It is estimated that 4% to 7% of the labor force in Canada, the United States and European countries are exposed to whole-body vibration (WBV) that increases their risk of harmful health effects [1
]. Changes in mining work practices and a demand for increased productivity has seen almost all heavy physical work in mining replaced with sedentary work and mechanized equipment operation [2
], the latter resulting in increased exposure to WBV. Operators of load-haul-dump (LHD) vehicles, in particular may be at increased injury risk due to postural demands and WBV exposure levels associated with harmful effects to the human body [3
Although the human body dampens most vibration frequencies transmitted through the operator-seat interface, WBV between 1 Hz and 20 Hz results in resonance of the spinal column, pelvis, internal organs and soft tissues [6
]. Health effects associated with short term WBV include muscle fatigue, discomfort, distorted motor performance, headache, loss of balance, motion sickness, increased heart rate, hyperventilation, decreased cognitive functions, as well as, diminished speech and vision [8
]. An even greater concern arises from chronic health effects associated with regular exposure to WBV, which include: spinal degeneration, spinal disc disease, disc failure, sciatic pain, herniated discs, low back pain, and gastrointestinal disorders [1
Previous research has demonstrated that WBV exposure can cause mechanical overloading of the spine [1
]. Subsequently, spinal degeneration occurs from increased internal forces where mechanical damage to the anatomical structure of the vertebrae is present [10
]. Furthermore, the magnitude of internal forces is also affected by muscular activity in response to WBV, whereby muscles alternate cycles of increased and decreased activity, which also increases the risk of spinal instability when muscles are in a relaxed phase [10
]. This behavior is further exaggerated when awkward bending postures are sustained, whereby activity of multifidus muscles (spinal stabilizing muscle) is decreased [11
]. In particular, operators of LHD vehicles frequently sustain awkward driving postures to maximize driver sight lines [12
], which increase their risk of musculoskeletal injury.
Measuring and assessing WBV of a seated mobile equipment driver, at the operator-seat interface, is most frequently accomplished with the International Organization of Standards report (ISO 2631-1) [13
]. Aside from clearly describing methods for quantifying WBV, the ISO 2631-1 standard also presents guidelines for safe exposure limits to WBV in relation to human health, comfort, perception and motion sickness [13
]. In accordance with ISO 2631-1 guidelines, WBV exposure is evaluated with the frequency weighted root-mean-square (r.m.s.) acceleration method when crest factors (CFs) are less than nine [13
]. However, this standard also presents an alternative method of analysis, the fourth power vibration dose value (VDV), which is suggested when CFs are greater than nine [13
]. Under this circumstance, the fourth power VDV is a superior indicator of WBV exposure since it is more sensitive to multiple shocks [14
]. When considering the 8-hour frequency-weighted r.m.s. acceleration value A(8), the ISO 2631-1 report defines the upper and lower limits of the 8-hour health guidance caution zone (HGCZ) as 0.45 m/s2
and 0.90 m/s2
]. Similarly, when considering the VDV, the upper and lower limits are 8.5 m/s1.75
and 17 m/s1.75
Another method for the evaluation of negative health effects associated with WBV exposure is outlined in the European Union Directive 2002/44/EC. The A(8) daily exposure limit value and daily exposure action values established in the European Union Directive 2002/44/EC are 0.5 m/s2 and 1.15 m/s2 respectively while the VDV daily exposure action value and daily exposure limit value are 9.1 m/s1.75 and 21 m/s1.75 respectively.
Despite the improved predicting power of the VDV in relation to human health, comfort, perception and motion sickness, recent research concluded that existing standards did not adequately describe human response to WBV when multiple shocks are present [14
]. Furthermore, it was argued that the ISO 2631-1 standard failed to identify upper and lower exposure limits and associated risk of injury based on the VDV [14
]. The United States Army Aeromedical Research Laboratory developed the Health Hazard Assessment (HHA) method, which they used to predict the risk of lumbar spinal injury to Tactical Ground Vehicle operators exposed to WBV with multiple shocks [14
Early success for evaluating the risk of adverse health effects from WBV containing multiple shocks with the HHA method was used to establish ISO 2631-5 [15
]. Health predictions using this assessment method are based on the biomechanical response of the lumbar spine to WBV; this contrasts with the VDV approach, which is based solely on the mathematical properties of the vibration signal [15
]. The ISO 2631-5 standard utilizes biodynamic models to predict spinal acceleration, regression models to predict peak L4/L5 compressive stress, and cumulative models to assess repeated shock and injury probability [16
]. Furthermore, two separate biodynamic models are utilized including a single degree-of-freedom model to predict the L4/L5 response to shocks in the X and Y-axes, and a recurrent neural network model that predicts L4/L5 response to shocks in the Z-axis only [16
These aforementioned models are used to calculate predictors of adverse health effects including the daily equivalent static compressive dose (Sed
), as well as, the cumulative risk factor (R). Over the course of a typical working day, a Sed
value less than 0.5 MPa indicates low probability of an adverse health effect, whereas a Sed
value greater than 0.8 MPa indicates high probability of an adverse health effect at lifetime exposure [15
]. Similarly, over an average career, an R-value less than 0.8 indicates low probability of adverse health effect, whereas an R value greater than 1.2 indicates high probability of an adverse health effect [15
While research suggests that LHD vehicle operators exposed to regular WBV are more likely to suffer from adverse health effects based on ISO 2631-1 guidelines, limited research has documented health risks predicted by the ISO 2631-5 standard [4
] or the newer EU Directive 2002/44/EC exposure guidelines. Reports of musculoskeletal discomfort associated with measured vibration exposures are also limited. Consequently, the primary objectives of this study are to (1) determine the body area associated with musculoskeletal discomfort during the operation of large and small LHD vehicles, (2) determine if there is a difference in probability of adverse health effects according to ISO 2631-1 and EU Directive 2002/44/EC when frequency weighted r.m.s. acceleration values are considered, (3) to determine if there is a difference in probability of adverse health effects from impulsive vibration when VDVs (ISO 2631-1; EU Directive 2002/44/EC) are compared to relative risk predicted by the Sed
and R factor values (ISO 2631-5); and (4) to determine if the probability of adverse health effects are different for small and large LHD vehicle operators.
4. Experimental Section
4.1. Selecting LHD Vehicles and Mine Sites
Researchers from Laurentian University and members of the technical advisory committee for underground equipment from the Mines and Aggregates Safety and Health Association (MASHA) of Ontario, Canada, approached several northern Ontario mine companies concerning their possible participation in this study. Testing occurred at five mining sites operated by three different mining companies. LHD vehicles were selected from a sample of convenience. Seven LHD vehicles (different makes and models) with a bucket haulage capacity greater than 4.6 m3 and six LHD vehicles (different makes and models) with haulage capacity less than 4.6 m3 were tested. LHDs are used to move ore and rock throughout an underground mine and the operator sits sideways to the direction of travel.
4.2. Selecting LHD Operators
Load-haul-dump vehicle operators were selected from a sample of convenience. Participating mine sites allowed researchers to notify LHD operators about the current study. Thirteen LHD operators (all male) with mean age, work experience, mass, and height of 47 ± 10 years, 19 ± 10 years, 90 ± 14 kg and 1.80 ± 0.06 m agreed to participate in the current study. Prior to their involvement, participants were asked to sign consent forms approved by Laurentian University’s Research Ethics Board. Participants also provided information in the form of a questionnaire detailing work experience (vehicle types driven; age of first exposure to vibration; daily vibration exposure; years of exposure to vibration), and work related musculoskeletal pain/discomfort. Operators were shown a picture with a body map indicating the body region for the head, neck, upper back, lower back, right/left shoulder, right/left elbow, right/left wrist/hand, right/left thigh, right/left knee, and right/left ankle/foot. Operators were then asked to mark the region on the body associated with work related discomfort in the last six-months and indicate the severe of the pain/discomfort between 1–4 with 1 representing mild pain/discomfort and 4 representing very severe pain/discomfort. To simplify presentation, reports were later grouped as follows along with the mean severity score; H/N = head/neck; UB = upper back; LB = lower back; R-Arm = right shoulder, right elbow and/or right wrist/hand; L-Arm = left shoulder, left elbow, and/or left wrist/hand; R-Leg = right thigh, right knee, and/or right foot; L-leg = left thigh, left knee, and/or left foot. The questionnaire was completed at the mine site but before starting vibration measurement.
4.3. WBV Measurement and Collection Procedures
Researchers asked the participating LHD vehicle operators to muck with the LHD vehicle in their designated work area for approximately 1-hour before returning to another designated area to remove the testing equipment. Mucking is a cyclical process that involves loading the LHD vehicle with ore and rock, driving the vehicle to a dumping zone, and then driving back to the loading zone empty in order to repeat the loading and dumping process. The time to complete one cycle varied according to the mine layout due to the relative locations of the development heading and dumping zone, tunnel width, road grade, and number of sharp left or right turns in the road bed. Most LHD vehicle operators completed one mucking cycle between 5 min and 10 min. Accordingly, the number of work cycles completed within the 1-hour window varied between participants and typically included 6 to 12 mucking cycles.
Whole-body vibration exposure measurements were recorded according to common international standards [13
]. Measurements were recorded at the operator-seat interface with a Series 2, 10 g tri-axial accelerometer (NexGen Ergonomics, Montreal, Quebec, Canada) in combination with a P3X8-2C DataLOG II datalogger (Biometrics, Gwent, United Kingdom). The accelerometer measured vibration in the fore-and-aft (x-axis), lateral (y-axis), and vertical (z-axis) axes with less than 5% cross talk.
The Series 2 tri-axial accelerometer was mounted in a rubber seat pad and secured to the seat surface so that it was fixed between the buttock of the operator and the seat. Vibration data were recorded at 500 Hz using a 13-bit analog-to-digital conversion with a resulting resolution of 0.0025 g at the ±10 g full-scale range. The raw WBV signals were recorded and saved to a SD memory card with a 512 MB storage capacity used by the datalogger. Subsequently, the raw data were transferred to an Intel® Pentium® 4 computer and analyzed with Vibration Analysis Tool Set (VATS) version 3.1.0 software developed by NegGen Ergonomics Incorporated (Montreal, Quebec, Canada).
4.4. WBV Analysis
4.4.1. Analyses Conducted in Accordance with ISO 2631-1 and EU Directive 2002/44
Whole-body vibration analyses were conducted in accordance with procedures and guidelines outlined in ISO 2631-1 [13
]. In compliance with this standard, frequency-weighted r.m.s accelerations were calculated for three orthogonal axes (awx
, and awz
]. Appropriate frequency-weighting curves (x-axis Wd
; y-axis Wd
; z-axis Wk
) and scaling factors for health (Kx,y
= 1.4 and Kz
= 1) were applied to each axis [13
]. A vector sum of the frequency-weighted r.m.s acceleration (av) was also calculated [13
The absence or presence of transient shocks was initially evaluated by calculating peak accelerations in each axis (x, y and z axes), which were used to calculate CFs (CFx
]. Next, vibration dose values (VDVs) in each translational axis (VDVx
) were calculated and considered whenever the crest factors exceeded nine [13
]. Although vibration at the operator-seat interface was continuously recorded, all aforementioned vibration measures (awx
, and awz
: peak accelerations; CFx
) were also calculated for successive 5-minute intervals and averaged over the entire trial to result in more representative crest factor values.
Lastly, the 8-hour equivalent frequency-weighted r.m.s. acceleration A(8) and the 8-hour equivalent vibration dose value (VDVtotal
) were calculated in accordance with the ISO 2631-1 report [13
] and previously reported assumptions [5
]. Eger and associates estimated that daily LHD vehicle operation is approximately 7 h in an 8-hour shift [5
]. The remaining time accounts for breaks, time traveling to and from the underground site (walking or personnel carrier), and moving to designated underground work areas [5
The 8-hour equivalent frequency-weighted r.m.s. acceleration value A(8) and VDVtotal
were subsequently compared to the ISO 2631-1 HGCZ and the EU Directive 2002/44/EC daily exposure action limit and limit value. The upper and lower limits of the ISO 2631-1 HGCZ are 0.45 m/s2
and 0.90 m/s2
respectively for A(8) and 8.5 m/s1.75
and 17 m/s1.75
respectively for VDVtotal
]. The limit values are slightly greater according to EU Directive 2002/44/EC. The A(8) daily exposure action value and daily exposure limit values are 0.5 m/s2
and 1.15 m/s2
respectively while the VDV daily exposure action value and daily exposure limit value are 9.1 m/s1.75
and 21 m/s1.75
4.4.2. Analyses Conducted in Accordance with ISO 2631-5
ISO 2631-5 is concerned with probable health risks to the lumbar spine from exposure to vibration containing multiple shocks. A tri-axial accelerometer at the operator/seat interface is used to determine the number and magnitude of peaks associated with the WBV exposure [16
]. These data are used to calculate the daily equivalent static compression dose at the lumbar spine (Sed
). A risk factor based on variables such as driver age, daily exposure, yearly exposure, lifetime exposure, and starting age was subsequently computed using two approaches. An individual operator profile (IOP), R factor was determined from each operator’s age, daily, yearly and lifetime exposure to WBV. Whereas a lifetime R factor was determined using a typical operator profile (TOP) estimated to represent WBV exposure levels accumulated for operation of a LHD over a lifetime [18
4.4.3. Comparison of Health Risks Probability
The probability of adverse health effects across the standards were compared for A(8), VDV, Sed
and R-factor according to Table 7
. Agreement of ISO 2631-1 and EU Directive 2002/44/EC with respect to probability of adverse health effects for A(8) were compared. The probability of health risks based on VDVtotal
, and R Factor values were also compared since each of these measures considers injury risk due to impulse shocks. These measured were selected as they represent a common method often reported in the literature (ISO 2631-1), a required standard when evaluating vibration exposure in the European Union (EU Directive 2002/44/EC), and a relatively new standard promoted to evaluate vibration containing impulse shocks (ISO 2631-5).
Summary of the measures calculated and compared to determine agreement with respect to probability of adverse health effects.
Summary of the measures calculated and compared to determine agreement with respect to probability of adverse health effects.
|Comparison||Vibration Exposure Evaluation Methods|
|ISO 2631-1||EU Directive 2002/44/EC||ISO 2631-5|
|1||X|| ||X|| || || |
|2|| ||X|| ||X||X||X|