The Ohio State University
Musculoskeletal disorders (MSDs) continue to be a common and costly problem12 in the manufacturing sector. The auto industry is one of several industries that have high incidence of MSDs15. Effective ergonomic interventions may reduce exposure to awkward postures and other risk factors resulting in lower risk of MSDs. One potential solution for reducing awkward posture is a rotating auto-body carrier conveyer system which allows the vehicle to be rotated at a variety of angles during the assembly process (Figure 1). However, it is unknown (from a biomechanical standpoint) if or how much the rotation of the vehicle will influence exposure to MSD risk factors. Thus, the goal of the project was to quantify exposure to MSD risk factors as a function of vehicle rotation angle during assembly tasks.
Figure 1: Car body rotated on ROFA system
Six experienced and six inexperienced workers were recruited in the study. On average, the experienced workers had 17 years of auto assembly experience. The average(standard deviation) age of the subjects was 35.8 years (13.1 years), while height and weight of the subjects were 178.5 cm (7.1 cm) and 80.6 kg (10.2 kg) respectively. The inexperienced subjects received three training sessions on all the tasks prior to testing. The experienced subjects also had one day of training in order to familiarize them with the rotated conditions.
Figure 2: Vehicle regions tested
There were two independent measures, region and rotation angle. The vehicle was divided into 7 representative working regions based on vertical height and horizontal reach distance (Figure 2). The second measure was auto rotation angle and was dependent upon the region of the vehicle.
There were a multitude of dependent measures including 1) spine load (compression, lateral shear, anterior/posterior shear), 2) spine posture (sagittal, lateral and twist), 3) normalized shoulder electromyography (EMG) of the lateral and anterior deltoids, 4) shoulder posture flexion and abduction, 5) normalized neck emg superior trapezius, 6) neck posture, and 7) wrist posture.
Figure 3: Instrumented subject peforming a task with the car rotated 45°
An automobile rotate carrier conveyor (Rosenheimer Forderanlagen ROFA)™ was used to rotate the vehicle from the standard zero or horizontal assembly-line condition to a maximum of ninety degrees. The lumbar motion monitor (LMM), a tri-axial electrogoniometer, was used to measure low back position, velocity and acceleration in all three planes of the body and has been previously validated12. A wired electromyography (EMG) system (Delsys, Boston MA) was used to measure muscle activity of the latissimus dorsi, erector spinae, rectus abdominus, external obliques, internal obliques, lateral deltoid, anterior deltoid, and superior trapezius muscle. All surface EMGs were collected on both the right and left side. The electrodes were applied using standard placement procedures14. Nine magnetic/gravitational sensors (Xsens Technologies,™ Enschede, The Netherlands) were placed on the torso, upper and lower legs, upper arm and head in order to track body posture during the experimental conditions. Finally, goniometers were used to measure elbow and wrist motion. Figures 3 and 4 show a fully instrumented subject performing the task in the engine room front bumper region at 45° and 90°, respectively.
Figure 4: Instrumented subject performing a task with the car rotated 90°
The order of the regions was completely randomized and each region had one installation task for the subjects to perform. The installation tasks were side airbag install, seat belt install, wiring harness install, wheel well liner install, fuel canister install, wiper motor install and brake-line install, for region 1-7, respectively. All tasks were actual assembly tasks simulated for the study and all subjects performed all trials. The study took place at the Center for Occupational Health in Automotive Manufacturing (COHAM), a laboratory at The Ohio State University. The installation tasks required 25-55 seconds depending on the task.
Figure 5: EMG-assisted biomechanical model
The raw EMG signals were processed and then imported, along with the kinematic data, into the EMG-assisted model created in the MD ADAMS software (MSC.Software) environment with the LifeMOD (LifeModeler, Inc.) biomechanical plug-in (Figure 5). While there is no practical way to directly measure spinal forces in vivo, using the ADAMS software to simulate dynamic loads on the body allows our EMG-assisted biomechanical model to estimate the spine forces resulting during the assembly tasks1,2,3,4,5,7,8,9,10,11. The model was also used to calculate shoulder, neck and wrist angles using cardan angles13. All shoulder and neck EMG data was normalized to maximum exertions for that muscle.
Figure 6: Rotation angle summary
Figure 6 shows the overall MSD risk assessment from the dependent measures for each of the tasks at each of the rotation angles. Red indicates the greatest risk, yellow indicates moderate risk and green indicates the least risk.
This study shows that our ability to rotate the car body can significantly reduce the worker’s risk exposure to several risk factors associated with MSDs. Rotating the car body significantly decreased spine load, spine posture, shoulder posture and neck posture, thus the risk to a multitude of MSDs could be reduced by rotating the car from the standard zero assembly condition.
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Sue A. Ferguson, Williams S. Marras, W. Gary Allread, Gregory G. Knapik, Kimberly A. Vandlen, Riley E. Splittstoesser, and Gang Yang
The Ohio State University Biodynamics Laboratory