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Back and neck extensor loading and back pain provocation in urban bus drivers with and without low back pain
Pathophysiology 12 (2005) 249–255
Back and neck extensor loading and back pain provocation in urban bus drivers with and without low back pain Ville Leinonen a,b,∗ , Markku Kankaanp¨aa¨ a,b , Heikki Vanharanta c , Olavi Airaksinen b , Osmo H¨anninen a a b
Department of Physiology, Kuopio University Hospital, P.O. Box 1777, FIN-70211 Kuopio, Finland Department of Physical Medicine and Rehabilitation, Kuopio University Hospital, Kuopio, Finland c Department of Physical Medicine and Rehabilitation, Oulu University Hospital, Oulu, Finland
Received 27 December 2004; received in revised form 8 September 2005; accepted 9 September 2005
Abstract This study assessed low back and trapezius muscle activity in bus drivers, with or without recurrent low back pain (LBP), during the long term driving. In addition, low back and neck–shoulder pain intensities and fatigue were measured and the effect of low back support was observed. Also the possible source of LBP was attempted to assess by vibration pain provocation test and lumbar MRI. Forty bus drivers (recurrent LBP n = 25) participated in this study. Low back and neck–shoulder pain and subjective fatigue intensity was assessed by visual analogue scales (VAS) before and after driving. Lumbar paraspinal and trapezius muscle activation during driving was measured by surface EMG. Vibration pain provocation test was applied for all subjects. Average paraspinal myoelectric activity during driving was approximately 1% of MVC in both groups. Average trapezius myoelectric activity during driving was from 2 to 4% of MVC. Trapezius muscle activity was higher in back healthy drivers than in those with LBP. The low back support had no effect either on paraspinal or trapezius EMG activity. Low back and neck–shoulder fatigue increased during driving in both groups especially in those subjects with positive vibration pain provocation. The neck–shoulder pain and fatigue were more severe in drivers suffering from LBP. Low back support had no effect on low back and neck–shoulder subjective fatigue and neck–shoulder pain but tended to limit the LBP increase during driving. Paraspinal muscle loading in urban bus drivers was very limited and either LBP or ergonomic low back support had no effect on it. Trapezius muscle seemed to be less active in drivers suffering from recurrent LBP. Internal disc disruptions may expose to pain and fatigability during driving. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: EMG; Bus driving; LBP; Vibration pain provocation
1. Introduction In past literature, problems associated with intervertebral discs and sick leaves caused by them are the most common in persons whose work requires long term sitting [1–4]. It has been estimated that men who spend over half of their working time by driving a motor vehicle have about three times higher probability to get lumbar disc herniation [1,2]. It has also been found that truck drivers have plenty of early degenerative changes of the spine [5] but according to a recent ∗
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0928-4680/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.pathophys.2005.09.004
twin-study occupational driving was not associated with disc degeneration [6]. Still, the driving may exacerbate the symptoms. An epidemiological survey [7] has been indicated that driving by any vehicle was more frequent in low back pain (LBP) patients. Back problems are more common in experienced bus drivers than in control subjects [8,9] and problems increase with age [10,11]. Up to 83% of bus drivers have reported low back problems [10,12]. However, the definite numbers of current back trouble prevalence in bus drivers do not exist and since the early findings, it is supposed that the work ergonomic and comfort has been emphasised and notably improved [13].
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Subjects suffering LBP often complain their inability to sit, and their pain intensity is associated with their ability to change their position during sitting [14,15]. Lumbar support with continuous passive motion improves comfort in simulated automobile seating both in healthy and LBP subjects [16]. Even people without clinical history of back trouble report pain with prolonged static sitting [17]. However, the sitting at work as a risk factor for LBP has been challenged [18] but e.g. driving with the exposure to whole body vibration is not comparable with the static sitting. Whole body vibration especially when driving [7,9,19] and unexpected movements or loads expose to low back pain [20–22]. Work requiring repeated forward bending and lifting expose to LBP [14,23]. Forward bending and lifting may provoke pain for LBP patients [4] and form a risk factor for acute disc herniation [24]. Disc pressure and myoelectric activity decrease when the backrest inclination is increased and the low back support is used [25]. Annular tears due to prolonged simulated sitting postures and cyclic compressive loading has been documented in young calf spines [26] and indicated a potential source of LBP in humans [27]. The low back injury process is not only associated with very high loads but even more often with lower repeated or continuous loads. Therefore it is important to study not only fast and high load but also longer lasting tissue loading [28]. The aims of the present study were to: 1. Compare low back and neck–shoulder muscle activities, subjectively assessed low back and neck–shoulder pain and fatigue intensities of bus drivers with recurrent LBP and healthy controls during the long term driving. 2. Assess the effect of ergonomic low back support on muscle activity, pain and fatigue during driving. 3. Observe the possible source of pain by vibration pain provocation test and lumbar MRI.
Table 1 Characteristics of the bus drivers with and without low back trouble (age, height, weight, MVC and Oswestry disability index (OSW)) participating in the study Healthy (n = 15) Age Height Weight BMI MVC (Nm) MVC (kg) OSW (0–100) Driving experience (years)
39.7 172.7 75.7 25.3 87.9 1.2 2.4 18.2
± ± ± ± ± ± ± ±
11.6 7.0 16.1 4.5 15.8 0.3 3.5 14.2
Low back trouble (n = 25) 41.4 176.4 87.7 28.2 98.0 1.1 11.6 18.3
± ± ± ± ± ± ± ±
11.2 6.0 11.8 3.5 19.9 0.2 7.6 12.8
Values are expressed as mean ± S.D. NS, non-significant; * P < 0.05, ‡ P < 0.001.
Sig. NS NS * *
NS NS ‡
NS † P < 0.01,
port was randomised. All driving sessions were performed by two similar busses (Kabus) with similar seats and automatic transmission. The low back support (Ikonen Matti Oy, Imatra, Finland) was ergonomically shaped, air filled and tied to the seat by sticker belt. The thickness of the support was freely adjustable by pump and valve. All drivers were allowed to choose freely the thickness of the support before driving. Questionnaires on low back and neck–shoulder pain intensities and subjective perception of fatigue were assessed by visual analogue scales (100 mm VAS [29]). Every subject evaluated his/her back and neck–shoulder pain, and fatigue intensities on a 100-mm where 0 denoted no pain or fatigue at all, and 100 the worst possible pain or fatigue. The result was given in millimetres. Low back pain induced disability was assessed by Oswestry disability index (ODI 0–100 [30]). The additional questionnaires for pain and fatigue intensities were filled before and after every driving session. Questionnaire included also pain during driving and a question whether the drivers benefited of the low back support or not. 2.2. Electromyography
2. Subjects and methods 2.1. Subjects Forty voluntary urban bus drivers, 25 (22 men and three women) with recurrent low back pain and 15 (14 men and one women) with healthy back participated in the study (Table 1). All subjects were bus drivers in local city-traffic. Daily working time lasted from 6 to 10 h including preparation of driving, approximately 5 h of driving with three breaks (lunch break and two coffee breaks). In addition to driving, drivers took fares from passengers and lifted luggage and baby carriages when needed. Before initiation of the study subjects signed an informed consent. Subjects were divided into back healthy and low back pain group according to their own assessments and presence of pain episodes during last 12 months. All subjects were studied twice and the use of ergonomic low back sup-
The averaged surface EMG was recorded bilaterally over the paraspinal muscles at L4–L5 level, and over the trapezius muscle. The skin was cleaned with an alcohol swap before pairs of disposable Ag/AgCl surface electrodes (Medicotest, Olstykke, Denmark) were attached over the investigated muscles. At the L4–L5 level of paraspinal muscles, electrodes were placed at a small angle from the sagittal plane, as has been previously suggested [31], 2 cm laterally from the spinal processes and with a 2 cm distance between electrodes. Trapezius muscle electrodes were placed at the upper part of the muscle at the C7 level. The ground electrode was placed on the skin approximately 9 cm laterally from each bipolar electrode pair. Portable ME 3000p (Mega Electronics Ltd., Kuopio, Finland) EMG system was used to record the bipolar surface EMG with four channels. The cables with preamplifiers were used to ensure good signal quality. A pair of 10-cm long
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cables connected the EMG recording electrodes to the preamplifier in each EMG channel. The preamplifier was secured on place by attaching it to the corresponding reference electrode. A single 2.5-m long cable connected the preamplifier to the amplifier box. The EMG signal was recorded at the sampling rate of 1000 Hz and (analogically frequency band-pass filtered (effective band width 7–500 Hz), amplified (differential amplifier, CMRR > 110 dB, gain 1000, noise < 1 V)), analoque-to-digital converted (12-bit). The raw EMG signal was converted into averaged EMG in 0.1 s. data epoch, and stored in a PC for later analysis. The maximum voluntary contraction and EMG amplitude of back and trapezius muscles were measured for every subject before driving. The EMG of investigated muscles was continuously recorded when driving and stored in PC for later analysis. The total length of each measurement was approximately 7 h, which included a half hour of preparation, 5 h driving with two short (5–15 min) breaks and from half to 1 h lunch break and another driving session of 1 h. The average muscle activity measurements during driving were taken from the 3 h of driving after preparation. The muscle activity was assessed as a percentage of MVC. 2.3. Pain provocation test Pain provocation test was made for 37 drivers, three drivers were excluded, one due to accident, one by work disability, other than back problem and one by unemployment in the bus company. Pain provocation test was made by using a modified electric toothbrush (Braun D-3, Braun AG, Germany), with a stable, blunt head 1 cm2 in side as described first by Yrj¨am¨a and Vanharanta [32] and have been proven to be sensitive and specific for internal disc disruptions. The frequency of the vibrator device was 42–50 Hz. Four lumbar (L2–L5) spinal processes were palpated and compressed with the blunt head of the device one by one at a right angle of the skin with the force of 1–3 kg/cm2 . Each spinal process was vibrated 2–5 s. The type of pain provoked was recorded using the model as described for discography [33]. The result of the vibration pain provocation test was classified as painless, exact reproduction of pain, similar pain or dissimilar pain as patients experience their back pain. There is strong evidence for internal disc disruption when the provoked pain is similar to the experienced LBP and the pain felt when compressing increased when vibrated [34]. 2.4. Magnetic resonance imaging (MRI) Fifteen drivers with LBP were examined by MRI of the lumbar spine (L1–S2) in sagittal plane by 1 T imager (Siemens Magnetom Impact Expert) with TSE T1, T2 and STIR. techniques and in axial plane (L3–S1) with T2 technique. The slice thickness was 4 mm, the field of view in sagittal 140 mm × 280 mm (126 × 256 pixels) and axial 188 mm × 250 mm (223 × 512 pixels) images. The earliest sign of disc degeneration was stated as decrease in signal
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density. The annular rupture was stated to be a peripheral light signal. 2.5. Statistical analysis The effect of LBP and low back support on low back and trapezius muscle activities and comparison of the groups were measured by Student’s t-test procedures. The effect of driving, LBP, low back support and pain provocation results on pain and fatigue intensities were evaluated by repeated measures ANOVA. The logarithmic transformation of VAS values was made because of non-normal distribution. Statistical significance was set as P < 0.05.
3. Results The study groups were similar as regard to age, height, and maximum voluntary contraction but the drivers with LBP were heavier (Table 1). Average lumbar paraspinal myoelectric activity during driving was approximately 1% of MVC in both groups (Table 2). The ergonomic low back support had no effect on paraspinal muscle activity in either healthy or LBP groups. Average trapezius myoelectric activity during driving was from 2 to 4% of MVC (Table 3). There was a side difference in trapezius muscle activity i.e. left side was loaded more than right side. There is evidence for higher trapezius muscle activity in non-LBP drivers compared with LBP drivers. The ergonomic low back support had no effect on trapezius muscle activity. Table 2 Low back muscle activity of healthy drivers and those having low back trouble (rms EMG) with and without low back support (percent of MVC) during urban bus driving Healthy (n = 15)
Low back trouble (n = 25)
Left No support Support
1.1 ± 0.8 1.3 ± 1.1
0.9 ± 0.6 0.9 ± 0.7
Right No support Support
0.9 ± 0.4 1.3 ± 1.2
1.2 ± 1.2 0.9 ± 0.7
Values are expressed as mean ± S.D. Table 3 Trapezius muscle activity of (rms EMG) with and without low back support (percent of MVC) during urban bus driving Healthy
Low back trouble
Left No support Support
4.8 ± 3.3 4.2 ± 2.6
3.1 ± 1.8* 3.4 ± 1.8
Right No support Support
3.3 ± 2.0 3.5 ± 2.0
2.3 ± 1.5 2.5 ± 1.4
Values are expressed as mean ± S.D. * P < 0.05.
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Table 4 Low back and trapezius muscle activity (EMG) of urban bus drivers during driving vs. break (% of MVC, n = 40) Driving Low back Left No support Support Right No support Support Trapezius Left No support Support Right No support Support
Break
0.9 ± 0.9 0.9 ± 1.0
2.2 ± 1.4‡ 2.5 ± 1.6‡
1.0 ± 0.9 1.0 ± 0.9
2.3 ± 1.4‡ 2.3 ± 1.2‡
3.6 ± 2.3 3.8 ± 1.9
5.2 ± 3.1‡ 4.8 ± 2.3‡
2.6 ± 1.7 3.0 ± 1.6
4.5 ± 2.2‡ 4.6 ± 2.1‡
Values are expressed as mean ± S.D. ‡ P < 0.001.
Both paraspinal and trapezius myoelectric activity was higher during breaks than during driving (Table 4). Low back and neck–shoulder fatigue increased significantly during driving in both groups (Tables 5 and 6). The neck–shoulder pain and fatigue were more severe in drivers suffering from low back trouble. Ergonomic low back support had no effect on low back and neck–shoulder fatigue and neck–shoulder pain. According to multiple-choice questionnaire 26 (65%) of all subjects, 9 (56%) from back pain group and 17 (71%) from back healthy drivers subjectively benefited for ergonomic low back support.
Table 5 Low back and neck–shoulder pain (VAS) in urban bus drivers before and after driving (low back support vs. no support) Baseline Low back pain (0–100 mm) Healthy (n = 15) No support Support Low back trouble (n = 25) No support Support Pooled (n = 40) No support Support
After driving
4.5 ± 4.0 2.4 ± 1.4
8.3 ± 8.8 5.1 ± 4.3*
19.6 ± 26.6 21.5 ± 23.9
23.8 ± 25.2 22.0 ± 22.2
14.0 ± 22.3 14.4 ± 21.0
18.0 ± 21.8* 15.7 ± 19.5
Neck–shoulder pain (0–100 mm) Healthy (n = 15) No support 10.5 Support 8.3 Low back trouble (n = 25) No support 18.5 Support 16.4 Pooled (n = 40) No support 15.5 Support 13.4 Values are expressed as mean ± S.D. * P < 0.05.
Table 6 Low back and neck–shoulder fatigue (VAS) in urban bus drivers before and after driving (low back support vs. no support)
± 14.8 ± 12.8
12.3 ± 11.9 7.9 ± 7.2
± 21.8 ± 25.2
20.8 ± 24.7 21.2 ± 20.9
± 19.7 ± 21.6
17.6 ± 21.1 16.2 ± 18.2
Baseline Low back fatigue (0–100 mm) Healthy (n = 15) No support Support Low back trouble (n = 25) No support Support Pooled (n = 40) No support Support
After driving
5.4 ± 3.9 3.9 ± 2.8
13.3 ± 13.2* 10.3 ± 12.3
16.2 ± 19.5 15.5 ± 16.5
29.2 ± 24.5‡ 32.1 ± 24.4‡
12.2 ± 16.3 11.2 ± 14.2
23.2 ± 22.2‡ 23.9 ± 23.1‡
Neck–shoulder fatigue (0–100 mm) Healthy (n = 15) No support 5.5 Support 8.6 Low back trouble (n = 25) No support 18.2 Support 15.4 Pooled (n = 40) No support 13.4 Support 12.9
± 4.4 ± 11.9
16.4 ± 13.6† 11.7 ± 13.4*
± 23.6 ± 15.9
25.5 ± 20.5* 28.4 ± 20.7‡
± 19.7 ± 14.7
22.1 ± 18.6‡ 22.1 ± 19.9‡
Values are expressed as mean ± S.D. * P < 0.05. † P < 0.01. ‡ P < 0.001. Table 7 Vibration pain provocation test, pain increased when vibrated in urban bus drivers
Painfull Painless
Healthy (n = 14)
Back disorder (n = 24)
0 14
12 12
Vibration pain provocation was positive in 12 and negative in 12 drivers with recurrent LBP and also negative in all back healthy drivers (Table 7). Vibration pain provocation test reproduced exact or similar pain as their back pain for seven subjects and dissimilar pain for 17 subjects (Table 8). The positive pain provocation indicated significantly increase of LBP (P = 0.029), neck–shoulder pain (P = 0.007), low back (P = 0.001) and neck–shoulder (P < 0.001) fatigue by driving (Table 9). The earliest signs of lumbar disc degeneration was found in all 15 subjects (pain provocation positive in nine cases) examined by MRI. The signs of annular rupture were seen in nine subjects, protrusion in eight, small disc herniation in two subjects and one spinal stenosis. The annular rupture was Table 8 Pain sensation in urban bus drivers when compressing and/or vibrating by vibrator
Similar or exact reproduction of pain Dissimilar pain No pain (pressure)
Healthy (n = 14)
Back disorder (n = 24)
0 2 12
7 16 1
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Table 9 Low back and neck–shoulder pain and fatigue (VAS) in urban bus drivers before and after driving in drivers having recurrent low back pain (low back support vs. no support, n = 24) Vibration positive (n = 12)
Vibration negative (n = 12)
Baseline
After driving
Baseline
After driving
Low back pain (0–100 mm) No support Support
14.3 ± 24.9 15.5 ± 22.4
20.6 ± 26.6 18.8 ± 19.2
25.7 ± 29.7 27.2 ± 26.7
25.3 ± 24.1 21.8 ± 23.1
Low back fatigue (0–100 mm) No support Support
12.4 ± 20.3 7.9 ± 8.2
29.8 ± 23.2* 29.9 ± 17.4†
19.8 ± 19.6 22.3 ± 19.6
25.9 ± 26.9 30.2 ± 29.2
Neck–shoulder pain (0–100 mm) No support Support
12.1 ± 22.6 11.6 ± 22.1
18.7 ± 26.2 24.2 ± 22.4*
24.3 ± 21.8 22.7 ± 29.1
20.9 ± 26.1 16.9 ± 21.2
Neck–shoulder fatigue (0–100 mm) No support 13.1 ± 21.3 Support 10.4 ± 13.1
25.5 ± 21.0† 32.2 ± 18.0†
22.3 ± 27.2 20.2 ± 18.4
22.4 ± 21.0 22.3 ± 23.3
Values are expressed as mean ± S.D. * P < 0.05. † P < 0.01.
found in five subjects having positive pain provocation and in four subjects with negative pain provocation.
4. Discussion To our knowledge, there is no previous field study on low back and neck–shoulder muscle loading in urban bus drivers. Subjects of this study represented average Finnish bus drivers, and only seven out of 40 drivers had never experienced back pain. This study showed that average paraspinal muscle loading in urban bus drivers during driving itself was minimal. Even average muscle activity during breaks was significantly higher. Low back pain and ergonomic lumbar support had no effect on paraspinal muscle activity. Trapezius muscle activity tended to be higher in drivers with LBP. Increased muscle activity was expected in drivers suffering low back pain due to protective muscle spasm or decreased due to reflexinhibition or adaptation [35] but this was not seen during driving. There seems to be considerable individual variation both in paraspinal and trapezius muscle activity according to the wide standard deviations in the mean values of the muscle activities. The left trapezius muscle activity varied from 1 to 12% of MVC and in right side from less than 1% to over 7% of MVC. The average paraspinal muscle activity varied from nearly 0% to over 3% of MVC between subjects of both groups. It was difficult to estimate how largely the muscle loading consists of dynamic and how largely of static load. The low paraspinal muscle loading would suggest to prevent fatigue. Apparently, however, the insufficient active stabilisation leads to the remarkable loading of spinal column especially of intervertebral discs and also the muscle spindle activation remains question. When sitting the lumbar spine is continuously in neutral zone when the stabilising function of
muscles is more important than that of ligaments and tendons [36–38]. The continuous loading of passive elements leads to increased stress and fatigue, expose to injury and eventually even pain. When the most important muscles stabilising back are mainly passive during driving, the importance of passive stabilising structures and also the significance of other stabilisers of back such as abdominal muscles must be emphasised. The disturbed function of abdominal muscles can be associated with low back pain and may expose to injury [35,39]. Since the stabilising muscles are passive during driving, the lumbar spine is supported poorly and exposed to the effect of whole body vibration and injuries caused by sudden loads [9,28]. Ergonomic low back support maintains back in physiologic posture when the pressure is concentrated on discs regularly. Disc pressure and myoelectric activity has been observed to decrease when the backrest inclination was increased and the low back support was used [25]. Thus, ergonomic lumbar support may prevent LBP. This is supported by the subjective beneficial effect. Overload of spine exposes to microinjuries such as internal annular ruptures, which can lead even to total annular rupture and disc herniation. Internal annular ruptures cause pain, which can be diagnosed by discographic pain provocation. This pain can be provoked non-invasively by bony vibration test [32,34,40,41]. According to this study there was evidence for internal disc disruption being one potential cause of low back pain in bus drivers. The prevalence of internal disc disruption in bus drivers suffering from recurrent low back pain was approximately the same as find earlier by discographic pain provocation [42]. Low back and neck–shoulder pain and fatigue increased more during driving in drivers who had positive vibration pain provocation test than those who had negative indicating that driving may expose for pain in those cases.
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The “hypersensitivity” to bony vibration must also be taken into consideration. The partial disagreement between MRI and pain provocation findings could be explained by the facts that all internal disc disruption are not visible in MRI and that all disc disruptions are not painful and the total annular rupture is not necessarily painful by the vibration stimulation test. Trapezius muscle loading during driving seems to be remarkably higher than paraspinal muscle loading in low back. It can be associated with neck–shoulder fatigue, which increased significantly in both groups and perhaps also with pain since as low as less than 1–2% MVC is recommended in sedentary work [43]. Muscle blood flow is related to neck–shoulder pain [44] and maybe a potential link between pain and muscle loading as well as muscle spindle excitability. However, the side difference in trapezius muscle was not associated with the location of pain in pain maps and the neck–shoulder pain did not increase significantly during driving. The increased left trapezius activity is probably due to that drivers mainly use their left hand in steering the bus and also by the EKG interference seen in some cases despite the filtering. In conclusion the paraspinal muscle loading in urban bus drivers was very small and neither low back pain nor ergonomic low back support had an effect on it, however, trapezius muscle seems to be less active in drivers suffering from recurrent low back pain. Internal disc disruptions may expose to pain and fatigability during driving.
Acknowledgements We would acknowledge Koiviston Auto Oy for financial support of this study and Kuopion Liikenne for making this study possible, Matti Ikonen for the ergonomic low back supports used in this study and Matti Suhonen MD for the observation of MRI.
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Back and neck extensor loading and back pain provocation in urban bus drivers with and without low back pain Ville Leinonen a,b,∗ , Markku Kankaanp¨aa¨ a,b , Heikki Vanharanta c , Olavi Airaksinen b , Osmo H¨anninen a a b
Department of Physiology, Kuopio University Hospital, P.O. Box 1777, FIN-70211 Kuopio, Finland Department of Physical Medicine and Rehabilitation, Kuopio University Hospital, Kuopio, Finland c Department of Physical Medicine and Rehabilitation, Oulu University Hospital, Oulu, Finland
Received 27 December 2004; received in revised form 8 September 2005; accepted 9 September 2005
Abstract This study assessed low back and trapezius muscle activity in bus drivers, with or without recurrent low back pain (LBP), during the long term driving. In addition, low back and neck–shoulder pain intensities and fatigue were measured and the effect of low back support was observed. Also the possible source of LBP was attempted to assess by vibration pain provocation test and lumbar MRI. Forty bus drivers (recurrent LBP n = 25) participated in this study. Low back and neck–shoulder pain and subjective fatigue intensity was assessed by visual analogue scales (VAS) before and after driving. Lumbar paraspinal and trapezius muscle activation during driving was measured by surface EMG. Vibration pain provocation test was applied for all subjects. Average paraspinal myoelectric activity during driving was approximately 1% of MVC in both groups. Average trapezius myoelectric activity during driving was from 2 to 4% of MVC. Trapezius muscle activity was higher in back healthy drivers than in those with LBP. The low back support had no effect either on paraspinal or trapezius EMG activity. Low back and neck–shoulder fatigue increased during driving in both groups especially in those subjects with positive vibration pain provocation. The neck–shoulder pain and fatigue were more severe in drivers suffering from LBP. Low back support had no effect on low back and neck–shoulder subjective fatigue and neck–shoulder pain but tended to limit the LBP increase during driving. Paraspinal muscle loading in urban bus drivers was very limited and either LBP or ergonomic low back support had no effect on it. Trapezius muscle seemed to be less active in drivers suffering from recurrent LBP. Internal disc disruptions may expose to pain and fatigability during driving. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: EMG; Bus driving; LBP; Vibration pain provocation
1. Introduction In past literature, problems associated with intervertebral discs and sick leaves caused by them are the most common in persons whose work requires long term sitting [1–4]. It has been estimated that men who spend over half of their working time by driving a motor vehicle have about three times higher probability to get lumbar disc herniation [1,2]. It has also been found that truck drivers have plenty of early degenerative changes of the spine [5] but according to a recent ∗
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0928-4680/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.pathophys.2005.09.004
twin-study occupational driving was not associated with disc degeneration [6]. Still, the driving may exacerbate the symptoms. An epidemiological survey [7] has been indicated that driving by any vehicle was more frequent in low back pain (LBP) patients. Back problems are more common in experienced bus drivers than in control subjects [8,9] and problems increase with age [10,11]. Up to 83% of bus drivers have reported low back problems [10,12]. However, the definite numbers of current back trouble prevalence in bus drivers do not exist and since the early findings, it is supposed that the work ergonomic and comfort has been emphasised and notably improved [13].
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Subjects suffering LBP often complain their inability to sit, and their pain intensity is associated with their ability to change their position during sitting [14,15]. Lumbar support with continuous passive motion improves comfort in simulated automobile seating both in healthy and LBP subjects [16]. Even people without clinical history of back trouble report pain with prolonged static sitting [17]. However, the sitting at work as a risk factor for LBP has been challenged [18] but e.g. driving with the exposure to whole body vibration is not comparable with the static sitting. Whole body vibration especially when driving [7,9,19] and unexpected movements or loads expose to low back pain [20–22]. Work requiring repeated forward bending and lifting expose to LBP [14,23]. Forward bending and lifting may provoke pain for LBP patients [4] and form a risk factor for acute disc herniation [24]. Disc pressure and myoelectric activity decrease when the backrest inclination is increased and the low back support is used [25]. Annular tears due to prolonged simulated sitting postures and cyclic compressive loading has been documented in young calf spines [26] and indicated a potential source of LBP in humans [27]. The low back injury process is not only associated with very high loads but even more often with lower repeated or continuous loads. Therefore it is important to study not only fast and high load but also longer lasting tissue loading [28]. The aims of the present study were to: 1. Compare low back and neck–shoulder muscle activities, subjectively assessed low back and neck–shoulder pain and fatigue intensities of bus drivers with recurrent LBP and healthy controls during the long term driving. 2. Assess the effect of ergonomic low back support on muscle activity, pain and fatigue during driving. 3. Observe the possible source of pain by vibration pain provocation test and lumbar MRI.
Table 1 Characteristics of the bus drivers with and without low back trouble (age, height, weight, MVC and Oswestry disability index (OSW)) participating in the study Healthy (n = 15) Age Height Weight BMI MVC (Nm) MVC (kg) OSW (0–100) Driving experience (years)
39.7 172.7 75.7 25.3 87.9 1.2 2.4 18.2
± ± ± ± ± ± ± ±
11.6 7.0 16.1 4.5 15.8 0.3 3.5 14.2
Low back trouble (n = 25) 41.4 176.4 87.7 28.2 98.0 1.1 11.6 18.3
± ± ± ± ± ± ± ±
11.2 6.0 11.8 3.5 19.9 0.2 7.6 12.8
Values are expressed as mean ± S.D. NS, non-significant; * P < 0.05, ‡ P < 0.001.
Sig. NS NS * *
NS NS ‡
NS † P < 0.01,
port was randomised. All driving sessions were performed by two similar busses (Kabus) with similar seats and automatic transmission. The low back support (Ikonen Matti Oy, Imatra, Finland) was ergonomically shaped, air filled and tied to the seat by sticker belt. The thickness of the support was freely adjustable by pump and valve. All drivers were allowed to choose freely the thickness of the support before driving. Questionnaires on low back and neck–shoulder pain intensities and subjective perception of fatigue were assessed by visual analogue scales (100 mm VAS [29]). Every subject evaluated his/her back and neck–shoulder pain, and fatigue intensities on a 100-mm where 0 denoted no pain or fatigue at all, and 100 the worst possible pain or fatigue. The result was given in millimetres. Low back pain induced disability was assessed by Oswestry disability index (ODI 0–100 [30]). The additional questionnaires for pain and fatigue intensities were filled before and after every driving session. Questionnaire included also pain during driving and a question whether the drivers benefited of the low back support or not. 2.2. Electromyography
2. Subjects and methods 2.1. Subjects Forty voluntary urban bus drivers, 25 (22 men and three women) with recurrent low back pain and 15 (14 men and one women) with healthy back participated in the study (Table 1). All subjects were bus drivers in local city-traffic. Daily working time lasted from 6 to 10 h including preparation of driving, approximately 5 h of driving with three breaks (lunch break and two coffee breaks). In addition to driving, drivers took fares from passengers and lifted luggage and baby carriages when needed. Before initiation of the study subjects signed an informed consent. Subjects were divided into back healthy and low back pain group according to their own assessments and presence of pain episodes during last 12 months. All subjects were studied twice and the use of ergonomic low back sup-
The averaged surface EMG was recorded bilaterally over the paraspinal muscles at L4–L5 level, and over the trapezius muscle. The skin was cleaned with an alcohol swap before pairs of disposable Ag/AgCl surface electrodes (Medicotest, Olstykke, Denmark) were attached over the investigated muscles. At the L4–L5 level of paraspinal muscles, electrodes were placed at a small angle from the sagittal plane, as has been previously suggested [31], 2 cm laterally from the spinal processes and with a 2 cm distance between electrodes. Trapezius muscle electrodes were placed at the upper part of the muscle at the C7 level. The ground electrode was placed on the skin approximately 9 cm laterally from each bipolar electrode pair. Portable ME 3000p (Mega Electronics Ltd., Kuopio, Finland) EMG system was used to record the bipolar surface EMG with four channels. The cables with preamplifiers were used to ensure good signal quality. A pair of 10-cm long
V. Leinonen et al. / Pathophysiology 12 (2005) 249–255
cables connected the EMG recording electrodes to the preamplifier in each EMG channel. The preamplifier was secured on place by attaching it to the corresponding reference electrode. A single 2.5-m long cable connected the preamplifier to the amplifier box. The EMG signal was recorded at the sampling rate of 1000 Hz and (analogically frequency band-pass filtered (effective band width 7–500 Hz), amplified (differential amplifier, CMRR > 110 dB, gain 1000, noise < 1 V)), analoque-to-digital converted (12-bit). The raw EMG signal was converted into averaged EMG in 0.1 s. data epoch, and stored in a PC for later analysis. The maximum voluntary contraction and EMG amplitude of back and trapezius muscles were measured for every subject before driving. The EMG of investigated muscles was continuously recorded when driving and stored in PC for later analysis. The total length of each measurement was approximately 7 h, which included a half hour of preparation, 5 h driving with two short (5–15 min) breaks and from half to 1 h lunch break and another driving session of 1 h. The average muscle activity measurements during driving were taken from the 3 h of driving after preparation. The muscle activity was assessed as a percentage of MVC. 2.3. Pain provocation test Pain provocation test was made for 37 drivers, three drivers were excluded, one due to accident, one by work disability, other than back problem and one by unemployment in the bus company. Pain provocation test was made by using a modified electric toothbrush (Braun D-3, Braun AG, Germany), with a stable, blunt head 1 cm2 in side as described first by Yrj¨am¨a and Vanharanta [32] and have been proven to be sensitive and specific for internal disc disruptions. The frequency of the vibrator device was 42–50 Hz. Four lumbar (L2–L5) spinal processes were palpated and compressed with the blunt head of the device one by one at a right angle of the skin with the force of 1–3 kg/cm2 . Each spinal process was vibrated 2–5 s. The type of pain provoked was recorded using the model as described for discography [33]. The result of the vibration pain provocation test was classified as painless, exact reproduction of pain, similar pain or dissimilar pain as patients experience their back pain. There is strong evidence for internal disc disruption when the provoked pain is similar to the experienced LBP and the pain felt when compressing increased when vibrated [34]. 2.4. Magnetic resonance imaging (MRI) Fifteen drivers with LBP were examined by MRI of the lumbar spine (L1–S2) in sagittal plane by 1 T imager (Siemens Magnetom Impact Expert) with TSE T1, T2 and STIR. techniques and in axial plane (L3–S1) with T2 technique. The slice thickness was 4 mm, the field of view in sagittal 140 mm × 280 mm (126 × 256 pixels) and axial 188 mm × 250 mm (223 × 512 pixels) images. The earliest sign of disc degeneration was stated as decrease in signal
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density. The annular rupture was stated to be a peripheral light signal. 2.5. Statistical analysis The effect of LBP and low back support on low back and trapezius muscle activities and comparison of the groups were measured by Student’s t-test procedures. The effect of driving, LBP, low back support and pain provocation results on pain and fatigue intensities were evaluated by repeated measures ANOVA. The logarithmic transformation of VAS values was made because of non-normal distribution. Statistical significance was set as P < 0.05.
3. Results The study groups were similar as regard to age, height, and maximum voluntary contraction but the drivers with LBP were heavier (Table 1). Average lumbar paraspinal myoelectric activity during driving was approximately 1% of MVC in both groups (Table 2). The ergonomic low back support had no effect on paraspinal muscle activity in either healthy or LBP groups. Average trapezius myoelectric activity during driving was from 2 to 4% of MVC (Table 3). There was a side difference in trapezius muscle activity i.e. left side was loaded more than right side. There is evidence for higher trapezius muscle activity in non-LBP drivers compared with LBP drivers. The ergonomic low back support had no effect on trapezius muscle activity. Table 2 Low back muscle activity of healthy drivers and those having low back trouble (rms EMG) with and without low back support (percent of MVC) during urban bus driving Healthy (n = 15)
Low back trouble (n = 25)
Left No support Support
1.1 ± 0.8 1.3 ± 1.1
0.9 ± 0.6 0.9 ± 0.7
Right No support Support
0.9 ± 0.4 1.3 ± 1.2
1.2 ± 1.2 0.9 ± 0.7
Values are expressed as mean ± S.D. Table 3 Trapezius muscle activity of (rms EMG) with and without low back support (percent of MVC) during urban bus driving Healthy
Low back trouble
Left No support Support
4.8 ± 3.3 4.2 ± 2.6
3.1 ± 1.8* 3.4 ± 1.8
Right No support Support
3.3 ± 2.0 3.5 ± 2.0
2.3 ± 1.5 2.5 ± 1.4
Values are expressed as mean ± S.D. * P < 0.05.
V. Leinonen et al. / Pathophysiology 12 (2005) 249–255
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Table 4 Low back and trapezius muscle activity (EMG) of urban bus drivers during driving vs. break (% of MVC, n = 40) Driving Low back Left No support Support Right No support Support Trapezius Left No support Support Right No support Support
Break
0.9 ± 0.9 0.9 ± 1.0
2.2 ± 1.4‡ 2.5 ± 1.6‡
1.0 ± 0.9 1.0 ± 0.9
2.3 ± 1.4‡ 2.3 ± 1.2‡
3.6 ± 2.3 3.8 ± 1.9
5.2 ± 3.1‡ 4.8 ± 2.3‡
2.6 ± 1.7 3.0 ± 1.6
4.5 ± 2.2‡ 4.6 ± 2.1‡
Values are expressed as mean ± S.D. ‡ P < 0.001.
Both paraspinal and trapezius myoelectric activity was higher during breaks than during driving (Table 4). Low back and neck–shoulder fatigue increased significantly during driving in both groups (Tables 5 and 6). The neck–shoulder pain and fatigue were more severe in drivers suffering from low back trouble. Ergonomic low back support had no effect on low back and neck–shoulder fatigue and neck–shoulder pain. According to multiple-choice questionnaire 26 (65%) of all subjects, 9 (56%) from back pain group and 17 (71%) from back healthy drivers subjectively benefited for ergonomic low back support.
Table 5 Low back and neck–shoulder pain (VAS) in urban bus drivers before and after driving (low back support vs. no support) Baseline Low back pain (0–100 mm) Healthy (n = 15) No support Support Low back trouble (n = 25) No support Support Pooled (n = 40) No support Support
After driving
4.5 ± 4.0 2.4 ± 1.4
8.3 ± 8.8 5.1 ± 4.3*
19.6 ± 26.6 21.5 ± 23.9
23.8 ± 25.2 22.0 ± 22.2
14.0 ± 22.3 14.4 ± 21.0
18.0 ± 21.8* 15.7 ± 19.5
Neck–shoulder pain (0–100 mm) Healthy (n = 15) No support 10.5 Support 8.3 Low back trouble (n = 25) No support 18.5 Support 16.4 Pooled (n = 40) No support 15.5 Support 13.4 Values are expressed as mean ± S.D. * P < 0.05.
Table 6 Low back and neck–shoulder fatigue (VAS) in urban bus drivers before and after driving (low back support vs. no support)
± 14.8 ± 12.8
12.3 ± 11.9 7.9 ± 7.2
± 21.8 ± 25.2
20.8 ± 24.7 21.2 ± 20.9
± 19.7 ± 21.6
17.6 ± 21.1 16.2 ± 18.2
Baseline Low back fatigue (0–100 mm) Healthy (n = 15) No support Support Low back trouble (n = 25) No support Support Pooled (n = 40) No support Support
After driving
5.4 ± 3.9 3.9 ± 2.8
13.3 ± 13.2* 10.3 ± 12.3
16.2 ± 19.5 15.5 ± 16.5
29.2 ± 24.5‡ 32.1 ± 24.4‡
12.2 ± 16.3 11.2 ± 14.2
23.2 ± 22.2‡ 23.9 ± 23.1‡
Neck–shoulder fatigue (0–100 mm) Healthy (n = 15) No support 5.5 Support 8.6 Low back trouble (n = 25) No support 18.2 Support 15.4 Pooled (n = 40) No support 13.4 Support 12.9
± 4.4 ± 11.9
16.4 ± 13.6† 11.7 ± 13.4*
± 23.6 ± 15.9
25.5 ± 20.5* 28.4 ± 20.7‡
± 19.7 ± 14.7
22.1 ± 18.6‡ 22.1 ± 19.9‡
Values are expressed as mean ± S.D. * P < 0.05. † P < 0.01. ‡ P < 0.001. Table 7 Vibration pain provocation test, pain increased when vibrated in urban bus drivers
Painfull Painless
Healthy (n = 14)
Back disorder (n = 24)
0 14
12 12
Vibration pain provocation was positive in 12 and negative in 12 drivers with recurrent LBP and also negative in all back healthy drivers (Table 7). Vibration pain provocation test reproduced exact or similar pain as their back pain for seven subjects and dissimilar pain for 17 subjects (Table 8). The positive pain provocation indicated significantly increase of LBP (P = 0.029), neck–shoulder pain (P = 0.007), low back (P = 0.001) and neck–shoulder (P < 0.001) fatigue by driving (Table 9). The earliest signs of lumbar disc degeneration was found in all 15 subjects (pain provocation positive in nine cases) examined by MRI. The signs of annular rupture were seen in nine subjects, protrusion in eight, small disc herniation in two subjects and one spinal stenosis. The annular rupture was Table 8 Pain sensation in urban bus drivers when compressing and/or vibrating by vibrator
Similar or exact reproduction of pain Dissimilar pain No pain (pressure)
Healthy (n = 14)
Back disorder (n = 24)
0 2 12
7 16 1
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Table 9 Low back and neck–shoulder pain and fatigue (VAS) in urban bus drivers before and after driving in drivers having recurrent low back pain (low back support vs. no support, n = 24) Vibration positive (n = 12)
Vibration negative (n = 12)
Baseline
After driving
Baseline
After driving
Low back pain (0–100 mm) No support Support
14.3 ± 24.9 15.5 ± 22.4
20.6 ± 26.6 18.8 ± 19.2
25.7 ± 29.7 27.2 ± 26.7
25.3 ± 24.1 21.8 ± 23.1
Low back fatigue (0–100 mm) No support Support
12.4 ± 20.3 7.9 ± 8.2
29.8 ± 23.2* 29.9 ± 17.4†
19.8 ± 19.6 22.3 ± 19.6
25.9 ± 26.9 30.2 ± 29.2
Neck–shoulder pain (0–100 mm) No support Support
12.1 ± 22.6 11.6 ± 22.1
18.7 ± 26.2 24.2 ± 22.4*
24.3 ± 21.8 22.7 ± 29.1
20.9 ± 26.1 16.9 ± 21.2
Neck–shoulder fatigue (0–100 mm) No support 13.1 ± 21.3 Support 10.4 ± 13.1
25.5 ± 21.0† 32.2 ± 18.0†
22.3 ± 27.2 20.2 ± 18.4
22.4 ± 21.0 22.3 ± 23.3
Values are expressed as mean ± S.D. * P < 0.05. † P < 0.01.
found in five subjects having positive pain provocation and in four subjects with negative pain provocation.
4. Discussion To our knowledge, there is no previous field study on low back and neck–shoulder muscle loading in urban bus drivers. Subjects of this study represented average Finnish bus drivers, and only seven out of 40 drivers had never experienced back pain. This study showed that average paraspinal muscle loading in urban bus drivers during driving itself was minimal. Even average muscle activity during breaks was significantly higher. Low back pain and ergonomic lumbar support had no effect on paraspinal muscle activity. Trapezius muscle activity tended to be higher in drivers with LBP. Increased muscle activity was expected in drivers suffering low back pain due to protective muscle spasm or decreased due to reflexinhibition or adaptation [35] but this was not seen during driving. There seems to be considerable individual variation both in paraspinal and trapezius muscle activity according to the wide standard deviations in the mean values of the muscle activities. The left trapezius muscle activity varied from 1 to 12% of MVC and in right side from less than 1% to over 7% of MVC. The average paraspinal muscle activity varied from nearly 0% to over 3% of MVC between subjects of both groups. It was difficult to estimate how largely the muscle loading consists of dynamic and how largely of static load. The low paraspinal muscle loading would suggest to prevent fatigue. Apparently, however, the insufficient active stabilisation leads to the remarkable loading of spinal column especially of intervertebral discs and also the muscle spindle activation remains question. When sitting the lumbar spine is continuously in neutral zone when the stabilising function of
muscles is more important than that of ligaments and tendons [36–38]. The continuous loading of passive elements leads to increased stress and fatigue, expose to injury and eventually even pain. When the most important muscles stabilising back are mainly passive during driving, the importance of passive stabilising structures and also the significance of other stabilisers of back such as abdominal muscles must be emphasised. The disturbed function of abdominal muscles can be associated with low back pain and may expose to injury [35,39]. Since the stabilising muscles are passive during driving, the lumbar spine is supported poorly and exposed to the effect of whole body vibration and injuries caused by sudden loads [9,28]. Ergonomic low back support maintains back in physiologic posture when the pressure is concentrated on discs regularly. Disc pressure and myoelectric activity has been observed to decrease when the backrest inclination was increased and the low back support was used [25]. Thus, ergonomic lumbar support may prevent LBP. This is supported by the subjective beneficial effect. Overload of spine exposes to microinjuries such as internal annular ruptures, which can lead even to total annular rupture and disc herniation. Internal annular ruptures cause pain, which can be diagnosed by discographic pain provocation. This pain can be provoked non-invasively by bony vibration test [32,34,40,41]. According to this study there was evidence for internal disc disruption being one potential cause of low back pain in bus drivers. The prevalence of internal disc disruption in bus drivers suffering from recurrent low back pain was approximately the same as find earlier by discographic pain provocation [42]. Low back and neck–shoulder pain and fatigue increased more during driving in drivers who had positive vibration pain provocation test than those who had negative indicating that driving may expose for pain in those cases.
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The “hypersensitivity” to bony vibration must also be taken into consideration. The partial disagreement between MRI and pain provocation findings could be explained by the facts that all internal disc disruption are not visible in MRI and that all disc disruptions are not painful and the total annular rupture is not necessarily painful by the vibration stimulation test. Trapezius muscle loading during driving seems to be remarkably higher than paraspinal muscle loading in low back. It can be associated with neck–shoulder fatigue, which increased significantly in both groups and perhaps also with pain since as low as less than 1–2% MVC is recommended in sedentary work [43]. Muscle blood flow is related to neck–shoulder pain [44] and maybe a potential link between pain and muscle loading as well as muscle spindle excitability. However, the side difference in trapezius muscle was not associated with the location of pain in pain maps and the neck–shoulder pain did not increase significantly during driving. The increased left trapezius activity is probably due to that drivers mainly use their left hand in steering the bus and also by the EKG interference seen in some cases despite the filtering. In conclusion the paraspinal muscle loading in urban bus drivers was very small and neither low back pain nor ergonomic low back support had an effect on it, however, trapezius muscle seems to be less active in drivers suffering from recurrent low back pain. Internal disc disruptions may expose to pain and fatigability during driving.
Acknowledgements We would acknowledge Koiviston Auto Oy for financial support of this study and Kuopion Liikenne for making this study possible, Matti Ikonen for the ergonomic low back supports used in this study and Matti Suhonen MD for the observation of MRI.
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