Association between Whole-Body Vibration exposure and musculoskeletal disorders among dumper operators: A case-control study in Indian iron ore mines

Abstract

BACKGROUND:

Dumper operators in mines worldwide are subjected to Musculoskeletal Disorders (MSDs) due to whole-body vibration exposure. This study evaluated the working-life-Whole-Body Vibration (WBV)-exposure and their association with various MSDs among dumper operators in mines which remains poorly addressed.

METHODS:

This case-control study in Indian iron ore mines was conducted to compare randomly selected 65 dumper operators and 65 office workers. Data were collected through face-to-face interviews using the Nordic Musculoskeletal Questionnaire (NMQ) and were analysed using logistic regression models.

RESULTS:

The study revealed that majority of the dumper operators were exposed to WBV exceeding the ISO-2631 limits. Compared with controls, the dumper operators had a much higher risk of upper back pain (age-overweight-adjusted odds ratio ORao = 5.37, 95% CI = 1.78–16.20), lower back pain (ORao = 2.72, 95% CI = 1.25–5.94), knee and leg pain (ORao = 3.68, 95% CI = 1.22–11.11), and having 2+ MSDs (ORao = 5.05, 95% CI = 1.88–13.51, vs. no MSDs). Working-life-WBV-exposure was higher among dumper operators having upper back pain (mean (SD) = 7.1 (1.91) vs. 5.7 (1.91), p < 0.01) and lower back pain (mean (SD) = 6.63 (2.10) vs. 5.55 (1.71), p < 0.01) compared to those without these MSDs. Older age was associated with higher risk of MSD pains.

CONCLUSION:

Dumper operators have excess MSDs due to high working-life-WBV-exposure. Their MSDs and working-life-WBV-exposure should be regularly evaluated and reduced.

Keywords

Working-life-WBV-exposure surface mining health hazard logistic regression model lower back pain

1 Introduction

Occupational exposure to whole-body vibration (WBV) is common and may contribute to severe health-related issues in many industries and is acknowledged as a potential occupational risk factor for musculoskeletal disorders (MSDs) [1]. A wide range of operations in a number of industries such as production, construction, agriculture and mining have exposed the workers to serious health hazards due to WBV exposure [2]. In the mining industry, rapid advancement in technology and growing demands of minerals across the globe have led to extensive use of heavy earth moving machineries (HEMM). The operators of HEMM may be highly exposed to WBV because during HEMM motion, the body is subjected to different forces like accelerations and decelerations, lateral swaying, and up and down vibrations [3]. In addition, while driving, the feet are actively used on accelerating, braking, and cannot be used to support and stabilize the body. WBV is transmitted either through seat in sitting position or through feet while standing on vibrating platform or screen.

Surface mining dominates the production of minerals worldwide for extraction of various minerals that include fuel minerals (such as coal and uranium), metallic minerals (such as iron, copper, gold, uranium and manganese), and non-metallic minerals (such as limestone, mica and potash). Currently, almost 95% of all-non-metallic minerals, around 90% of the metallic minerals and more than 60% of the coal are mined by surface mining methods [4]. Of the over 30 billion tonnes of ore and waste materials that are mined each year, surface mining accounts for nearly 25 billion tonnes [4]. India, China, Australia, South Africa, Canada, USA, Indonesia, Kazakhstan and Chile are some of the most mining-intensive countries that produce bulk of the minerals for internal consumption and export. Heavy earth moving machineries such as drills, shovels, road headers, rock breakers, bulldozers, and heavy-duty rollers are mostly used for production of ore/mineral in opencast mines. Surface mining therefore highly exposes the workers to WBV and leads to MSDs which may be a public and occupational concern in many countries.

This transportation in surface mining is usually accomplished through dumpers. Dumpers of various size and capacity (ranges from 100 tons to 400 tons) are used in India. The dumper operators in surface mines are highly exposed to WBV from running engine, and these machines generate further vibration due to their operation on unpaved and undulated natural surfaces. The dumpers run on irregular and undulated haul road surfaces with continuous engine vibration for about 90% of the 8-h shifts with very limited breaks, which continuously exposes the dumper operators to both WBV and shocks. Because of these occupational hazards, the WBV exposure and the risk of various MSDs associated with dumper operation need to be evaluated and possibly prevented by appropriate measures.

The United States Bureau of Labor Statistics reported an incidence rate of work-related MSDs including all mining sectors to be 26.9 per 10,000 full time employees [5]. Studies in opencast mines in Finland, Norway, Russia and Sweden showed that vehicle drivers experienced a higher prevalence of MSDs than the other miners [6]. Some other studies have also shown the high prevalence of self-reported musculoskeletal symptoms among mine vehicle operators than among the non-exposed workers [7, 8]. In India, one study reported that drivers of earth-moving machines in mines were highly exposed to MSDs [9]. Another study in India also showed that dumper and dozer operators in coal mines had a high exposure to MSDs [10, 11]. The MSDs have also been investigated in construction, agriculture and mining industries [12 –14]. Some investigations showed that occupational exposure to WBV is associated with elevated risk of lower back pain, shoulder pain, neck pain, and knee and leg pain [15, 16]. An elevated risk of MSDs was reported among truck drivers and commercial travellers [17 –20]. Other studies showed potential health risk associated with WBV exposure among operators of dumpers, transport trucks and front-end loaders [21 –26]. Several studies have confirmed the association of MSDs with long-term exposure to WBV [25–31 , 55]. It may be noted that potential risk factors of MSDs also include age, years of exposure in the job, body mass index (BMI), gender, posture of operators and WBV exposure [12 , 32–55]. However, exposure of dumper operators to MSDs due to WBV exposure and its associated risk factors have little been evaluated and known [56 –59]. This case-control study therefore aimed to assess the WBV exposure and the associated risk of MSDs among dumper operators of Indian iron ore mines compared with office workers from the same mines. We furthermore investigated whether working-life-WBV-exposure, job experience, advancing age and being overweight were associated with various categories of MSDs.

2 Methods

2.1 Study mines

This case-control study was conducted in two opencast iron ore mines located in eastern part of India for a period of six months. In the previous year, the iron ore production from eastern part of India was 59% of the total production in India. In India and especially in the two mines where this study was carried out, most of the iron ore deposits are hill-top deposits. Both the mines are fully mechanized and have similar infrastructural facilities. Their annual production of iron ore was more than 10 million tons. The two mines are being operated by a single company having experience of iron ore mining for about 100 years. The haul roads in these mines are quite uneven and irregular with presence of puddles, potholes, debris ruts and bumps. As the terrain is hilly, the haul road had sharp turns with an average slope of 6 degrees (Fig. 1).

Fig. 1

(a) Mine layout; (b) haul road; (c) dumper; (d) and (e) operator sitting in the dumper cabin.

2.2 Study design

This study included 65 cases and 65 controls. The cases were dumper operators who were daily exposed to machine-induced vibration. The controls were workers who were never exposed to the vibration from the same mines; they had been employed as weighbridge operator, service assistant, draftsman, office staff (for planning, purchase, and accounts), maintenance department staff, and at the survey office. They were working in sedentary office jobs in the mines with an average work length of 10 years (range: 1–35 years). All the operators contacted were willing to participate in the study. The study was approved by the Department of Mining Engineering of the Indian Institute of Technology Kharagpur. The purpose of the study and scope of their participation was explained to the subjects. Voluntary consent for participation was obtained before conducting the survey.

All the dumpers used were of same model (Komatsu-HD-785) with 100 tonne capacity and were procured in four fleets (2008, 2009, 2010 and 2015). The dumpers transported iron ore from the mines to a pit-head processing plant and the overburden to a subgrade waste dump. The lead distance varied between 1.5 and 5.2 km. Komatsu dumpers are designed specifically for mining, quarrying and construction. They are very reliable in hauling applications. The mechanical, electrical and hydraulic power specifications and the seat design parameters of all dumpers are the same. All the dumpers had a mechanical seat suspension system.

The study protocol consisted of a request for participation of subjects and data collection using a face-to-face interview as most of the dumper operators could not read and write. The mine management was informed prior to the interview so that appropriate arrangements could be made to make the participants available without hampering their work. For most workers the interview was done in shift breaks. Interviews were conducted in the local language so that workers could comprehend the queries and respond effectively. In this study, the participation rate of dumper operators from the two mines was 93%. Seat dimensions of the dumpers were also measured to assess the compatibility of seat with the operator’s body.

The following data were gathered: personal characteristics (name, age, height, and weight), job characteristics, years with job, duration of the work exposure prior to the current job, employment data, and MSDs (self-reported complaints for neck, upper limb, elbow, upper back, lower back, knee and leg) using the Nordic Musculoskeletal Questionnaire (NMQ). The participants were further asked whether MSDs prevented them from performing their work normally during the past 12 months. The MSDs for each body part were defined as the presence of pain for the location. Cumulative MSDs was defined as the cumulated number of neck pain, upper limb and elbow pain, upper back pain, lower back pain, and knee and leg pain.

2.3 Assessment of WBV exposure

WBV exposure was measured in accordance with the standard protocol of the ISO 2631-1 : 1997; Amendment: 2010 guidelines. A tri-axial seat pad accelerometer (Model number: Nor-136) was positioned on the dumper operators’ seat based on the direction and anatomical positioning. A three-axis coordinate system was used to measure the vibration: anterior-posterior direction was considered as the x-axis, the medial-lateral direction was considered as the y-axis, and vertical direction was considered as the z-axis. The WBV parameters that were estimated included: (1) frequency-weighted root-mean-squire (RMS) acceleration which is the most basic evaluation method for the measurement of WBV exposure; (2) crest factor which is defined as the ratio of peak and RMS acceleration [60] and a measure of shock within the vibration signal (dumper operators are exposed to multiple shocks during operation); and (3) vibration dose value (VDV) for the exposures when crest factor is greater than 9 [60 –62]. In this study, WBV exposure for each operator was considered as the average of six-measurement cycles, each of an average duration of 40 minutes to get a representative value of WBV exposure for a total exposure duration of 8 hours (daily vibration exposure). According to ISO 2631–1 : 1997 guidelines, the upper and lower limits of the health-guidance caution zone for 8 hours of exposure are 0.9 m s^–2 and 0.45 m s^–2 in terms of frequency weighted RMS acceleration. Similarly, the upper and lower limits of the health-guidance caution based on vibration dose value are 17 m s^–1.75 and 8.5 m s^–1.75. The values of the frequency weighted RMS acceleration and VDV not exceeding ISO lower limits indicate that a worker is at low health risk, the values between lower limit and upper limit indicate the worker is exposed to moderate level of health risk due to vibration, and the values above the upper limit make the worker vulnerable to increased risk of health problems. Health risk was assessed with the 8-h equivalent RMSA (noted A(8)) and 8-h equivalent VDV (noted VDV(8)) according to ISO 2631- 1 : 1997/Amendment 2010 standard [61, 62].

The total vibration exposure for over the career for various dumper operators was estimated in terms of frequency-weighted RMS acceleration and VDV using the method proposed by Dannerlein [63]. The exposure was first computed for one month, then for a 1-year period, and next for over the career duration by assuming that the exposure did not change significantly in terms of nature, magnitude, vehicles conditions, type of roads and season. The cumulated WBV exposure, defined as A_w and VDV_w, was defined as the sum of the corresponding standardized variables (divided by their respective standard deviation). It is noted as A_w+VDV_w throughout the paper.

2.4 Statistical analysis

The comparison of the prevalence of different types of MSDs and individual characteristics between dumper operators and controls was made using the chi-square test or variance analysis. The risk of each type of MSDs for the dumper operators versus the controls was evaluated using odds ratio adjusted for age and overweight (ORao) and 95% confidence interval (95% CI). The association between age and overweight with various types of MSDs were assessed with crude and adjusted odds ratios (ORao) which were computed using logistic regression models. All tests were two-sided with a probability of < 0.05 considered as significant. The analyses were performed using IBM SPSS ver. 21.0.

3 Results

Table 1 shows that the dumper operators were highly exposed to WBV including shocks. The magnitude of RMSA was much greater in z-axis with mean value of 0.48 m s^–2 (range 0.31–0.81 m s^–2) than in x-axis (mean = 0.21 m s^–2, range = 0.15–0.30 m s^–2) and y-axis (mean = 0.21 m s^–2, range = 0.16–0.27 m s^–2). The mean of vector sum awv was 0.63 m s^–2 (range 0.43–0.97 m s^–2). The crest factor magnitude was also highest in z-axis having values for majority of the operators greater than 9 suggesting a high exposure to shocks. The mean A_w was 5.13 m s^–2 (range 1.20–10.12 m s^–2). The VDV value was also much higher in z-axis with mean value of 5.24 m s^–1.75 (range 2.92–9.25 m s^–1.75) than in x-axis (mean = 2.37 m s^–1.75, range 1.71–3.36 m s^–1.75) and y-axis (mean = 2.34 m s^–1.75, range 1.63–3.02 m s^–1.75). The mean value of VDV_sum was 5.38 m s^–1.75 (range 3.15–9.31 m s^–1.75). The mean VDV_w was 128.2 m s^–1.75 (range 57.1–226.5 m s^–1.75). For A (8), 59% of operators were exposed to a value exceeding the lower limit (0.45 m s^–2) of “Health Guidance Caution Zone” (HGCZ). Similarly, for VDV (8), 90.8% of the dumper operators were exposed to a value above the lower limit of HGCZ (8.5 m s^–1.75). The mean A_w + VDV_w was 6.10 (range 2.31–11.24).

Table 1
Occupational whole-body vibration exposure of dumper operators (n = 65)

Mean (SD) Median Skewness Range < ISO lower limit^a (%) Within ISO limit^a (%) > ISO upper limit^a (%)

Whole body vibration exposure

RMSA

a_wx (anterior-posterior x-axis), ms^–2 0.21 (0.03) 0.21 0.44 0.15–0.30 100.0 0.0 0.0

a_wy (medial-lateral y-axis), ms^–2 0.21 (0.02) 0.21 0.09 0.16–0.27 100.0 0.0 0.0

z-axis (vertical z-axis), ms^–2 0.48 (0.10) 0.47 0.81 0.31–0.81 41.5 58.5 0.0

A_wv (total)_, ms^–2 0.63 (0.10) 0.62 0.78 0.43–0.97 0.0 96.9 3.1

A(8) (daily exposure), ms^–2 0.48 (0.10) 0.47 0.81 0.31–0.81 41.5 58.5 0.0

A_w, ms^–2 5.13 (2.17) 4.67 0.27 1.20–10.12 – – –

VDV exposure

VDV_x (anterior-posterior 2.37 (0.37) 2.36 0.34 1.71–3.36 100.0 0.0 0.0

x-axis), ms^–1.75

VDV_y (medial-lateral 2.34 (0.29) 2.36 -0.12 1.63–3.02 100.0 0.0 0.0

y-axis), ms^–1.75

VDV_z (vertical z-axis), ms^–1.75 5.24 (1.22) 5.13 1.07 2.92–9.25 96.9 3.1 0.0

VDV_sum (total)_, ms^–1.75 5.38 (1.18) 5.26 1.09 3.15–9.31 96.9 3.1 0.0

VDV(8) (daily exposure), ms^–1.75 10.90 (2.25) 10.56 1.11 7.32–17.99 9.2 90.8 0.0

VDV_w, ms^–1.75 128.2 (2.17) 127.7 0.30 57.1–226.5 – – –

A_w+VDV_w 6.10 (1.98) 5.97 0.28 2.31–11.24 – – –

CF

CF_x (anterior-posterior x-axis) 11.10 (1.89) 10.60 1.17 8.58–18.74 12.30 87.69

CF_y (medial-lateral y-axis) 10.50 (1.57) 10.47 0.18 6.45–14.65 16.92 83.07

CF_z (vertical z-axis) 15.10 (4.79) 14.15 1.08 8.97–28.65 0.0 100.0

	Mean (SD)	Median	Skewness	Range	< ISO lower limit^a (%)	Within ISO limit^a (%)	> ISO upper limit^a (%)
Whole body vibration exposure
RMSA
a_wx (anterior-posterior x-axis), ms^–2	0.21 (0.03)	0.21	0.44	0.15–0.30	100.0	0.0	0.0
a_wy (medial-lateral y-axis), ms^–2	0.21 (0.02)	0.21	0.09	0.16–0.27	100.0	0.0	0.0
z-axis (vertical z-axis), ms^–2	0.48 (0.10)	0.47	0.81	0.31–0.81	41.5	58.5	0.0
A_wv (total)_, ms^–2	0.63 (0.10)	0.62	0.78	0.43–0.97	0.0	96.9	3.1
A(8) (daily exposure), ms^–2	0.48 (0.10)	0.47	0.81	0.31–0.81	41.5	58.5	0.0
A_w, ms^–2	5.13 (2.17)	4.67	0.27	1.20–10.12	–	–	–
VDV exposure
VDV_x (anterior-posterior	2.37 (0.37)	2.36	0.34	1.71–3.36	100.0	0.0	0.0
x-axis), ms^–1.75
VDV_y (medial-lateral	2.34 (0.29)	2.36	-0.12	1.63–3.02	100.0	0.0	0.0
y-axis), ms^–1.75
VDV_z (vertical z-axis), ms^–1.75	5.24 (1.22)	5.13	1.07	2.92–9.25	96.9	3.1	0.0
VDV_sum (total)_, ms^–1.75	5.38 (1.18)	5.26	1.09	3.15–9.31	96.9	3.1	0.0
VDV(8) (daily exposure), ms^–1.75	10.90 (2.25)	10.56	1.11	7.32–17.99	9.2	90.8	0.0
VDV_w, ms^–1.75	128.2 (2.17)	127.7	0.30	57.1–226.5	–	–	–
A_w+VDV_w	6.10 (1.98)	5.97	0.28	2.31–11.24	–	–	–
CF
CF_x (anterior-posterior x-axis)	11.10 (1.89)	10.60	1.17	8.58–18.74	12.30		87.69
CF_y (medial-lateral y-axis)	10.50 (1.57)	10.47	0.18	6.45–14.65	16.92		83.07
CF_z (vertical z-axis)	15.10 (4.79)	14.15	1.08	8.97–28.65	0.0		100.0

Abbreviations: SD, standard deviation; n, number of subjects, RMSA, frequency-weighted root mean square acceleration; VDV, frequency-weighted vibration dose value; CF, crest factor, A_w: cumulated exposure in terms of RMSA for over the career, VDV_w: cumulated exposure in terms of vibration dose value for over the career, A_w+VDV_w: sum of the corresponding standardized variables divided by their respective standard deviation).^a ISO 2631-1 Health guidance caution zone (HGCZ) (1997) for RMSA: lower limit = 0.45 ms^–2, upper limit = 0.9 ms^–2; for VDV: lower limit = 8.5 ms^–1.75, upper limit = 17 ms^–1.75. All machines were KOMATSU HD-785-7 dump trucks.

Table 2 reveals that, compared with controls, the dumper operators had similar age but a higher proportion of subjects with more than 10 years in the job (52.3% vs. 26.2%, p = 0.018) and overweight (81.5% vs. 66.1%, p = 0.046). The dumper operators were much more affected than the controls for lower back pain (50.8% vs. 29.2% among the controls, p = 0.010), upper back pain (29.2% vs. 7.7%, p = 0.002) and knee pain (20.0% vs. 6.2%, p = 0.019).

Table 2

Characteristics of dumper operators (cases) and controls in iron ore mines (65 pairs)

	Cases (n = 65) % or mean (SD)	Controls (n = 65) % or mean (SD)	p-value^a
Musculoskeletal disorders^b
Neck pain	7.7	9.2	0.753
Upper limb and shoulder pain	13.8	9.2	0.410
Shoulder pain	12.3	4.6	0.115
Upper limb pain	4.6	4.6	0.698
Upper back pain	29.2	7.7	0.002
Lower back pain	50.8	29.2	0.010
Knee and leg pain	21.5	9.2	0.050
Knee pain	20.0	6.2	0.019
Leg pain	9.2	3.1	0.144
Age (yr)			0.183
20–29	7.7	1.5
30–39	21.5	24.6
40–49	27.7	36.9
50–60	43.1	36.9
Mean (SD)	46.3 (9.7)	45.4 (8.1)	0.571
More than 10 years with the job	52.3	26.2	0.018
Height (cm)			0.562
< 176	90.8	92.3
176–181	4.62	1.5
> 181	4.6	6.1
Overweight (body mass index 23 kg/m² or over)	81.5	66.1	0.046
Mean body mass index (standard deviation)	26.1 (3.5)	24.2 (2.8)	0.001

Abbreviations: SD, standard deviation; n, number of subjects Cases: dumper operators in iron ore mines, controls: mine office workers in the same mines matched for age. ^aChi² test or variance analysis. ^bBased on the Nordic Musculoskeletal Questionnaire (NMQ).

As shown in Table 3, we found that based on odds ratio adjusted for age and overweight, the dumper operators had a much higher MSDs risk than the controls: ORao = 5.37 (95% CI 1.78–16.20, p = 0.003) for upper back pain, ORao = 2.72 (95% CI 1.25–5.94, p = 0.012) for lower back pain, and ORao = 3.68 (95% CI 1.22–11.11, p = 0.021) for knee and leg pain. The ORao was close to significance for shoulder pain (p < 0.10). Importantly, the dumper operators had a much higher risk for MSDs affecting two or more body parts (ORao = 5.05, 95% CI 1.88–13.51, p = 0.001).

Table 3

Comparison of the prevalence of musculoskeletal disorders ^a in dumper operators (cases) and controls (65 pairs): odds ratio adjusted for age and overweight and 95% confidence interval (95% CI)

	Cases (n = 65)	Controls (n = 65) %	Odds ratio	p-value	95% CI
Musculoskeletal disorders (outcome variables)^a
Neck pain	7.7	9.2	1.08	0.906	0.30–3.95
Upper limb and shoulder pain	13.8	9.2	2.00	0.246	0.62–6.39
Shoulder pain	12.3	4.6	3.25	0.092	0.77–13.72
Upper limb pain	4.6	4.6	1.17	0.846	0.26–5.73
Upper back pain	29.2	7.7	5.37	0.003	1.78–16.20
Lower back pain	50.8	29.2	2.72	0.012	1.25–5.94
Knee and leg pain	21.5	9.2	3.68	0.021	1.22–11.11
Knee pain	20.0	6.2	5.05	0.012	1.43–17.80
Leg pain	9.2	3.1	4.51	0.079	0.88–24.18
Number of affected body parts^b
0 (reference group)	46.2	67.7	1.00	1.00	1.00
1 only	16.9	18.5	1.44	0.473	0.53–3.95
2 or more	36.9	13.8	5.05	0.001	1.88–13.51

Cases: dumper operators in iron ore mines, controls: mine office workers in the same mines. ^aBased on the Nordic Musculoskeletal Questionnaire (NMQ). ^bDefined as the cumulated number of shoulder pain, upper back pain, lower back pain, knee pain and leg pain (coded 0 if absent or 1 if present) which had odds ratios with p < 0.10 (range 0 to 5).

We found that the dumper operators had a high risk of accelerated ageing for upper limb and shoulder pain (adjusted odds ratio for a 10-year age increase ORa = 4.00, p < 0.05), upper back pain (ORa = 3.23, p < 0.01), lower back pain (ORa = 2.16, p < 0.01) and for knee and leg pain (ORa = 2.18, p < 0.05) (Table 4). Being overweight was significantly associated with lower back pain only (ORo = 3.97, p < 0.05) (Table 4). The cumulated number of MSDs was associated with age only (ORa for a 10-year age increase = 3.37, p < 0.01, Table 5).

Table 4

Associations between musculoskeletal disorders^a and age and overweight in dumper operators (n = 65): odds ratio (OR) and 95% confidence interval (95% CI)

		Musculoskeletal disorders
	Neck pain		Upper limb and shoulder pain		Upper back pain		Lower back pain		Knee and leg pain
	OR	95% CI	OR	95% CI	OR	95% CI	OR	95% CI	OR	95% CI
		Crude odds ratio and 95% confidence interval
Age for a 10-year increase	2.75	(0.71 to 10.6)	3.97^*	(1.18 to 13.3)	3.10^**	(1.43 to 6.67)	2.15^**	(1.22 to 3.80)	2.08^*	(1.00 to 4.37)
Overweight (body mass index kg/m² or over)	0.90	(0.09 to 8.84)	1.96	(0.22 to 17.3)	2.36	(0.46 to 11.9)	3.91^*	(1.05 to 16.1)	0.46	(0.12 to 1.85)

	Adjusted odds ratio (ORa) and 95% confidence interval
Age for a 10-year increase	2.76	(0.71 to 10.6)	4.00^*	(1.17 to 13.5)	3.23^**	(1.42 to 7.13)	2.16^**	(1.18 to 3.92)	2.18^*	(1.02 to 4.64)
Overweight (body mass index kg/m² or over)	0.77	(0.07 to 8.20)	1.82	(0.18 to 18.1)	2.23	(0.40 to 13.1)	3.97^*	(1.01 to 17.8)	0.38	(0.08 to 1.65)

Bold types: significant OR: ^*p < 0.05, ^**p < 0.01. Cases: dumper operators in iron ore mines, controls: mine office workers in the same mines. ^aBased on the Nordic Musculoskeletal Questionnaire (NMQ).

Table 5

Associations between cumulated number of musculoskeletal disorders^a and age and overweight in dumper operators (n = 65): odds ratio and 95% confidence interval (95% CI)

	Cumulated number of musculoskeletal disorders ^b
	One only (vs. 0)			2 or more (vs. 0)
	OR	95% CI	OR	95% CI
	Crude odds ratio (cOR) and 95% CI
Age for a 10-year increase	1.21	(0.58 to 2.53)	3.40^**	(1.61 to 7.16)
Overweight (body mass index kg/m² or over	3.64	(0.40 to 33.1)	2.54	(0.59 to 10.91)
	Adjusted odds ratio (aOR) and 95% CI
Age for a 10-year increase	1.19	(0.56 to 2.53)	3.37^**	(1.54 to 7.14)
Overweight (body mass index kg/m² or over)	3.57	(0.39 to 32.25)	2.47	(0.48 to 12.5)

Bold types: significant OR: ^**p< 0.01. Cases: dumper operators in iron ore mines, controls: mine office workers in the same mines. ^aBased on the Nordic Musculoskeletal Questionnaire (NMQ). ^bDefined as the cumulated number of shoulder pain, upper back pain, lower back pain, knee pain and leg pain (coded 0 if absent or 1 if present) which had close-to-significance odds ratios with p < 0.10 (range 0 to 5).

Finally, Table 6 shows that A_w was higher among the dumper operators with upper back pain (mean (SD) = 6.22 (2.08) vs. 4.67(2.06), p < 0.01) and lower back pain (mean (SD) = 5.76 (2.29) vs. 4.47(1.85), p < 0.01) compared to those without these pains. Similarly, VDV_w was higher among the dumper operators with upper back pain (mean (SD) = 141.9 (33.6) vs. 122.6 (34.6), p < 0.05) and lower back pain (mean (SD) = 136.3 (36.9) vs. 119.9 (31.8), p < 0.05) compared to those who are not suffering from these pains. The A_w + VDV_w was higher among the dumper operators with upper back pain (mean (SD) = 7.1 (1.91) vs. 5.7 (1.91), p < 0.05) and lower back pain (mean (SD) = 6.63 (2.10) vs. 5.55 (1.71), p < 0.01) compared to those without these pains. The association between Aw or A_w+VDV_w with neck pain was close to significance (mean (SD) = 6.35 (1.29) vs. 5.02(2.20), p = 0.08; 7.24 (1.15) vs. 6.01 (2.01), p = 0.07, respectively). All types of MSDs were associated with the subject’s age. Job duration was higher among the subjects with MSDs than those free from MSDs but the difference was significant for upper and lower back pain only.

Table 6

Associations between WBV exposure, age and years in the job with musculoskeletal disorders in dumper operators (n = 65): mean (standard deviation)

	Musculoskeletal disorders
	Neck pain			Upper limb and shoulder pain			Upper back pain			Lower back pain			Knee and leg pain			All combined
	No	Yes	p	No	Yes	p	No	Yes	p	No	Yes	p	No	yes	p	No	yes	p
Age (year)	45.7 (9.8)	52.8 (5.4)	0.04	45.1 (9.8)	53.8 (4.8)	0.001	43.9 (9.5)	52.1 (7.6)	0.001	42.9 (9.4)	49.5 (8.9)	0.006	45.0 (10.1)	50.8 (6.5)	0.014	42.6 (9.3)	49.4 (9.0)	0.004
Years in the job (year)	12.7 (8.8)	16.4 (8.1)	0.385	12.6 (9.1)	15.7 (5.9)	0.196	10.4 (7.3)	19.5 (8.9)	0.001	9.8 (7.0)	16.2 (9.2)	0.003	12.5 (8.9)	14.9 (8.0)	0.350	9.7 (6.9)	15.9 (9.2)	0.003
A_w, ms^–2	5.02 (2.20)	6.35 (1.29)	0.080	5.05 (2.23)	5.63 (1.43)	0.322	4.67 (2.06)	6.22 (2.08)	0.008	4.47 (1.85)	5.76 (2.29)	0.016	5.03 (2.20)	5.47 (2.10)	0.505	4.43 (1.85)	5.72 (2.27)	0.014
VDV_w, ms^–1.75	126.6 (35.7)	147.8 (23.2)	0.113	127.8 (36.5)	131.3 (27.1)	0.741	122.6 (34.6)	141.9 (33.6)	0.044	119.9 (31.8)	136.3 (36.9)	0.050	127.1 (35.6)	132.4 (34.8)	0.623	119.3 (32.2)	135.8 (36.3)	0.050
A_w+VDV_w	6.01 (2.01)	7.24 (1.15)	0.070	6.05 (2.06)	6.42 (1.39)	0.503	5.7 (1.91)	7.1 (1.91)	0.017	5.55 (1.71)	6.63 (2.10)	0.028	6.02 (2.00)	6.38 (1.96)	0.557	5.51 (1.72)	6.60 (2.07)	0.025

The dumper operators who were experiencing two or more types of MSDs had higher A_w (mean (SD) = 5.72 (2.27) vs. 4.43 (1.85), p < 0.05), higher VDV_w (mean (SD) = 135.8 (36.3) vs. 119.3 (32.2), p < 0.10) and A_w + VDV_w (mean (SD) = 6.60 (2.07) vs. 5.51 (1.72), p < 0.05) compared to those without any MSD.

4 Discussion

4.1 Main findings

This study in Indian iron ore mines shows that dumper operators are highly exposed to WBV over the working life and have a 2.7–5.6-time higher risk for upper back pain, lower back pain, knee and leg pain, and cumulating two or more affected body parts. Furthermore, a 10-year age increase exposes the dumper operators to a 2–4-time higher risk for upper limb and shoulder pain, upper back pain, lower back pain and “knee and leg pain”. Overweight operators have a 4-time-higher risk for lower back pain. These findings may help to understand the risk patterns of MSDs among dumper operators, identify the operators most at risk and establish preventive measures in India and a number of countries worldwide.

4.2 High daily vibration exposure and health risk among dumper operators

The present study demonstrates that most dumper operators (59%) were daily exposed to vibration exposure that exceeded the ISO lower limit (0.45 m s-2) based on A(8) values. They were also exposed to multiple shocks (as indicated high crest factor values). We found that for all dumper operators, the crest factor in vertical axis (z-axis) exceeded the limit specified in ISO- 2631 : 1997/Amendment: 2010. Similarly, nearly all dumper operators (91%) were daily exposed to vibration dose value, VDV (8), exceeding the ISO lower limit (8.5 m s^–1.75). It may be noted that the mean values of RMSA, VDV and CF were much lower in the x- and y-axes than in z-axis. These results are consistent with other WBV exposure studies which have also reported higher vibration exposure in z-axis than in the x- and y-axes on track locomotive in United States [64], trucks on test tracks in Canada [65], and haul trucks in an aggregate stone mine operation in United States [66]. The high vibration magnitude in the vertical axis can be attributed to the poorly graded haul roads in the mines. In addition, there were a number of potholes, ruts and bumps on the haul roads, which increased the magnitude of vibration. Furthermore, all the dumpers have mechanical seat suspension system, which are less effective than pneumatic and semi-active magnetorheological damper suspension system in attenuating and isolating the vibration transmitted to operators [67]. A laboratory research conducted by Mayton et al. [67] revealed that application of pneumatic based rheonetic technology might provide heavy vehicles with improved isolation from seat transmitted vibration and will result in a subsequent reduction of MSD problems.

4.3 High exposure of dumper operators to musculoskeletal disorders

The findings indicate that the dumper operators have a high prevalence of multiple MSDs affecting several body parts including upper and lower back pain, shoulder pain and “knee and leg pain” compared to the control group. Lower back pain was the most frequent MSDs affecting half of dumper operators (51%), followed by the upper back pain (30%) and knee and leg pain (22%). For these types of MSDs, the risks, as measured with ORao in reference to the control group, were 2.72, 5.37 and 3.68, respectively. Because of these multiple MSDs, the dumper operators suffered much more frequently from 2 or more affected body parts than the control group (36.9% vs. 13.8%, ORao = 5.05, p < 0.001). The risk of various types of MSDs among dumper operators may be attributed to the presence of a wide range of risk factors including poor haul road condition, awkward posture, exposure to WBV and prolonged sitting position of dumper operators. In addition, the dumper’s seat dimensions did not match with the dumper operator’s anthropometric body dimensions. Despite the fact that dumper operator’s seat height is adjustable and varies from 37 to 45 cm, the backrest of dumper operators’ seat was shorter than the sitting height for 90% of the operators. Hence, a part of the back portions of most operators is unsupported which results in awkward posture and may lead to MSDs. The seat is somewhat curved, its length is larger than the buttock popliteal length for 90% of the dumper operators. Seat’s width is also larger than the buttock width for most dumper operators. The maximum seat height (45 cm) as measured are also shorter than the popliteal height of 95% of the dumper operators. It results in an inappropriate sitting posture for the operators and consequently in an elevated health risk of upper back, lower back, and knee and leg pain. Inclined and uneven haul roads, constructed in the hilly terrain, enhance frequent jerks and shocks and contribute to MSDs. Our results are in agreement with the findings of a few previous studies [68 –71]. Urwin et al. [68] reported that the common locations of pain among the construction workers are back, knee and leg, and shoulder. In a study by Backman [69], around 70% of the truck drivers experienced lower and upper back pain. Netterstrom and Juel [70] reported the prevalence of recurrent upper and lower back disorders among 57% of Denmark’s bus drivers. Mirzaei and Mohammadi [71] observed that the pain among the tractor drivers in Zahedan city concerned mostly back (56.8%) as well as knee and leg (29.5%), and that the WBV was a cause of musculoskeletal pain. Patterson et al. [72] found that the most bus drivers studied suffered from upper and lower back pain and all of them complained about the seat design. Our findings are also in agreement with other researches on drivers and office workers [73 –75].

4.4 Accelerated ageing of dumper operators and role of overweight in MSDs

Our study reveals that the dumper operators had a strong relationship between ageing and MSDs affecting various body parts including upper back, lower back, knee and leg, and upper limb and shoulder (the prevalence increased between 2 to 4 times for each 10-year age increase). It also shows that the dumper operators with overweight had a 4-time higher risk of lower back pain. Immediate action is needed by mine management and it may be targeted to older operators and those with overweight. However, the other operators could not be neglected. Preventive measures at an early age may reduce the MSD problems which are arising due to ageing. In the present study, length of job did not have a significant odds ratio. This suggests that the MSDs could occur even among the operators with a few years with job. Importantly, our study reveals that the MSDs among the dumper operators and the controls could be distinguished by the number of affected body parts. Indeed, having 2 or more affected body parts concerned much more to the dumper operators than the controls (36.9% vs. 13.8%, ORao = 5.05, p < 0.01). For older operators, more muscle force acts across the joints which results in an increased risk of MSDs [77]. The role of being overweight in lower back pain may be explained by a higher-pressure force which acts across the vertebral disc in the spinal system. Due to high pressure force, vertebral discs remain in the compressed position and if an operator remains seated in the dumper seat for a longer period of time while working in an awkward posture, the high compressive force across the vertebral discs results in more severe pain. A number of studies have confirmed the association between age and the prevalence of MSDs [70 , 77–81]. A study carried out by Bihari et al. [79] in the residents of national capital region in India reported that the prevalence of musculoskeletal pain increased with age. A literature review by Woolf and Pfleger [80] highlighted an increase of the prevalence of musculoskeletal complaints with ageing. A study conducted by Anderson et al. [81] among workers in industrial and service companies in Denmark reported a correlation between age and lower back problems. In a study among bus drivers, Sadeghi et al. [82] found a substantial association of lower back pain with worker’s age and weight. Several other studies also showed that overweight workers had more frequently a problem in upper and lower back [83 –88].

4.5 Association of WBV exposure with musculoskeletal disorders

Our study shows that the dumper operators who had higher working-life-WBV-exposure as measured by A_w, VDV_w and A_w + VDV_w had a higher risk of upper and lower back pain. This is a result of interest as it is generally hard to find a significant result with a relatively small number of subjects. They also had a higher risk for neck pain but the result was close to significance, probably due to the number of subjects. Our findings are consistent with those of a few available studies. A review of literature by Bovenzi and Hulshof [14] also reported the relationship between WBV exposure and MSDs affecting neck, upper back and lower back. Several studies reported that increased WBV exposure increased the risk of neck disorders [89 –91].

4.6 Strengths and limitations

The present study is an original investigation, which shows that the dumper operators are highly exposed to both the WBV and multiple MSDs. All the operators contacted participated in the study. By investigating the MSDs for various body parts, we found that the dumper operators were more exposed to cumulated affected body parts, which are strongly correlated with age. Our study has some limitations. It used self-reported data gathered using face-to-face interviews. The sample was rather small but this limitation has been observed in most available studies in the literature. Several tests were significant at the 0.01 level, with high odds ratio values. Our findings need to be confirmed by further research in India as well as in other countries.

5 Conclusion

This case-control study among dumper operators in Indian iron ore mines shows that they are highly exposed to WBV and the risk for a wide range of MSDs including upper back pain, lower back pain, knee and leg pain, and two or more affected locations. It further shows that older workers have a higher risk of most types of MSDs and overweight workers had a higher risk of lower back pain only. Hence, the WBV and the MSDs should be evaluated and monitored over the career, especially for older and overweight workers. Preventive measures are needed to reduce the WBV exposure. They include better machine design, seat design with appropriate seat suspension system to attenuate the vibration transmission, and ergonomic design of seat to provide comfortable posture to the operators. The seat dimensions should be matched with operator’s body dimensions. The duration of WBV exposure needs to be reduced. The altered working environment needs to be improved. The dumper operators should be more aware of their risks for MSDs and accordingly take necessary precaution. The overweight operators may be helped to reduce their body weight by taking proper diet, and physical and sports activities. Our findings are useful not only for dumper operators in mines but also for truck operators who are working in construction sites. These findings may help implementing preventive measures to reduce their MSDs.

Footnotes

Acknowledgments

The authors wish to acknowledge the support received from the management, staff and workers of the case study mines.

Conflict of interest

None to report.

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