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Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight
Accepted Manuscript Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight Mike Murray, Britt Lange, Shadi Samir Chreiteh, Henrik Baare Olsen, Bo Riebeling Nørnberg, Eleanor Boyle, Karen Søgaard, Gisela Sjøgaard PII: DOI: Reference:
S1050-6411(15)00243-6 http://dx.doi.org/10.1016/j.jelekin.2015.12.009 JJEK 1933
To appear in:
Journal of Electromyography and Kinesiology
Received Date: Revised Date: Accepted Date:
3 July 2015 16 December 2015 19 December 2015
Please cite this article as: M. Murray, B. Lange, S.S. Chreiteh, H.B. Olsen, B.R. Nørnberg, E. Boyle, K. Søgaard, G. Sjøgaard, Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight, Journal of Electromyography and Kinesiology (2016), doi: http://dx.doi.org/10.1016/ j.jelekin.2015.12.009
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Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight
1
Mike Murray, 2Britt Lange, 1Shadi Samir Chreiteh, 1Henrik Baare Olsen, 3Bo Riebeling Nørnberg, 1,4
1
Eleanor Boyle, 1Karen Søgaard, 1Gisela Sjøgaard,
Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Denmark 2
Department of Anesthesia and Intensive Care Medicine, Odense University Hospital, Denmark 3
Royal Danish Air Force, Air Force Staff, Defence Command Denmark, Denmark 4
Dalla Lana School of Public Health, University of Toronto, Canada
Keywords: electromyography; muscle activity; workload; neck; shoulder; pilots; crew-members; helicopter; pain
Corresponding author: Mike Murray Institute of sport science and clinical biomechanics University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Telephone number: (45) 27 20 50 58 E-mail: [email protected] 1
Abstract Neck pain among helicopter pilots and crew-members is common. This study quantified the physical workload on neck and shoulder muscles using electromyography (EMG) measures during helicopter flight. Nine standardized sorties were performed, encompassing: cruising from location A to location B (AB) and performing search and rescue (SAR). SAR was performed with night vision goggles (NVG), while AB was performed with (AB+NVG) and without NVG (AB-NVG). EMG was recorded for: trapezius (TRA), upper neck extensors (UNE), and sternocleido-mastoid (SCM). Maximal Voluntary Contractions (MVC) were performed for normalization of EMG (MVE). Neck posture of pilots and crew-members was monitored and pain intensity of neck, shoulder, and back was recorded. Mean muscle activity for UNE was ~10% MVE and significantly higher than TRA and SCM, and SCM was significantly lower than TRA. There was no significant difference between AB-NVG and AB+NVG. Muscle activity in the UNE was significantly higher during SAR+NVG than AB-NVG. Sortie time (%) with non-neutral neck posture for SAR+NVG and AB-NVG was: 80.4%, 74.5% (flexed), 55.5%, 47.9% (rotated), 4.5%, 3.7% (lateral flexed). Neck pain intensity increased significantly from pre- (0.7 ± 1.3) to post-sortie (1.6 ± 1.9) for pilots (p = 0.028). If sustained, UNE activity of ~10% MVE is high, and implies a risk for neck disorders.
Introduction Neck pain is common among helicopter pilots and crew-members (Forde et al., 2011; Salmon et al., 2011; Van den Oord et al., 2010). The 1-year prevalence for helicopter pilots has been reported to be 43-48% (Bridger et al., 2002; Van den Oord, De Loose 2010). In another study a 3-month prevalence was reported to be 57% (Ang and Harms-Ringdahl 2006). For crew-members, the 1-year prevalence has been estimated to be 62% (Van den Oord et al., 2014). These prevalence rates are high compared to the general population with values for the 1-year prevalence of 26% (Hoy et al., 2010) and 37% (Fejer et al., 2006). In a survey, approximately 15% of the pilots and 28–56% of the crew-members
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(depending on helicopter type) had been grounded at least once during their career due to flight-related neck pain (Adam 2004). Approximately 10% of the pilots indicated that on at least one occasion their ability to perform normal flying duties was disrupted or compromised because of in-flight neck pain. Severe neck pain may result in grounding of pilots and crew-members influencing operational capability and flight planning (Harrison et al., 2009) and induce financial costs due to loss of manpower and litigation (Salmon, Harrison 2011). A variety of factors have been found to be associated with flight-related neck pain. Both the weight of the flight helmet and the Night Vision Goggles (NVG) have often been reported to be perceived causes (Harrison et al., 2015; Van den Oord et al., 2012). NVG are mounted on the front of the helmet, which increases the helmet’s weight and further shifts the head’s center of mass forward and upward in relation to the motion axis of the cervical spine (Butler 1996; Thuresson et al., 2005). To counterbalance this, pilots and crew-members may use counterweights (CW) attached to the back of their helmet (Thuresson, Ang 2005). Increased external loading of the cervical spine may play an important role regarding flight-related neck pain. However, NVG also reduces peripheral vision and pilots have been found to spend significantly more time in mid to severe deviations from neutral neck posture with NVG, as compared to without NVG (Forde, Albert 2011). Pilots and crew-members may position themselves differently depending on the type of sortie. Cruising from point A to B (AB) may potentially call for different neck postures as compared with a search and rescue (SAR) sortie. This might affect the physical workload on neck and shoulder muscles. Flightrelated neck pain may therefore be partly explained from a biomechanical perspective with a positive relationship between cervical loading, posture, and neck pain (Thuresson et al., 2003). The aim of this study was to quantify the activity of neck and shoulder muscles among helicopter pilots and crewmembers during operative sorties with and without NVG. Our main hypotheses were: 1) sorties with 3
NVG result in significantly higher levels of muscle activity as compared to sorties without NVG, and 2) SAR sorties with NVG result in the highest level of muscle activity measured. Additional hypotheses tested were: 1) pilots and crew-members have different levels of muscle activity and 2) some muscles have higher activity than others.
Methods Subjects Nine pilots (eight males/one female) and nine crew-members (all males) from the Royal Danish Air Force participated in this study. Inclusion criteria were: 1) profession as either pilot in command (1P), second pilot (2P), first technician (1T), or second technician (2T), 2) having been on active flight duty within the past 30 days, and 3) trained in using NVG-equipment. Exclusion criteria were: 1) intensity of neck, shoulder, or back pain ≥3 at present rated on an 11 point numeric box scale (0 = no pain, 10 = worst pain imaginable), 2) previous trauma to the neck, shoulder, or back. One additional crew-member had been recruited but was excluded from participation due to a pre-sortie intensity of neck pain equal to five. Participants were provided with verbal and written information regarding the study before giving their informed written consent. The study was approved by the local ethical committee of Southern Denmark (S-20120121).
Experimental design and equipment Nine standardized sorties were completed each lasting around 1.5 hours. Each sortie included three parts 1) AB with NVG (AB+NVG), 2) SAR always with NVG (SAR+NVG), and 3) AB without NVG (AB-NVG). Sorties were randomized regarding AB+NVG and AB-NVG being flown first or last, respectively, such that the sequences were either: AB+NVG, SAR+NVG, AB-NVG or AB-NVG, 4
SAR+NVG, AB+NVG. Participants were balanced according to profession (1P, 2P, 1T or 2T). Sorrtie parts represented regular work tasks within the squadron. Sorties were flown with the AgustaWestland EH101 Merlin helicopter (AgustaWestland, EH Industries Ltd, England) and executed from October to November 2012. Participants were equipped with a flight suit, a dry suit, a safety harness, a flight helmet weighing 1.858 kg (Alpha-221, Helmet Integrated Systems Ltd., England), and NVGequipment weighing 0.987 kg (Night Vision & Communications solutions, USA) (Figure 1).
Questionnaire Pilots and crew-members reported flight hours within the past year, one month, and 7 days. NVG hours were reported within the past year, three months, and 7 days. Episodes of neck, shoulder or back pain were reported pre and post sorties using a modified version of the standardized Nordic questionnaire for the analysis of musculoskeletal symptoms (Kuorinka et al., 1987). Questions were: 1) do you have pain in the neck, shoulder or back at present? (yes/no), and 2) if yes, please rate the intensity of pain in your neck, shoulder or back (11 point numeric box scale). The latter question was asked pre as well as post flight.
Instrumentation and procedure Before each sortie, one pilot (1P or 2P) and one crew-member (1T or 2T) were equipped with six electromyographic (EMG) surface electrodes (Ag/AgC1, Ambu Blue Sensor, N-00-S/25, Denmark). EMG-electrodes were positioned 1) bilaterally above the Trapezius muscle (TRA) 20% medial to half the length between the lateral part of the acromion and the seventh cervical vertebra (Holtermann et al., 2009), 2) above the upper neck extensor (UNE) muscles, at the level of the fourth cervical vertebra (Gosselin et al., 2004; Juul-Kristensen et al., 2004), and 3) above the Sternocleido-mastoid muscle 5
(SCM) at the cranial 2/3 of the muscle (Falla et al., 2002). Placement sites were shaved, scrubbed with cleansing gel, and cleaned with 70% alcohol. EMG-electrodes were positioned with an inter-electrode distance of 20 mm and checked for resistance <20 kΩ. The EMG-signal was pre-amplified with a gain of 400 through a built-in amplifier in the wireless EMG transmission probes (EMG probe, Direct Transmission System (DTS), Noraxon Inc, USA). The EMG-signal was low-pass filtered at 500 Hz and high-pass filtered at 10 Hz, and sent to a belt-worn receiver by Wi-Fi (TELEMyo™ 2400T, DTS, Noraxon Inc, USA). The EMG-signal was sampled at 1500 Hz (16 bit A/D converted) using a 12 channel configuration and transferred to a laptop (HP EliteBook 6930p notebook). Event markers were placed within the EMG-signal to enable distinction between sortie parts.
Maximal Voluntary Contractions and electromyography normalization The EMG signal was normalized using isometric maximal voluntary contractions (MVC) previously validated (Essendrop et al., 2001; Faber et al., 2006) and maximally activating the targeted muscles (Blouin et al., 2007; Mayoux-Benhamou et al., 1997; Schomacher et al., 2012). Shoulder elevation was chosen for normalization of TRA, cervical flexion for SCM, and cervical extension for UNE. During shoulder elevation the participants were positioned in a standardized chair with the back straight and no contact to the floor, arms relaxed, and looking straight ahead. Two force dynamometers (Bofors Electronics, Sweden) were placed bilaterally above the shoulders one centimeter medially to the lateral edge of the acromion. Both shoulders were simultaneously elevated during the test (Jensen et al., 1993). During neck flexion, participants were positioned seated with their front against the chair’s backrest and feet on the floor. The head was positioned anatomically neutral and a force transducer was placed above the eyebrows. Participants were instructed to maintain this position and press against the force transducer by leaning forward. During neck extension, the force transducer was placed above the 6
external occipital protuberance. Participants were instructed to maintain an anatomical neutral head position and press against the force transducer by leaning backwards. A minimum of three MVC´s were performed. If the third MVC was >5% higher than either of the previous two, a fourth MVC was performed. If the fourth MVC exceeded the previous three MVC´s by >5% a fifth and final MVC was performed (Essendrop, Schibye 2001). MVC´s were separated by 30 seconds for recovery. EMG-data was stored in Noraxon software (Noraxon Inc, USA), converted in Matlab (MathWorks Inc. USA), and analyzed in Hedera 2.0 (University of Southern Denmark, Denmark). A window size of 1 second with moving steps of 0.1 seconds was used. The EMG root mean square voltage (μV) corresponding to the highest MVC was used as the reference against which EMG-activity was normalized (% of Maximal Voluntary Electrical signal (MVE)).
Observations During sorties, a modified version of the observational method for posture, activity, tools, and handling (PATH) was used to monitor the pilots and crew-members equipped with EMG-sensors (Buchholz et al., 1996). Pilots and crew-members were observed (visual inspection performed by the first author for all observations in this study) every 60 seconds during the three sortie parts according to the orientation of the neck: neutral (<30° flexed/extended), flexed (>30° from neutral), extended (>30° from neutral), rotated (>30° from neutral), and laterally flexed (>30° from neutral), and trunk: neutral (flexion, lateral bending, and twisting < 20°), moderate flexion (>20° to <45° from neutral), severe flexion (>45° from neutral), lateral bending/twisting (flexion < 20°, lateral bending, or twisting >20°), and flexion/twisting (flexion and twisting >20°) (Buchholz, Paquet 1996)
Statistical analysis 7
Differences between pilots and crew-members regarding age, weight, height, flight, and NVG hours, intensity of neck, shoulder and back pain, MVC, and observations were analyzed using a Studentʹs ttest or Wilcoxon rank-sum test depending on normality of data. Pre- and post-sortie changes regarding intensity of neck, shoulder, and back pain was analyzed using a Wilcoxon matched pairs signed-rank test. The level of statistical significance was p <0.05. The hypotheses regarding the muscle activity (%MVE) was analyzed using a multilevel linear regression model. The model included the following parameters: 1) type of sortie part (AB-NVG, AB+NVG, SAR+NVG); 2) profession (pilots (1P and 2P) versus crew-members (1T and 2T)); 3) muscle group (TRA, UNE, SCM); and 4) electrode position (right or left side). A full model was first fitted and parameters with a p-value >0.1 were excluded from the model. Test for interactions between parameters was performed and the final model was fitted with an interaction between muscle group and type of sortie. Normality of the residuals was assessed using a Q-Q plot, and a Shapiro Wilk´s test. Statistical analysis was performed in Stata Statistics/Data Analysis version 13.0 (StataCorp LP, USA). Results are presented as mean±SD if not otherwise specified.
Results Subjects There were no significant differences between pilots and crew-members regarding participant characteristics (Table 1).
Intensity of neck, shoulder, and back pain Five out of eighteen (28%) participants experienced neck pain pre-sortie and 9/18 (50%) post-sortie. For the shoulders, numbers were 4/18 (22%) and 3/18 (17%), and for the back 4/18 (22%) and 4/18 (22%), respectively. There was no significant difference in pain intensity between pilots and crew8
members pre-sortie. Intensity of neck pain was; however, significantly increased from pre- to postsortie for pilots (p = 0.028) but not for crew-members (p = 0.187) (Table 2).
Maximal voluntary contraction MVC for cervical extension was significantly higher (p = 0.009) among crew-members (281.6 ± 40.7 N) than pilots (208.8 ± 54.3 N) (Table 3).
Observations Eighteen observations were conducted: 6 observations (1 pilot and 5 crew-members) during AB-NVG, 6 observations (1 pilot and 5 crew-members) during AB+NVG, and 6 observations (3 pilots and 3 crew-members) during SAR+NVG. Unfortunately, not all observations were performed as planned and inadequate data did not allow for comparison between professions nor between sortie parts. As a result, observations for pilots and crew-members were merged. During sorties, participants positioned themselves within a predominantly flexed and rotated neck position (Figure 2). A trend was observed towards more time (% of sortie time) in non-neutral neck positions during SAR+NVG as compared to AB-NVG: 80.4% vs. 74.5 % (flexed), 55.5% vs. 47.9% (rotated), 4.5% vs. 3.7 % (lateral flexed) and 12.4% vs. 17.4% (neutral).
Muscle activity Sampling time was throughout the flight, but duration analyzed for each sortie parts AB-NVG, AB+NVG and SAR+NVG was: 16.9 ± 5.1, 22.7 ± 10.8 and 21.3 ± 11.1 minutes. EMG-measurements were terminated ahead of time during two sorties due to incoming SAR missions. Sortie parts analyzed included eight AB-NVG (7 pilots and 8 crew-members), nine AB+NVG (9 pilots and 9 crew-members) 9
and eight SAR+NVG (8 pilots and 8 crew-members). Parameters within the multilevel linear regression model were tested for relevance, and profession (pilots (1P and 2P) versus crew-members (1T and 2T)) was excluded (p = 0.543). No significant evidence was found against the assumption of an interaction between muscle group and type of sortie (p = 0.080). Mean muscle activity in the UNE was significantly higher (p = <0.001) as compared to TRA, and muscle activity in SCM was significantly lower as compared to TRA (p = 0.045) when adjusted for type of sortie, electrode position, and muscle group (Table 4). No significant differences regarding muscle activity in TRA, UNE or SCM were found between sortie parts AB-NVG and AB+NVG. Muscle activity in the UNE was, however, significantly higher during SAR+NVG as compared to AB-NVG (p = 0.020) (Figure 3). Muscle activity was ~10% MVE or above in the UNE during 50% of SAR+NVG as illustrated by the cumulative distribution function (CDF) (Figure 4). The UNE were also significantly higher than TRA and SCM during 10%, 50%, and 90% of SAR+NVG.
Discussion The main finding of this study was a sustained muscle activity of approximately 10% MVE in the UNE during sorties. This sustained muscle activity may cause ischemic muscular pain, and localized muscle fatigue has been shown to occur in sustained contractions of as low as 5% MVC (Sjogaard et al., 1986). Low-level static exertions have also been reported to be a risk factor for the development of cumulative trauma disorders and repetitive strain injuries (Sjogaard and Jensen 2006). Our findings should be interpreted relative to the duration of an average military sortie, which has been reported to be approximately 2 hours long with the average of the longest sorties reaching up to 3.5 hours (Harrison et al., 2007). This duration is considerably longer compared to the sortie parts in our study, which each were approximately 20 minutes long. We found a sustained muscle activity of 10% 10
MVE in the UNE which is considered high, especially in relation to the duration of an average military sortie (Sjogaard and Jensen 2006). To our knowledge, there has not been a published study that has quantified the physical workload on neck and shoulder muscles using EMG measures in helicopter pilots and crew-members during real-flight scenarios. In-flight EMG-measurements in fighter pilots may be comparable, if measured during a normal gravitational level (+1G z). Green et al. reported muscle activity within the cervical part of the erector spinae muscle to be approximately ~10% MVE (Green and Brown 2004). Similarly, Hamalainen et al. addressed upper neck muscle activity among fighter pilots and found a slightly lower level of muscle activity equal to approximately 6.4% MVE (Hamalainen and Vanharanta 1992). EMG-recordings were measured at +1Gz in both studies, with neutral head positioning wearing a helmet, oxygen mask, and anti-gravitational suite (Hamalainen and Vanharanta 1992). Even though these results are similar to ours, regarding the magnitude of muscle activity, studies conducted in helicopter pilots within laboratory settings may be more appropriate as a reference, in order to limit differences regarding flight environments, ergonomics and equipments. Thuresson et al. previously found a level of muscle activity between 7% and 10% MVE in the left upper neck muscles and bilaterally in the lower neck muscles during ipsilateral neck flexion and rotation. A level of approximate 5% MVE was also found during trunk inclination and ipsilateral rotation (Thuresson, Ang 2003). Overall, these results are similar to our findings. It has been shown that if a contraction is to be maintained for one hour, it may have to be as low as 8% MVC with an acceptable level for continuous contractions equal to a few percent of MVC (Bjorksten and Jonsson 1977). A suggested level of acceptance of maximal 2-5% MVC has also been suggested (Sjogaard and Jensen 2006). The association between low-level static muscle contractions and musculoskeletal disorders is; however, not fully identified (Sjogaard and Jensen 2006), and musculoskeletal disorders have been found to be high in professions with exposure within the recommended levels of static 11
muscle contraction. As a consequence, it has been suggested that static loads are not acceptable at all, if sustained frequently or over long durations of time (Bjorksten and Jonsson 1977; Sjogaard and Jensen 2006). Excessive loading of the UNE, as illustrated by our findings could potentially lead to musculoskeletal pain in the neck (Ang et al., 2005). We found no significant difference in muscle activity between sortie parts AB-NVG and AB+NVG. The increase in helmet mass due to NVG was approximately 50%. However, the change in muscle activity was approximately 1% MVE in the TRA, UNE, and SCM. This might be due to a non-linear relationship between force and muscle activity. Schuldt et al. previously demonstrated that an exerted force of up to 40% MVC was produced by a level of muscle activity of 10 – 15% MVE (Schuldt and Harms-Ringdahl 1988). These findings are further supported by Thuresson et al. (Thuresson, Ang 2003), and are probably resulting from a large synergy in neck muscles sharing the load of neck extension. The load induced by NVG equipment accounts for a small relative load in relation to the maximal force demonstrated during the normalization procedure using MVCs. A non-linear relationship between force and muscle activity may therefore, to some extent, explain the relatively small change in muscle activity between sortie parts AB-NVG and AB+NVG. Knight et al. studied changes in neck muscle activity due to added head load and static head positions, and found that changes in head position significantly influenced the magnitude of cervical loading (Knight and Baber 2004). Thuresson et al. examined neuromuscular activity in the upper neck, lower neck, and upper Trapezius muscle in relation to different trunk and head positions, taking head worn equipment into account (Thuresson, Ang 2003). When assessing muscle activity of all positions and sides, Thuresson et al. found significantly higher levels of muscle activity in the upper neck muscles using a helmet and NVG or helmet, NVG, and CW as compared to a helmet alone. However, when assessing the activity levels for every position and side separately no significant difference in muscle activity level between 12
any of the head-worn items of equipment was found (Thuresson, Ang 2003). Head and trunk positions may therefore have more influence on muscle activity than the load of head worn equipment alone (Thuresson, Ang 2003). These findings are partly concurrent with our results and third hypothesis. The sortie part SAR+NVG was accompanied by significantly higher values of muscle activity in the UNE compared to AB-NVG, but not AB+NVG. Given that SAR+NVG and AB+NVG were both flown with NVG, head and trunk positioning may have influenced muscle activity to a greater extent compared to the load of the NVG per se. Our observations underlined that pilots and crew-members are predominantly positioned with flexed and rotated head positions during sorties, which has also been confirmed previously (Butler 1996; Forde, Albert 2011). Unfortunately, our observational data were insufficient to determine possible differences in neck and trunk posture between sortie parts. The subjects in our study are representative of other helicopter pilots and crew-members (Harrison, Neary 2007). MVC for cervical flexion and extension are also representative for crew-members (Harrison, Neary 2009). However, while our values for cervical flexion are comparable for other helicopter pilots, considerably higher values have been reported previously for cervical extension (52 ± 11.4 Nm) (Ang, Linder 2005). Compared to gender and age matched subjects, our MVC values for shoulder elevation and cervical flexion are similar (Essendrop, Schibye 2001) (Faber, Hansen 2006). However, values for cervical extension are below average (Vasavada et al., 2001) (Jordan et al., 1999). A low level of strength in the UNE will result in a large relative load due to the mass of helmet and NVG. This might explain the significant increase in neck pain intensity among helicopter pilots from pre- to post-sortie, but not crew-members that were found significantly stronger in the UNE. An increase in individual capacity by increased UNE strength would potentially reduce overload that may lead to neck pain. Based on this, it would be beneficial for helicopter pilots and crew-members to engage in regular exercise training targeting neck and shoulder muscles. 13
Conclusion The mean muscle activity during sorties was overall ~10% MVE in the UNE and even higher during SAR+NVG. This is considered to be a high sustained activity level, especially in relation to the duration of an average military sortie. Such high workload in combination with a flexed and/or rotated positioning of the head may play a role for the high prevalence of neck pain among this occupational group. The present exposure-assessment suggests that strengthening exercises for the UNE, lowering the relative load during flight, could potentially alleviate the high prevalence of neck pain among this occupational group.
Acknowledgement The authors of this study would like to thank the Royal Danish Air Force for financial support and the helicopter pilots and crew-members from the 722 squadron for participation.
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Figure legends
Figure 1 1A. Standard helicopter helmet model Alpha-221 (1.858 kg in size medium/broad) 1B. Standard helicopter helmet with night configuration. Night Vision Goggles (NVG): A (0.508 kg), bracket: B (0.086 kg), battery pack: C (0.242 kg): counter weights: D (0.151 kg).
Figure 2 Positioning of the neck and trunk during sortie parts.
Figure 3 Mean muscle activity (%MVE) during sortie parts. Trapezius m. (TRA). Upper neck extensors (UNE). Sternocleido-mastoid m. (SCM). *Significant difference between muscle groups. †Significant difference between SAR+NVG and AB-NVG for the UNE.
Figure 4 Cumulative distribution function (CDF), during SAR+NVG. Muscle activity is reported in percentage of maximal voluntary evoked signal (%MVE) as function of sortie time in percent. *Significant difference between the UNE as compared to TRA and SCM during 10%, 50% and 90% of total sortie time.
20
Table legends
Table 1 Participant characteristics, flight and NVG experience. AgustaWestland Merlin helicopter (EH101), Night Vision Goggles (NVG). Values are presented in hours as mean±SD.
Table 2 Intensity of neck, shoulder, and back pain pre- and post-sorties. Intensity of pain is rated on an 11 point numeric box scale (0 = no pain, 10 = worst pain imaginable). Values are presented as mean±SD. *Significant change pre- and post-sortie.
Table 3 Maximal voluntary contraction (MVC) values for pilots and crew-members during cervical flexion, extension, and shoulder elevation. Values are presented in newton (N) and newton-meter (Nm) as mean±SD. *Significant difference between pilots and crew-members.
Table 4 Mean muscle activity (%MVE) for pilots and crew-members during sortie parts. Values are presented as mean±SD with 95% confidence intervals (95% CI).
21
Figure 1 1A
1B C
B D
A A
22
Figure 2
23
Neck and trunk positioning during sorties
Sortie time (%)
120.0 100.0 80.0 60.0 40.0 20.0 0.0
AB-NVG AB+NVG SAR+NVG
24
Figure 3
Mean muscle activity for type of sortie and muscle group
*
14.00
† Muscle actitivy (%MVE)
12.00 10.00 8.00
* *
6.00 4.00 2.00 0.00
TRA
UNE
SCM
25
Figure 4
Cumulative distribution function during search and rescue with night vision goggles 60
Mean muscle activity (%MVE)
50
40
30
*
20
*
10
TRA UNE SCM
*
0 0
10
20
30
40
50
60
70
80
90
100
Sortie time (%)
26
Table 1
Characteristic
Pilots (n=9)
Crew (n=9)
p-value
Age (years)
43.7 ± 4.6
46.7 ± 8.0
p = 0.370
Weight (kg)
77.7 ± 7.1
84.3 ± 9.7
p = 0.137
Height (m)
1.8 ± 0.1
1.8 ± 0.1
p = 0.560
EH101 in total (hours)
865.2 ± 566.2
705.0 ± 332.1
p = 0.475
EH101 last 12 months (hours)
185.2 ± 79.3
110.5 ± 74.1
p = 0.055
EH101 last 30 days (hours)
17.7 ± 8.9
11.7 ± 8.7
p = 0.168
EH101 last 7 days (hours)
5.4 ± 2.3
4.2 ± 2.7
p = 0.311
Other rotary-wing aircraft in total (hours)
1943.1 ± 1163.2 1800.2 ± 1683.7
p = 0.837
Other fixed-wing aircraft in total (hours)
546.4 ± 805.1
0.5 ± 1.1
p = 0.059
NVG in total (hours)
169.6 ± 108.9
154.3 ± 77.8
p = 0.737
NVG past 12 months (hours)
38 ± 19.4
33.8 ± 11.6
p = 0.587
NVG last 3 months (hours)
13.6 ± 5.4
10.3 ± 6.2
p = 0.252
NVG last 7 days (hours)
2.7 ± 2.4
1.9 ± 2.2
p = 0.471
Flight experience
NVG experience
27
28
Table 2
Pilots (n = 9)
Pre-sortie
Post-sortie
Change
p-value
Neck pain
0.7 ± 1.3
1.6 ± 1.9
0.9 ± 1.1
P = 0.028*
Shoulder pain
0.4 ± 1.0
0.6 ± 1.0
0.1 ± 1.5
P = 0.615
Back pain
0.4 ± 1.0
0.2 ± 0.7
- 0.2 ± 0.4
P = 0.157
Neck pain
0.7 ± 0.9
1.8 ± 2.2
1.1 ± 2.1
P = 0.187
Shoulder pain
0.2 ± 0.4
0.6 ± 1.7
0.3 ± 1.4
P = 0.934
Back pain
1.0 ± 1.8
1.4 ± 2.2
0.4 ± 1.3
P = 0.317
Crew (n = 9)
29
Table 3
Maximal Voluntary Contraction (MVC)
Pilots (n=9)
Crew (n=9)
p-value
Cervical flexion (N)
150.5 ± 51.9
216.4 ± 71.2
P = 0.063
Cervical flexion (Nm)
26.0 ± 8.6
37.2 ± 14.9
P = 0.107
Cervical extension (N)
208.8 ± 54.3
281.6 ± 40.7
P = 0.009*
Cervical extension (Nm)
36.8 ± 7.7
49.2 ± 8.7
P = 0.010*
Shoulder elevation (right) (N)
706.5 ± 191.2
881.6 ± 187.8
P = 0.073
Shoulder elevation (right) (Nm)
119.8 ± 38.3
152.5 ± 37.5
P = 0.097
Shoulder elevation (left) (N)
681.3 ± 140.3
836.5 ± 186.5
P = 0.081
Shoulder elevation (left) (Nm)
115.0 ± 25.7
144.6 ± 40.9
P = 0.105
30
Table 4
Sorties
Muscle
Mean ± SD
95% CI
AB-NVG (%MVE)
TRA
3.8 ± 3.1
2.4 - 5.3
AB+NVG (%MVE)
TRA
4.7 ± 2.9
3.4 - 6.0
SAR+NVG (%MVE)
TRA
3.5 ± 3.0
2.1 - 4.9
AB-NVG (%MVE)
UNE
9.0 ± 3.1
7.6 - 10.5
AB+NVG (%MVE)
UNE
10.4 ± 2.9
9.0 - 11.7
SAR+NVG (%MVE)
UNE
11.5 ± 3.0
10.1 - 12.9
AB-NVG (%MVE)
SCM
2.2 ± 3.1
0.8 - 3.7
AB+NVG (%MVE)
SCM
2.8 ± 2.9
1.5 - 4.2
SAR+NVG (%MVE)
SCM
3.2 ± 3.0
1.8 - 4.6
31
Mike Murray Mike Murray received the M.Sc. in Sports and Exercise Science in 2011 from the Department of Sports Science and Clinical Biomechanics, Faculty of Health Sciences, University of Southern Denmark, and is currently enrolled as a PhD student in the department’s research unit for Physical Activity and Health at work. His main area of research is focused on methods of quantifying work exposures and risk factors in the development of musculoskeletal disorders and the use of physical exercise training as prevention and/or rehabilitation of work related musculoskeletal disorders.
Britt Lange
Britt Lange completed a bachelor of medicine (US: MD) at the University of Copenhagen in 1997 and became a specialist in Anesthesia and Intensive Care Medicine in 2005. Currently she is a senior doctor at the university hospital in Odense, Denmark. She earned in 2013 her PhD in human physiology at the University of Southern Denmark. Since 1987 she has served as an officer of the reserve, and from 2000 been assigned to the Royal Danish Air Force (RDAF) focusing on search and rescue missions and fighter pilots working environment. She is a member of the Aerospace Medical Association (AsMA) and among others responsible for the education of doctors in the RDAF. She is still involved in prevention and rehabilitation for musculoskeletal disorders in the RDAF and active as researcher.
Shadi Samir Chreiteh
Shadi Samir Chreiteh received his M.Sc. degree in Biomedical Engineering and Informatics from University of Aalborg, Denmark, in 2009. In 2009-2013 he worked at the Institute of Sports Science and Clinical Biomechanics, University of Sourthern Denmark, Odense, Denmark. He was involved in development and design of clinical experiments, hardware and software focused on kinetics, motor unit activity, motor coordination and muscle fatigue in humans and the relation to musculoskeletal disorders. Currently he is doing a PhD at the Technical University of Denmark. He is developing a new wireless medical device (patch) for monitoring several vital signs. .
Henrik Baare Olsen
Henrik Baare Olsen received his MS degree in Electrical from the Technical University of Denmark in 1993. Form 1995-2009 he has been working at the National Institute of Occupational Health, Denmark. He has developed hardware and software for intra-muscular data acquisition and decomposition techniques. Currently he is working at Institute of Sport Sciences and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark. His research interests are focused on understanding mechanisms of the upper extremity motor control and the risk of developing muscular-skeletal disorders during sport and work.
Bo Riebeling Nørnberg
Bo Riebeling Nørnberg: M.D. 1986 from University of Copenhagen. Finished medical internship 1989. Following further clinical education, he joined the Center of Aviation Medicine at Rigshospitalet, University Hospital, Copenhagen, from 1991 to 2002. From 2002 to 2010 he was Head of Aviation Physiology at Royal Danish Air Force/Chief physician and spent 7 months at United States Air Force School of Aerospace Medicine, San Antonio, Texas, completing Aerospace Officers Physiology Course and Advanced Aerospace Medicine for International Medical Officers. Having further completed Danish Defence Staff Officers Course he has served as medical adviser to the Danish Air Staff since 2010.
Eleanor Boyle
Eleanor Boyle completed her MSc degree in epidemiology and biostatistics from The University of Western Ontario and her PhD in health outcomes from the University of Toronto. She is currently an associate professor at the University of Southern Denmark in the Department of Sports Science and Clinical Biomechanics and an assistant professor at Dalla Lana School of Public Health, University of Toronto in the Epidemiology Division. She has published over 50 articles.
Professor Karen Søgaard Professor Karen Søgaard, received the M.Sc. in Physical Education from the August Krogh Institute, University of Copenhagen and at the same institution she pursued a Ph.D. in Human Physiology in 1994. She spent 8 months as a research fellow at the Department of Kinesiology at Simon Fraser University, Vancouver, Canada in 1995 and in 2001 5 months at Prince of Wales Medical Research Institute, Sydney, Australia. Her main field of competence is human exercise physiology with focus on muscle mechanics, metabolism and fatigue. She is involved in experiments focused on kinetics, motor unit activity, motor coordination and muscle fatigue in humans and the relation to musculoskeletal disorders. She has more than 150 original papers in international peer reviewed scientific journals. Currently, she is professor at Center for Muscle and Joint Health, University of Southern Denmark, Odense, Denmark and elected member of ISEK council since 2010. Recently, she has mainly been involved in large randomized controlled trial interventions focused on physical activity as prevention and rehabilitation for musculoskeletal disorders.
Professor Gisela Sjøgaard Professor Gisela Sjøgaard completed M.S. degrees in mathematics and physical education and earned in 1979 her Ph.D. in muscle physiology at the faculty of natural science and her Dr.Med.Sc. in 1990 at the faculty of medicine at the University of Copenhagen. She was professor and head of the department of physiology at the National Institute of Occupational Health in Denmark, visiting professor at the University of Guelph, Canada and at the University of Michigan, USA, and holds presently a professorship in Sports and Health Sciences at University of Southern Denmark. She has published more than 170 original papers in international peer reviewed scientific journals as well as numerous educational publications. She has participated actively with presentations at more than 200 conferences including more than 100 invited lectures. Her main field of competence is human exercise physiology with focus on muscle mechanics, metabolism and fatigue. Special area of interest is neuromuscular control and muscle biochemistry, as well as their applications to work related musculoskeletal disorders.
S1050-6411(15)00243-6 http://dx.doi.org/10.1016/j.jelekin.2015.12.009 JJEK 1933
To appear in:
Journal of Electromyography and Kinesiology
Received Date: Revised Date: Accepted Date:
3 July 2015 16 December 2015 19 December 2015
Please cite this article as: M. Murray, B. Lange, S.S. Chreiteh, H.B. Olsen, B.R. Nørnberg, E. Boyle, K. Søgaard, G. Sjøgaard, Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight, Journal of Electromyography and Kinesiology (2016), doi: http://dx.doi.org/10.1016/ j.jelekin.2015.12.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Neck and shoulder muscle activity and posture among helicopter pilots and crew-members during military helicopter flight
1
Mike Murray, 2Britt Lange, 1Shadi Samir Chreiteh, 1Henrik Baare Olsen, 3Bo Riebeling Nørnberg, 1,4
1
Eleanor Boyle, 1Karen Søgaard, 1Gisela Sjøgaard,
Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Denmark 2
Department of Anesthesia and Intensive Care Medicine, Odense University Hospital, Denmark 3
Royal Danish Air Force, Air Force Staff, Defence Command Denmark, Denmark 4
Dalla Lana School of Public Health, University of Toronto, Canada
Keywords: electromyography; muscle activity; workload; neck; shoulder; pilots; crew-members; helicopter; pain
Corresponding author: Mike Murray Institute of sport science and clinical biomechanics University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Telephone number: (45) 27 20 50 58 E-mail: [email protected] 1
Abstract Neck pain among helicopter pilots and crew-members is common. This study quantified the physical workload on neck and shoulder muscles using electromyography (EMG) measures during helicopter flight. Nine standardized sorties were performed, encompassing: cruising from location A to location B (AB) and performing search and rescue (SAR). SAR was performed with night vision goggles (NVG), while AB was performed with (AB+NVG) and without NVG (AB-NVG). EMG was recorded for: trapezius (TRA), upper neck extensors (UNE), and sternocleido-mastoid (SCM). Maximal Voluntary Contractions (MVC) were performed for normalization of EMG (MVE). Neck posture of pilots and crew-members was monitored and pain intensity of neck, shoulder, and back was recorded. Mean muscle activity for UNE was ~10% MVE and significantly higher than TRA and SCM, and SCM was significantly lower than TRA. There was no significant difference between AB-NVG and AB+NVG. Muscle activity in the UNE was significantly higher during SAR+NVG than AB-NVG. Sortie time (%) with non-neutral neck posture for SAR+NVG and AB-NVG was: 80.4%, 74.5% (flexed), 55.5%, 47.9% (rotated), 4.5%, 3.7% (lateral flexed). Neck pain intensity increased significantly from pre- (0.7 ± 1.3) to post-sortie (1.6 ± 1.9) for pilots (p = 0.028). If sustained, UNE activity of ~10% MVE is high, and implies a risk for neck disorders.
Introduction Neck pain is common among helicopter pilots and crew-members (Forde et al., 2011; Salmon et al., 2011; Van den Oord et al., 2010). The 1-year prevalence for helicopter pilots has been reported to be 43-48% (Bridger et al., 2002; Van den Oord, De Loose 2010). In another study a 3-month prevalence was reported to be 57% (Ang and Harms-Ringdahl 2006). For crew-members, the 1-year prevalence has been estimated to be 62% (Van den Oord et al., 2014). These prevalence rates are high compared to the general population with values for the 1-year prevalence of 26% (Hoy et al., 2010) and 37% (Fejer et al., 2006). In a survey, approximately 15% of the pilots and 28–56% of the crew-members
2
(depending on helicopter type) had been grounded at least once during their career due to flight-related neck pain (Adam 2004). Approximately 10% of the pilots indicated that on at least one occasion their ability to perform normal flying duties was disrupted or compromised because of in-flight neck pain. Severe neck pain may result in grounding of pilots and crew-members influencing operational capability and flight planning (Harrison et al., 2009) and induce financial costs due to loss of manpower and litigation (Salmon, Harrison 2011). A variety of factors have been found to be associated with flight-related neck pain. Both the weight of the flight helmet and the Night Vision Goggles (NVG) have often been reported to be perceived causes (Harrison et al., 2015; Van den Oord et al., 2012). NVG are mounted on the front of the helmet, which increases the helmet’s weight and further shifts the head’s center of mass forward and upward in relation to the motion axis of the cervical spine (Butler 1996; Thuresson et al., 2005). To counterbalance this, pilots and crew-members may use counterweights (CW) attached to the back of their helmet (Thuresson, Ang 2005). Increased external loading of the cervical spine may play an important role regarding flight-related neck pain. However, NVG also reduces peripheral vision and pilots have been found to spend significantly more time in mid to severe deviations from neutral neck posture with NVG, as compared to without NVG (Forde, Albert 2011). Pilots and crew-members may position themselves differently depending on the type of sortie. Cruising from point A to B (AB) may potentially call for different neck postures as compared with a search and rescue (SAR) sortie. This might affect the physical workload on neck and shoulder muscles. Flightrelated neck pain may therefore be partly explained from a biomechanical perspective with a positive relationship between cervical loading, posture, and neck pain (Thuresson et al., 2003). The aim of this study was to quantify the activity of neck and shoulder muscles among helicopter pilots and crewmembers during operative sorties with and without NVG. Our main hypotheses were: 1) sorties with 3
NVG result in significantly higher levels of muscle activity as compared to sorties without NVG, and 2) SAR sorties with NVG result in the highest level of muscle activity measured. Additional hypotheses tested were: 1) pilots and crew-members have different levels of muscle activity and 2) some muscles have higher activity than others.
Methods Subjects Nine pilots (eight males/one female) and nine crew-members (all males) from the Royal Danish Air Force participated in this study. Inclusion criteria were: 1) profession as either pilot in command (1P), second pilot (2P), first technician (1T), or second technician (2T), 2) having been on active flight duty within the past 30 days, and 3) trained in using NVG-equipment. Exclusion criteria were: 1) intensity of neck, shoulder, or back pain ≥3 at present rated on an 11 point numeric box scale (0 = no pain, 10 = worst pain imaginable), 2) previous trauma to the neck, shoulder, or back. One additional crew-member had been recruited but was excluded from participation due to a pre-sortie intensity of neck pain equal to five. Participants were provided with verbal and written information regarding the study before giving their informed written consent. The study was approved by the local ethical committee of Southern Denmark (S-20120121).
Experimental design and equipment Nine standardized sorties were completed each lasting around 1.5 hours. Each sortie included three parts 1) AB with NVG (AB+NVG), 2) SAR always with NVG (SAR+NVG), and 3) AB without NVG (AB-NVG). Sorties were randomized regarding AB+NVG and AB-NVG being flown first or last, respectively, such that the sequences were either: AB+NVG, SAR+NVG, AB-NVG or AB-NVG, 4
SAR+NVG, AB+NVG. Participants were balanced according to profession (1P, 2P, 1T or 2T). Sorrtie parts represented regular work tasks within the squadron. Sorties were flown with the AgustaWestland EH101 Merlin helicopter (AgustaWestland, EH Industries Ltd, England) and executed from October to November 2012. Participants were equipped with a flight suit, a dry suit, a safety harness, a flight helmet weighing 1.858 kg (Alpha-221, Helmet Integrated Systems Ltd., England), and NVGequipment weighing 0.987 kg (Night Vision & Communications solutions, USA) (Figure 1).
Questionnaire Pilots and crew-members reported flight hours within the past year, one month, and 7 days. NVG hours were reported within the past year, three months, and 7 days. Episodes of neck, shoulder or back pain were reported pre and post sorties using a modified version of the standardized Nordic questionnaire for the analysis of musculoskeletal symptoms (Kuorinka et al., 1987). Questions were: 1) do you have pain in the neck, shoulder or back at present? (yes/no), and 2) if yes, please rate the intensity of pain in your neck, shoulder or back (11 point numeric box scale). The latter question was asked pre as well as post flight.
Instrumentation and procedure Before each sortie, one pilot (1P or 2P) and one crew-member (1T or 2T) were equipped with six electromyographic (EMG) surface electrodes (Ag/AgC1, Ambu Blue Sensor, N-00-S/25, Denmark). EMG-electrodes were positioned 1) bilaterally above the Trapezius muscle (TRA) 20% medial to half the length between the lateral part of the acromion and the seventh cervical vertebra (Holtermann et al., 2009), 2) above the upper neck extensor (UNE) muscles, at the level of the fourth cervical vertebra (Gosselin et al., 2004; Juul-Kristensen et al., 2004), and 3) above the Sternocleido-mastoid muscle 5
(SCM) at the cranial 2/3 of the muscle (Falla et al., 2002). Placement sites were shaved, scrubbed with cleansing gel, and cleaned with 70% alcohol. EMG-electrodes were positioned with an inter-electrode distance of 20 mm and checked for resistance <20 kΩ. The EMG-signal was pre-amplified with a gain of 400 through a built-in amplifier in the wireless EMG transmission probes (EMG probe, Direct Transmission System (DTS), Noraxon Inc, USA). The EMG-signal was low-pass filtered at 500 Hz and high-pass filtered at 10 Hz, and sent to a belt-worn receiver by Wi-Fi (TELEMyo™ 2400T, DTS, Noraxon Inc, USA). The EMG-signal was sampled at 1500 Hz (16 bit A/D converted) using a 12 channel configuration and transferred to a laptop (HP EliteBook 6930p notebook). Event markers were placed within the EMG-signal to enable distinction between sortie parts.
Maximal Voluntary Contractions and electromyography normalization The EMG signal was normalized using isometric maximal voluntary contractions (MVC) previously validated (Essendrop et al., 2001; Faber et al., 2006) and maximally activating the targeted muscles (Blouin et al., 2007; Mayoux-Benhamou et al., 1997; Schomacher et al., 2012). Shoulder elevation was chosen for normalization of TRA, cervical flexion for SCM, and cervical extension for UNE. During shoulder elevation the participants were positioned in a standardized chair with the back straight and no contact to the floor, arms relaxed, and looking straight ahead. Two force dynamometers (Bofors Electronics, Sweden) were placed bilaterally above the shoulders one centimeter medially to the lateral edge of the acromion. Both shoulders were simultaneously elevated during the test (Jensen et al., 1993). During neck flexion, participants were positioned seated with their front against the chair’s backrest and feet on the floor. The head was positioned anatomically neutral and a force transducer was placed above the eyebrows. Participants were instructed to maintain this position and press against the force transducer by leaning forward. During neck extension, the force transducer was placed above the 6
external occipital protuberance. Participants were instructed to maintain an anatomical neutral head position and press against the force transducer by leaning backwards. A minimum of three MVC´s were performed. If the third MVC was >5% higher than either of the previous two, a fourth MVC was performed. If the fourth MVC exceeded the previous three MVC´s by >5% a fifth and final MVC was performed (Essendrop, Schibye 2001). MVC´s were separated by 30 seconds for recovery. EMG-data was stored in Noraxon software (Noraxon Inc, USA), converted in Matlab (MathWorks Inc. USA), and analyzed in Hedera 2.0 (University of Southern Denmark, Denmark). A window size of 1 second with moving steps of 0.1 seconds was used. The EMG root mean square voltage (μV) corresponding to the highest MVC was used as the reference against which EMG-activity was normalized (% of Maximal Voluntary Electrical signal (MVE)).
Observations During sorties, a modified version of the observational method for posture, activity, tools, and handling (PATH) was used to monitor the pilots and crew-members equipped with EMG-sensors (Buchholz et al., 1996). Pilots and crew-members were observed (visual inspection performed by the first author for all observations in this study) every 60 seconds during the three sortie parts according to the orientation of the neck: neutral (<30° flexed/extended), flexed (>30° from neutral), extended (>30° from neutral), rotated (>30° from neutral), and laterally flexed (>30° from neutral), and trunk: neutral (flexion, lateral bending, and twisting < 20°), moderate flexion (>20° to <45° from neutral), severe flexion (>45° from neutral), lateral bending/twisting (flexion < 20°, lateral bending, or twisting >20°), and flexion/twisting (flexion and twisting >20°) (Buchholz, Paquet 1996)
Statistical analysis 7
Differences between pilots and crew-members regarding age, weight, height, flight, and NVG hours, intensity of neck, shoulder and back pain, MVC, and observations were analyzed using a Studentʹs ttest or Wilcoxon rank-sum test depending on normality of data. Pre- and post-sortie changes regarding intensity of neck, shoulder, and back pain was analyzed using a Wilcoxon matched pairs signed-rank test. The level of statistical significance was p <0.05. The hypotheses regarding the muscle activity (%MVE) was analyzed using a multilevel linear regression model. The model included the following parameters: 1) type of sortie part (AB-NVG, AB+NVG, SAR+NVG); 2) profession (pilots (1P and 2P) versus crew-members (1T and 2T)); 3) muscle group (TRA, UNE, SCM); and 4) electrode position (right or left side). A full model was first fitted and parameters with a p-value >0.1 were excluded from the model. Test for interactions between parameters was performed and the final model was fitted with an interaction between muscle group and type of sortie. Normality of the residuals was assessed using a Q-Q plot, and a Shapiro Wilk´s test. Statistical analysis was performed in Stata Statistics/Data Analysis version 13.0 (StataCorp LP, USA). Results are presented as mean±SD if not otherwise specified.
Results Subjects There were no significant differences between pilots and crew-members regarding participant characteristics (Table 1).
Intensity of neck, shoulder, and back pain Five out of eighteen (28%) participants experienced neck pain pre-sortie and 9/18 (50%) post-sortie. For the shoulders, numbers were 4/18 (22%) and 3/18 (17%), and for the back 4/18 (22%) and 4/18 (22%), respectively. There was no significant difference in pain intensity between pilots and crew8
members pre-sortie. Intensity of neck pain was; however, significantly increased from pre- to postsortie for pilots (p = 0.028) but not for crew-members (p = 0.187) (Table 2).
Maximal voluntary contraction MVC for cervical extension was significantly higher (p = 0.009) among crew-members (281.6 ± 40.7 N) than pilots (208.8 ± 54.3 N) (Table 3).
Observations Eighteen observations were conducted: 6 observations (1 pilot and 5 crew-members) during AB-NVG, 6 observations (1 pilot and 5 crew-members) during AB+NVG, and 6 observations (3 pilots and 3 crew-members) during SAR+NVG. Unfortunately, not all observations were performed as planned and inadequate data did not allow for comparison between professions nor between sortie parts. As a result, observations for pilots and crew-members were merged. During sorties, participants positioned themselves within a predominantly flexed and rotated neck position (Figure 2). A trend was observed towards more time (% of sortie time) in non-neutral neck positions during SAR+NVG as compared to AB-NVG: 80.4% vs. 74.5 % (flexed), 55.5% vs. 47.9% (rotated), 4.5% vs. 3.7 % (lateral flexed) and 12.4% vs. 17.4% (neutral).
Muscle activity Sampling time was throughout the flight, but duration analyzed for each sortie parts AB-NVG, AB+NVG and SAR+NVG was: 16.9 ± 5.1, 22.7 ± 10.8 and 21.3 ± 11.1 minutes. EMG-measurements were terminated ahead of time during two sorties due to incoming SAR missions. Sortie parts analyzed included eight AB-NVG (7 pilots and 8 crew-members), nine AB+NVG (9 pilots and 9 crew-members) 9
and eight SAR+NVG (8 pilots and 8 crew-members). Parameters within the multilevel linear regression model were tested for relevance, and profession (pilots (1P and 2P) versus crew-members (1T and 2T)) was excluded (p = 0.543). No significant evidence was found against the assumption of an interaction between muscle group and type of sortie (p = 0.080). Mean muscle activity in the UNE was significantly higher (p = <0.001) as compared to TRA, and muscle activity in SCM was significantly lower as compared to TRA (p = 0.045) when adjusted for type of sortie, electrode position, and muscle group (Table 4). No significant differences regarding muscle activity in TRA, UNE or SCM were found between sortie parts AB-NVG and AB+NVG. Muscle activity in the UNE was, however, significantly higher during SAR+NVG as compared to AB-NVG (p = 0.020) (Figure 3). Muscle activity was ~10% MVE or above in the UNE during 50% of SAR+NVG as illustrated by the cumulative distribution function (CDF) (Figure 4). The UNE were also significantly higher than TRA and SCM during 10%, 50%, and 90% of SAR+NVG.
Discussion The main finding of this study was a sustained muscle activity of approximately 10% MVE in the UNE during sorties. This sustained muscle activity may cause ischemic muscular pain, and localized muscle fatigue has been shown to occur in sustained contractions of as low as 5% MVC (Sjogaard et al., 1986). Low-level static exertions have also been reported to be a risk factor for the development of cumulative trauma disorders and repetitive strain injuries (Sjogaard and Jensen 2006). Our findings should be interpreted relative to the duration of an average military sortie, which has been reported to be approximately 2 hours long with the average of the longest sorties reaching up to 3.5 hours (Harrison et al., 2007). This duration is considerably longer compared to the sortie parts in our study, which each were approximately 20 minutes long. We found a sustained muscle activity of 10% 10
MVE in the UNE which is considered high, especially in relation to the duration of an average military sortie (Sjogaard and Jensen 2006). To our knowledge, there has not been a published study that has quantified the physical workload on neck and shoulder muscles using EMG measures in helicopter pilots and crew-members during real-flight scenarios. In-flight EMG-measurements in fighter pilots may be comparable, if measured during a normal gravitational level (+1G z). Green et al. reported muscle activity within the cervical part of the erector spinae muscle to be approximately ~10% MVE (Green and Brown 2004). Similarly, Hamalainen et al. addressed upper neck muscle activity among fighter pilots and found a slightly lower level of muscle activity equal to approximately 6.4% MVE (Hamalainen and Vanharanta 1992). EMG-recordings were measured at +1Gz in both studies, with neutral head positioning wearing a helmet, oxygen mask, and anti-gravitational suite (Hamalainen and Vanharanta 1992). Even though these results are similar to ours, regarding the magnitude of muscle activity, studies conducted in helicopter pilots within laboratory settings may be more appropriate as a reference, in order to limit differences regarding flight environments, ergonomics and equipments. Thuresson et al. previously found a level of muscle activity between 7% and 10% MVE in the left upper neck muscles and bilaterally in the lower neck muscles during ipsilateral neck flexion and rotation. A level of approximate 5% MVE was also found during trunk inclination and ipsilateral rotation (Thuresson, Ang 2003). Overall, these results are similar to our findings. It has been shown that if a contraction is to be maintained for one hour, it may have to be as low as 8% MVC with an acceptable level for continuous contractions equal to a few percent of MVC (Bjorksten and Jonsson 1977). A suggested level of acceptance of maximal 2-5% MVC has also been suggested (Sjogaard and Jensen 2006). The association between low-level static muscle contractions and musculoskeletal disorders is; however, not fully identified (Sjogaard and Jensen 2006), and musculoskeletal disorders have been found to be high in professions with exposure within the recommended levels of static 11
muscle contraction. As a consequence, it has been suggested that static loads are not acceptable at all, if sustained frequently or over long durations of time (Bjorksten and Jonsson 1977; Sjogaard and Jensen 2006). Excessive loading of the UNE, as illustrated by our findings could potentially lead to musculoskeletal pain in the neck (Ang et al., 2005). We found no significant difference in muscle activity between sortie parts AB-NVG and AB+NVG. The increase in helmet mass due to NVG was approximately 50%. However, the change in muscle activity was approximately 1% MVE in the TRA, UNE, and SCM. This might be due to a non-linear relationship between force and muscle activity. Schuldt et al. previously demonstrated that an exerted force of up to 40% MVC was produced by a level of muscle activity of 10 – 15% MVE (Schuldt and Harms-Ringdahl 1988). These findings are further supported by Thuresson et al. (Thuresson, Ang 2003), and are probably resulting from a large synergy in neck muscles sharing the load of neck extension. The load induced by NVG equipment accounts for a small relative load in relation to the maximal force demonstrated during the normalization procedure using MVCs. A non-linear relationship between force and muscle activity may therefore, to some extent, explain the relatively small change in muscle activity between sortie parts AB-NVG and AB+NVG. Knight et al. studied changes in neck muscle activity due to added head load and static head positions, and found that changes in head position significantly influenced the magnitude of cervical loading (Knight and Baber 2004). Thuresson et al. examined neuromuscular activity in the upper neck, lower neck, and upper Trapezius muscle in relation to different trunk and head positions, taking head worn equipment into account (Thuresson, Ang 2003). When assessing muscle activity of all positions and sides, Thuresson et al. found significantly higher levels of muscle activity in the upper neck muscles using a helmet and NVG or helmet, NVG, and CW as compared to a helmet alone. However, when assessing the activity levels for every position and side separately no significant difference in muscle activity level between 12
any of the head-worn items of equipment was found (Thuresson, Ang 2003). Head and trunk positions may therefore have more influence on muscle activity than the load of head worn equipment alone (Thuresson, Ang 2003). These findings are partly concurrent with our results and third hypothesis. The sortie part SAR+NVG was accompanied by significantly higher values of muscle activity in the UNE compared to AB-NVG, but not AB+NVG. Given that SAR+NVG and AB+NVG were both flown with NVG, head and trunk positioning may have influenced muscle activity to a greater extent compared to the load of the NVG per se. Our observations underlined that pilots and crew-members are predominantly positioned with flexed and rotated head positions during sorties, which has also been confirmed previously (Butler 1996; Forde, Albert 2011). Unfortunately, our observational data were insufficient to determine possible differences in neck and trunk posture between sortie parts. The subjects in our study are representative of other helicopter pilots and crew-members (Harrison, Neary 2007). MVC for cervical flexion and extension are also representative for crew-members (Harrison, Neary 2009). However, while our values for cervical flexion are comparable for other helicopter pilots, considerably higher values have been reported previously for cervical extension (52 ± 11.4 Nm) (Ang, Linder 2005). Compared to gender and age matched subjects, our MVC values for shoulder elevation and cervical flexion are similar (Essendrop, Schibye 2001) (Faber, Hansen 2006). However, values for cervical extension are below average (Vasavada et al., 2001) (Jordan et al., 1999). A low level of strength in the UNE will result in a large relative load due to the mass of helmet and NVG. This might explain the significant increase in neck pain intensity among helicopter pilots from pre- to post-sortie, but not crew-members that were found significantly stronger in the UNE. An increase in individual capacity by increased UNE strength would potentially reduce overload that may lead to neck pain. Based on this, it would be beneficial for helicopter pilots and crew-members to engage in regular exercise training targeting neck and shoulder muscles. 13
Conclusion The mean muscle activity during sorties was overall ~10% MVE in the UNE and even higher during SAR+NVG. This is considered to be a high sustained activity level, especially in relation to the duration of an average military sortie. Such high workload in combination with a flexed and/or rotated positioning of the head may play a role for the high prevalence of neck pain among this occupational group. The present exposure-assessment suggests that strengthening exercises for the UNE, lowering the relative load during flight, could potentially alleviate the high prevalence of neck pain among this occupational group.
Acknowledgement The authors of this study would like to thank the Royal Danish Air Force for financial support and the helicopter pilots and crew-members from the 722 squadron for participation.
14
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19
Figure legends
Figure 1 1A. Standard helicopter helmet model Alpha-221 (1.858 kg in size medium/broad) 1B. Standard helicopter helmet with night configuration. Night Vision Goggles (NVG): A (0.508 kg), bracket: B (0.086 kg), battery pack: C (0.242 kg): counter weights: D (0.151 kg).
Figure 2 Positioning of the neck and trunk during sortie parts.
Figure 3 Mean muscle activity (%MVE) during sortie parts. Trapezius m. (TRA). Upper neck extensors (UNE). Sternocleido-mastoid m. (SCM). *Significant difference between muscle groups. †Significant difference between SAR+NVG and AB-NVG for the UNE.
Figure 4 Cumulative distribution function (CDF), during SAR+NVG. Muscle activity is reported in percentage of maximal voluntary evoked signal (%MVE) as function of sortie time in percent. *Significant difference between the UNE as compared to TRA and SCM during 10%, 50% and 90% of total sortie time.
20
Table legends
Table 1 Participant characteristics, flight and NVG experience. AgustaWestland Merlin helicopter (EH101), Night Vision Goggles (NVG). Values are presented in hours as mean±SD.
Table 2 Intensity of neck, shoulder, and back pain pre- and post-sorties. Intensity of pain is rated on an 11 point numeric box scale (0 = no pain, 10 = worst pain imaginable). Values are presented as mean±SD. *Significant change pre- and post-sortie.
Table 3 Maximal voluntary contraction (MVC) values for pilots and crew-members during cervical flexion, extension, and shoulder elevation. Values are presented in newton (N) and newton-meter (Nm) as mean±SD. *Significant difference between pilots and crew-members.
Table 4 Mean muscle activity (%MVE) for pilots and crew-members during sortie parts. Values are presented as mean±SD with 95% confidence intervals (95% CI).
21
Figure 1 1A
1B C
B D
A A
22
Figure 2
23
Neck and trunk positioning during sorties
Sortie time (%)
120.0 100.0 80.0 60.0 40.0 20.0 0.0
AB-NVG AB+NVG SAR+NVG
24
Figure 3
Mean muscle activity for type of sortie and muscle group
*
14.00
† Muscle actitivy (%MVE)
12.00 10.00 8.00
* *
6.00 4.00 2.00 0.00
TRA
UNE
SCM
25
Figure 4
Cumulative distribution function during search and rescue with night vision goggles 60
Mean muscle activity (%MVE)
50
40
30
*
20
*
10
TRA UNE SCM
*
0 0
10
20
30
40
50
60
70
80
90
100
Sortie time (%)
26
Table 1
Characteristic
Pilots (n=9)
Crew (n=9)
p-value
Age (years)
43.7 ± 4.6
46.7 ± 8.0
p = 0.370
Weight (kg)
77.7 ± 7.1
84.3 ± 9.7
p = 0.137
Height (m)
1.8 ± 0.1
1.8 ± 0.1
p = 0.560
EH101 in total (hours)
865.2 ± 566.2
705.0 ± 332.1
p = 0.475
EH101 last 12 months (hours)
185.2 ± 79.3
110.5 ± 74.1
p = 0.055
EH101 last 30 days (hours)
17.7 ± 8.9
11.7 ± 8.7
p = 0.168
EH101 last 7 days (hours)
5.4 ± 2.3
4.2 ± 2.7
p = 0.311
Other rotary-wing aircraft in total (hours)
1943.1 ± 1163.2 1800.2 ± 1683.7
p = 0.837
Other fixed-wing aircraft in total (hours)
546.4 ± 805.1
0.5 ± 1.1
p = 0.059
NVG in total (hours)
169.6 ± 108.9
154.3 ± 77.8
p = 0.737
NVG past 12 months (hours)
38 ± 19.4
33.8 ± 11.6
p = 0.587
NVG last 3 months (hours)
13.6 ± 5.4
10.3 ± 6.2
p = 0.252
NVG last 7 days (hours)
2.7 ± 2.4
1.9 ± 2.2
p = 0.471
Flight experience
NVG experience
27
28
Table 2
Pilots (n = 9)
Pre-sortie
Post-sortie
Change
p-value
Neck pain
0.7 ± 1.3
1.6 ± 1.9
0.9 ± 1.1
P = 0.028*
Shoulder pain
0.4 ± 1.0
0.6 ± 1.0
0.1 ± 1.5
P = 0.615
Back pain
0.4 ± 1.0
0.2 ± 0.7
- 0.2 ± 0.4
P = 0.157
Neck pain
0.7 ± 0.9
1.8 ± 2.2
1.1 ± 2.1
P = 0.187
Shoulder pain
0.2 ± 0.4
0.6 ± 1.7
0.3 ± 1.4
P = 0.934
Back pain
1.0 ± 1.8
1.4 ± 2.2
0.4 ± 1.3
P = 0.317
Crew (n = 9)
29
Table 3
Maximal Voluntary Contraction (MVC)
Pilots (n=9)
Crew (n=9)
p-value
Cervical flexion (N)
150.5 ± 51.9
216.4 ± 71.2
P = 0.063
Cervical flexion (Nm)
26.0 ± 8.6
37.2 ± 14.9
P = 0.107
Cervical extension (N)
208.8 ± 54.3
281.6 ± 40.7
P = 0.009*
Cervical extension (Nm)
36.8 ± 7.7
49.2 ± 8.7
P = 0.010*
Shoulder elevation (right) (N)
706.5 ± 191.2
881.6 ± 187.8
P = 0.073
Shoulder elevation (right) (Nm)
119.8 ± 38.3
152.5 ± 37.5
P = 0.097
Shoulder elevation (left) (N)
681.3 ± 140.3
836.5 ± 186.5
P = 0.081
Shoulder elevation (left) (Nm)
115.0 ± 25.7
144.6 ± 40.9
P = 0.105
30
Table 4
Sorties
Muscle
Mean ± SD
95% CI
AB-NVG (%MVE)
TRA
3.8 ± 3.1
2.4 - 5.3
AB+NVG (%MVE)
TRA
4.7 ± 2.9
3.4 - 6.0
SAR+NVG (%MVE)
TRA
3.5 ± 3.0
2.1 - 4.9
AB-NVG (%MVE)
UNE
9.0 ± 3.1
7.6 - 10.5
AB+NVG (%MVE)
UNE
10.4 ± 2.9
9.0 - 11.7
SAR+NVG (%MVE)
UNE
11.5 ± 3.0
10.1 - 12.9
AB-NVG (%MVE)
SCM
2.2 ± 3.1
0.8 - 3.7
AB+NVG (%MVE)
SCM
2.8 ± 2.9
1.5 - 4.2
SAR+NVG (%MVE)
SCM
3.2 ± 3.0
1.8 - 4.6
31
Mike Murray Mike Murray received the M.Sc. in Sports and Exercise Science in 2011 from the Department of Sports Science and Clinical Biomechanics, Faculty of Health Sciences, University of Southern Denmark, and is currently enrolled as a PhD student in the department’s research unit for Physical Activity and Health at work. His main area of research is focused on methods of quantifying work exposures and risk factors in the development of musculoskeletal disorders and the use of physical exercise training as prevention and/or rehabilitation of work related musculoskeletal disorders.
Britt Lange
Britt Lange completed a bachelor of medicine (US: MD) at the University of Copenhagen in 1997 and became a specialist in Anesthesia and Intensive Care Medicine in 2005. Currently she is a senior doctor at the university hospital in Odense, Denmark. She earned in 2013 her PhD in human physiology at the University of Southern Denmark. Since 1987 she has served as an officer of the reserve, and from 2000 been assigned to the Royal Danish Air Force (RDAF) focusing on search and rescue missions and fighter pilots working environment. She is a member of the Aerospace Medical Association (AsMA) and among others responsible for the education of doctors in the RDAF. She is still involved in prevention and rehabilitation for musculoskeletal disorders in the RDAF and active as researcher.
Shadi Samir Chreiteh
Shadi Samir Chreiteh received his M.Sc. degree in Biomedical Engineering and Informatics from University of Aalborg, Denmark, in 2009. In 2009-2013 he worked at the Institute of Sports Science and Clinical Biomechanics, University of Sourthern Denmark, Odense, Denmark. He was involved in development and design of clinical experiments, hardware and software focused on kinetics, motor unit activity, motor coordination and muscle fatigue in humans and the relation to musculoskeletal disorders. Currently he is doing a PhD at the Technical University of Denmark. He is developing a new wireless medical device (patch) for monitoring several vital signs. .
Henrik Baare Olsen
Henrik Baare Olsen received his MS degree in Electrical from the Technical University of Denmark in 1993. Form 1995-2009 he has been working at the National Institute of Occupational Health, Denmark. He has developed hardware and software for intra-muscular data acquisition and decomposition techniques. Currently he is working at Institute of Sport Sciences and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark. His research interests are focused on understanding mechanisms of the upper extremity motor control and the risk of developing muscular-skeletal disorders during sport and work.
Bo Riebeling Nørnberg
Bo Riebeling Nørnberg: M.D. 1986 from University of Copenhagen. Finished medical internship 1989. Following further clinical education, he joined the Center of Aviation Medicine at Rigshospitalet, University Hospital, Copenhagen, from 1991 to 2002. From 2002 to 2010 he was Head of Aviation Physiology at Royal Danish Air Force/Chief physician and spent 7 months at United States Air Force School of Aerospace Medicine, San Antonio, Texas, completing Aerospace Officers Physiology Course and Advanced Aerospace Medicine for International Medical Officers. Having further completed Danish Defence Staff Officers Course he has served as medical adviser to the Danish Air Staff since 2010.
Eleanor Boyle
Eleanor Boyle completed her MSc degree in epidemiology and biostatistics from The University of Western Ontario and her PhD in health outcomes from the University of Toronto. She is currently an associate professor at the University of Southern Denmark in the Department of Sports Science and Clinical Biomechanics and an assistant professor at Dalla Lana School of Public Health, University of Toronto in the Epidemiology Division. She has published over 50 articles.
Professor Karen Søgaard Professor Karen Søgaard, received the M.Sc. in Physical Education from the August Krogh Institute, University of Copenhagen and at the same institution she pursued a Ph.D. in Human Physiology in 1994. She spent 8 months as a research fellow at the Department of Kinesiology at Simon Fraser University, Vancouver, Canada in 1995 and in 2001 5 months at Prince of Wales Medical Research Institute, Sydney, Australia. Her main field of competence is human exercise physiology with focus on muscle mechanics, metabolism and fatigue. She is involved in experiments focused on kinetics, motor unit activity, motor coordination and muscle fatigue in humans and the relation to musculoskeletal disorders. She has more than 150 original papers in international peer reviewed scientific journals. Currently, she is professor at Center for Muscle and Joint Health, University of Southern Denmark, Odense, Denmark and elected member of ISEK council since 2010. Recently, she has mainly been involved in large randomized controlled trial interventions focused on physical activity as prevention and rehabilitation for musculoskeletal disorders.
Professor Gisela Sjøgaard Professor Gisela Sjøgaard completed M.S. degrees in mathematics and physical education and earned in 1979 her Ph.D. in muscle physiology at the faculty of natural science and her Dr.Med.Sc. in 1990 at the faculty of medicine at the University of Copenhagen. She was professor and head of the department of physiology at the National Institute of Occupational Health in Denmark, visiting professor at the University of Guelph, Canada and at the University of Michigan, USA, and holds presently a professorship in Sports and Health Sciences at University of Southern Denmark. She has published more than 170 original papers in international peer reviewed scientific journals as well as numerous educational publications. She has participated actively with presentations at more than 200 conferences including more than 100 invited lectures. Her main field of competence is human exercise physiology with focus on muscle mechanics, metabolism and fatigue. Special area of interest is neuromuscular control and muscle biochemistry, as well as their applications to work related musculoskeletal disorders.