Combined stresses due to whole-body vibrations and awkward postures when operating straddle carriers

Status:

completed 12/2022

Aims:

Professional drivers are subjected to a wide range of stresses while carrying out their duties. This includes physical exposures such as whole-body vibrations, noise and awkward postures, as well as psychosocial stresses. These multi-factorial stresses can cause musculoskeletal problems and can even lead to a chronic, irreversible impairment. Because of this, investigations into these combined stresses are very important for prevention in this area. While the effects of individual stresses such as whole-body vibrations, noise and posture have been investigated time and time again, insights into the effects caused by the combination of these stresses are very limited.

The effects of posture and whole-body vibrations in several types of vehicle have been measured and investigated over the course of some projects (IFA 4157, FF-FP0306). During these studies, the effects of the combined stresses associated with such driving activities were presented as examples for some vehicles for the first time (IFA Report 2/2018). In port facilities, in addition to industrial trucks and cranes, straddle carriers (also known as straddle tracks) are a significant element of the transportation industry that has not yet been carefully examined. Straddle carriers are used to transport ISO standard containers from the gantry crane to the stacking area, or to a lorry or train. During this process, the containers are lifted and transported inside the approximately 11-m-high chassis frame after the spreader is locked into place in all four corner fittings on the container. The driver's cab is always positioned on one side of the straddle carrier. As a result of this, straddle carrier drivers are frequently forced to twist their upper body to one side while driving or twist their body around or bent forward while reversing. Musculoskeletal problems in the area of the neck and back are also frequently reported. There are several technical possibilities to minimise these unfavourable postures, such as the use of cameras, automatic spreader operation (spreader automatically locks on to container) or the use of rotatable seats. We were previously unaware of whether these preventative measures were being used and how much of an effect they had on the wellbeing of drivers.

The aim of this project was to investigate the level of exposure to whole-body vibrations and stresses resulting from adopting awkward postures while driving a straddle carrier and to analyse which measures are being used in companies to reduce these stresses. To this end, the impact of these prevention measures are to be analysed by means of a questionnaire focused on the personal experience of the drivers and their current state of musculoskeletal health.

Activities/Methods:

The postures were recorded using the sensor-based Xsens motion capture system, which was able to be incorporated into the work clothes of the drivers. Following this, the recorded body angles were analysed and evaluated using the "Widaan" software, which was developed by the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA).

Whole-body vibration was measured and analysed in the three orthogonal axes on the seat and on the seat mounting point using a sensor solution developed in the IFA. In this way, it was possible to measure the extent to which the seats reduced or amplified the acceleration values transferred to the driver.

Furthermore, a video recording was taken and used for the evaluation of the activities. As well all data, this was used anonymously.

To record the health complaints and the personal experience of the drivers with regard to exposures, a simplified survey consisting of a Nordic questionnaire and the Borg scale was used.

Results:

A total of 53 straddle carrier operators working at three different port facilities were studied with regard to the physical stresses they are subjected to as a result of exposure to vibration and awkward postures. 21 male straddle carrier drivers and 2 female straddle carrier drivers were recorded using the sensor technology while performing their routine duties. In addition, 30 straddle carrier drivers volunteered to complete a survey in relation to their exposure to stresses and musculoskeletal problems.

The vibration stresses experienced during the operation of a straddle carrier were comparable in all three port facilities (awz=0.26 ms^-2 to 0.30 ms^-2 on the seat in the vertical direction). Due to the low acceleration values, it was only possible to reduce the vibrations transferred to the drivers by a small amount. The vibration values during a driving shift, which typically lasted six hours, were below the action values for prevention as specified in the Noise and Vibrations Occupational Safety and Health Ordinance (LärmVibrationsArbSchV).

In contrast to the low vibration exposure, the posture analysis showed a high proportion of time spent in unfavourable, non-neutral position for almost every postural position that was investigated. In the sagittal plane, high percentages of time in a non-neutral position were observed at all facilities for head inclination (up to 49%), neck flexion (up to 41%) and inclination of the thoracic spine (up to 33%). In the coronal plane, high percentages of time in a non-neutral position were measured for the neck flexion (up to 43%) and the back flexion (up to 30%). The required postures largely came about during activities in which the test subjects had to look to the side and downwards at the containers. A notable finding is the percentages of neck and back torsion in facility 2 (up to 46%), which, unlike at the other facilities, occurred in both direction (to the left and to the right). A reason for this could be the position of the driver's cab, which, unlike in facility 1 and facility 3, is positioned on the right-hand side. During the driving activities (driving with a load and without a load), the test subjects in facility 2 usually sat at an angle to the driving direction, while this seat setting was not possible or not used in the other facilities. Another notable finding was a certain position that some test subjects in facility 2 adopted in which they rested their arm towards the back of the side handle. In the context of this study, it was not possible to investigate whether this posture should be preferred from an ergonomic standpoint. Another laboratory study should be carried out on the basis of electrical muscular activity and stress questionnaires to answer this question.

The use of technical aids such as cameras or rotatable seats could not be accurately investigated, as these were very rarely fitted or not used at all. The level of acceptance for rotatable seats (when present) was very low for reasons of familiarity. A future intervention study would be useful in order to investigate the effectiveness of this measure and promote it as a preventive aid.

With regard to the health-related wellbeing of drivers, a very high level of complaints relating to the area of the lower back (up to 60%) and neck (up to 50%) were recorded in all three facilities. A high level of incapacity for work was also reported as a result of back problems. The self-reported ratings of vibration exposure by the test subjects were overestimated in all three facilities. A significant overestimation was observed in facility 3, which was once again confirmed by the rating of the overall exposure according to the Borg scale (median of 5; heavy exposure). The overall exposure in facilities 1 and 3 was rated as medium (median of 3.4-4).

As part of this study, a personal report was created as a form of feedback. This personal report can be made available to each test subject in future. The report provides information about physical exposures (vibration and awkward postures) and notable findings during the measurement process. In addition, corrective exercises (featuring an image and description) were found to counteract the stresses recorded during the measurement process. These exercises show movements that can be performed in the vehicle during waiting periods and outside of the vehicle.

In summary, all straddle carrier activities involve high levels of posture-related stress, although the vibration exposure was at a similarly low level at all three facilities. The high level of back and neck complaints reported and the resulting level of incapacity for work show the importance of considering the impact of awkward postures during exposure to vibrations. Therefore, further studies are required to trial and present preventive measures.

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