033
2000; Lariviere et al., 2003). Results of identification of fatigue in this study show that right trapezius muscles are conclusive while for other muscles it is inconclusive. These variations in median frequency can be attributed to other factors during flight like posture or the design of cockpit interiors. Results of this study imply that right trapezius muscle is more significantly recruited than the left while the recruitment in erector spinae is generally uniform. On observing the drills and flying posture adopted by the pilots, it was seen that the right arm is used to hold the stick for the entire duration of the flight. Thus the right gleno-femoral joint is in a state of flexion and the elbow joint is in the state of extension throughout the flight. Since the right trapezius is recruited for the movement of the gleno-femoral joint, these muscles are more likely to be additionally fatigued compared to left trapezius. The fatigue rate in left erector spinae has shown excellent correlation with the average duration of the flight in mean amplitude and an average correlation in rms amplitude. It can be concluded that the duration of a flight contributes to transient back pain. It shows similarities with the results reported by Bonger (Bonger et al., 1990) who stated that average hours of flight per day seemed most strongly related to transient back pain. In median frequency, an excellent correlation has been obtained between the fatigue rate of right trapezius muscle and the total service as a Chetak helicopter pilot. It implies that the fatiguing pattern of the trapezius muscles have excellent correlation with the service career of a pilot.
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5. Conclusion Various studies have covered the aspect of low back pain among helicopter pilots through questionnaire based studies. However, its relationship with flight induced muscle fatigue has not been explored. In this study, we have performed MVC study on select muscle groups on helicopters pilots flying real sorties. Pre- and postflight sEMG analysis indicates fatigue in both trapezius and erector spinae muscle groups. Statistical analysis showed that right trapezius muscle fatigues more than left trapezius muscle, which correlates with the right handed handling of the helicopter stick by the pilots. However there were lateral difference in fatigue of the back muscle (erector spinae), which indicates that the back muscle fatigue and hence the pain could result from the posture and whole body vibration exposure. Correlation studies between pain grades obtained using RBGPS and attributes of pilots evidently point out that total flying hours and average flying duration are two important basis factors for the occurrence of low back pain among helicopter pilots. References Balasubramanian, V., Adalarasu, K., 2007. sEMG-based analysis of change in muscle activity during simulated driving. Journal of Bodywork and Movement Therapies 11, 151e158. Balasubramanian, V., Prasad, G.S., 2006. Ergonomic assessment of bar cutting process in construction. International Journal of Systems and Industrial Engineering, 321e332. Balasubramanian, V., Srinivasan, J., 2009. Surface EMG based muscle activity analysis for aerobic cyclist. Journal of Bodywork and Movement Therapies 13, 34e42. Balasubramanian, V., Adalarasu, K., Regulapati, R., 2009. Comparing dynamic and stationary standing postures in an assembly task. International Journal of Industrial Ergonomics 39, 649e654. Berzuini, C., Figini, M.M., Bernardinelli, L., 1985. Evaluation of the effectiveness of sEMG parameters in the study of neurogenic diseases e a statistical approach using clinical and simulated data. IEEE Transactions on Biomedical Engineering 32, 15e27. Bonger, P.M., Hulshof, C.T.J., Dijkstra, L., Boshuizen, H.C., 1990. Back pain and exposure to whole-body vibration in helicopter pilots. Ergonomics 33, 1007e1026. De Luca, C.J., 1993. The Use of Surface Electromyography in Biomechanics, Wartenweiler Memorial Lecture. The International Society for Biomechanics.
De Oliveira, C.G., Nadal, J., 2004. Back muscle sEMG of helicopter pilots in flight: effects of fatigue, vibration and posture. Aviation, Space, and Environmental Medicine 75, 317e322. De Oliveira, C.G., Simpson, D.M., Nadal, J., 2001. Lumbar back muscle activity of helicopter pilots and whole-body vibration. Journal of Biomechanics 34, 1309e1315. Froom, P., Froom, J., Van Dyk, D., Caine, Y., Ribak, J., Margaliot, S., Floman, Y., 1984. Lytic spondylolisthesis in helicopter pilots. Aviation, Space, and Environmental Medicine 55, 556e557. Froom, P., Barzilay, J., Caine, Y., Margaliot, S., Forecast, D., Gross, M., 1986. Low back pain in pilots. Aviation, Space, and Environmental Medicine 57, 694e695. Froom, P., Hanegbi, R., Ribak, J., Gross, M., 1987. Low back pain in the AH-1 Cobra helicopter. Aviation, Space, and Environmental Medicine 58, 315e318. Gaudreault, N., Arsenault, A.B., Lariviere, C., DeSerres, S.J., Rivard, C.H., 2005. Assessment of the paraspinal muscles of subjects presenting an idiopathic scoliosis: an EMG pilot study. BMC Musculoskeletal Disorders 6, 14. Griffin, M.J., 1990. Handbook of Human Vibration. Academic Press Limited. ISBN 0123030404. Hagg, G.M., Luttmann, A., Jager, M., 2000. Methodologies for evaluating electromyographic field data in ergonomics. Journal of Electromyography and Kinesiology 10, 301e312. Hansen, O.B., Wagstaff, A.S., 2001. Low back pain in Norwegian helicopter aircrew. Aviation, Space, and Environmental Medicine 72, 161e164. Krogh-Lund, C., Jørgensen, K., 1993. Myo-electric fatigue manifestations revisited: power spectrum, conduction velocity, and amplitude of human elbow flexor muscles during isolated and repetitive endurance contractions at 30% maximal voluntary contraction. European Journal of Applied Physiology and Occupational Physiology 66 (2), 161e173. Lariviere, C., Arsenault, A.B., Gravel, D., Gagnon, D., Loisel, P., 2003. Surface electromyography assessment of back muscle intrinsic properties. Journal of Electromyography and Kinesiology 13, 305e318. Mansfield, N.J., 2005. Impedance methods (apparent mass, driving point mechanical impedance and absorbed power) for assessment of the biomechanical response of the seated person to whole-body vibration. Industrial Health 43 (3), 378e389. Merletti, R., Lo Conte, L.R., Orizo, C., 1991. Indices of muscle fatigue. Journal of Electromyography and Kinesiology 1, 20e33. Srinivasan, J., Balasubramanian, V., 2007. Low back pain and muscle fatigue due to road cycling - an sEMG study. Journal of Bodywork and Movement Therapies 11 (3), 260e266. Toren, A., 2001. Muscle activity and range of motion during active trunk rotation in a sitting posture. Applied Ergonomics 32, 583e591. Acceleration and vibration, United States Naval Flight Surgeons Manual, third ed., 1991 Chapter 2. Wilder, D.G., Pope, M.H., 1996. Epidemiological and aetiological aspects of low back pain in vibration environments e an update. Clinical Biomechanics 11 (2), 61e73.