Effect of prolonged sitting on body-seat contact pressures among quay crane operators: A pilot study

1 Introduction

In modern commercial ports, containers are transferred to and from vessels by means of large quay cranes. Crane operators work inside a cabin suspended on a trolley at 30–40 m above ground. This working environment can be challenging, owing to high noise and vibration levels associated with trolley movement [1] and high levels of performance pressure to maximize economic efficiency. Moreover, operators adopt relatively constrained sitting postures for prolonged periods (typically 4–6 consecutive hours), while controlling the container position with short hand movements using joysticks and receiving feedback on correct operation through direct visual contact through transparent walls and floor of the control station (Fig. 1). As such, operators are required to adopt prolonged static and non-neutral postures, particularly of the neck and trunk, conditions that represent a risk factor of musculoskeletal problems [2, 3].

The particular position assumed during container handling tasks requires that a majority of body weight is supported by the seat pan, while additional portions are transferred to the armrests, seat back, backrest, and two footrests built in the control station and suspended over the transparent floor [4]. Seat design is thus important, as non-optimal seat shapes may generate undesirable stress concentrations which, in cases of long exposures, may cause discomfort, pain or, in the worst scenario, pressure sores [5]. Such adverse outcomes may result in deterioration of the performance [6] in terms of operational efficiency (i.e., reduction in the number of containers loaded/unloaded) and the safety of materials or other workers involved in the process.

Previous studies show that there are different experimental methods (surface electromyography, posture assessment and body-seat interface pressure analysis) able to integrate the subjective perceptions of comfort of the workers to supply a more effective assessment of the design goodness of seats and chairs [7]. Among them, contact pressure at the body-seat interface has been found to be the parameter most clearly related to subjective ratings [7–9], and thus is widely used to characterize comfort levels in the automotive industry [10–15] as well as in office chairs [16, 17].

However, it must be remarked that most experiments performed to assess pressure distribution in the sitting posture (both among workers and recreational/professional drivers) were often limited in duration (5 to 45–65 minutes, with some exceptions [11, 19]). While in some cases this is fully justified by the actual duration of the sitting task (for example, most car trips in the United States last 20 minutes or less, [9]) some authors have stated that at least 2 hours of test time are needed to have a reliable perception and assessment of comfort [20].

In the specific case of crane operators, having available information about the whole duration of their shift would be of some interest, as they are forced to maintain the same constrained sitting posture for several hours. Thus, the present study aimed to perform long-term monitoring of the body-seat interface contact pressure in a sample of professional quay crane operators by collecting pressure data under conditions similar to those encountered in the actual working environment. Given the limitations imposed by security measures (which prevent data collection in commercial ports during regular vessel loading/unloading operations) the experimental tests were performed in a dedicated simulator where the quay crane operator performs container movement tasks using the same control station and for a shift duration equal to those adopted in the real working environment. The main objective of the study was to investigate the variations in contact pressure in the seat pan and backrest that occur across a realistic work shift. Results are discussed to highlight the possibility of this approach to detect postural changes associated with a prolonged constrained sitting posture.

2 Methods

3 Results

The results obtained from experimental tests, which refer to the mean values across the 8 participants calculated from linear fitting performed on the raw pressure data, are summarized in Fig. 4. An example of pressure distribution on the seat-pan mat during a trial is provided in Fig. 5.

The mean contact pressure calculated on the whole surface of the two mats (Fig. 4, top) shows that the pressure on the backrest decreased from 5.5 to 5.2 kPa, while in the seat pan the pressure increased from 7.4 to 8.2 kPa. The detailed analysis of the 4 sub-regions (Fig. 4, bottom), shows that at the beginning of the experiment, the contact pressure on the buttocks was 9.1 and 10.6 kPa respectively for the left and right side, while lower values were observed on the thighs (4.4 kPa left, 5.6 kPa right). As the trial progressed, the buttock mean pressure tended to decrease, with a more marked trend on the left side (a 6% reduction was observed after 4 hours). In parallel, pressure on the thighs tended to increase (10 and 20% respectively for right and left side). It is worth noting that the right limb had higher contact pressures in both sub-regions.

4 Discussion

The purpose of the present study was to assess the feasibility of long-term monitoring of seat-body interface pressure in a simulated environment, with a goal of evaluating this method to identify the existence of postural modifications during a regular work shift in quay crane operators. Given that measurements under actual conditions are difficult to perform, given the limited space in the quay cockpit and the critical conditions in which the task develops, it was considered important to have preliminary data acquired in simulated environments to support and justify future investigations.

To the authors’ knowledge, this is the first study that attempted to investigate body-seat pressures in the case of long-duration work shifts, and thus it is difficult to compare our data with those of previous studies.

Our results suggest that as work progresses, operators modify their posture in such a way as to reduce stress on the buttocks (likely to attenuate discomfort). This appears to occur by shifting a larger part of their body weight to the thighs which, not have bony tuberosities (as occurs in the ischial region), are less prone to stress concentrations. However, from a global point of view, the net effect is represented by an increase in the mean contact pressure on the seat pan over time. The latter effect is consistent with the findings of Callaghan et al. [13], who observed increases in average seat pan pressure in one hour of simulated driving. Nevertheless, when the analysis focused on the buttocks region, the same authors also detected a marked linear increase in contact pressure over time (44%), while in our case, the pressure tended to decrease.

Of note is that data from the present study related to the buttocks region are in agreement with those of Na et al. [11] and Jin et al. [18], who reported respective reductions in contact pressure of 5.5–8.6% during 45 and 100 min tests using a driving simulator. In both studies, this phenomenon was explained by a change in posture consisting of a forward movement of the hip joint.

In other similar situations, no specific trends in body-seat pressures were observed, as reported by Albert et al. [19] in the case of 15 bus drivers tested for 65 minutes under actual working conditions. The authors speculated that more relevant information about postural strategies and adaptation during the work shift can be extracted from the analysis of the position of the center of pressure (COP), rather than by pressure distribution alone.

It is also interesting to observe that differences were found between the left and right sides of the body in both the thighs and buttocks, with contact pressure values being higher in the latter. This can be explained by considering the command arrangement of the control station: the right-hand joystick controls four maneuvers (left/right crane movement, container/spreader engagement/disengagement), while the left joystick controls only spreader hoisting/lowering. Thus, it is reasonable that as more operations are performed with the right arm, some kind of postural shift towards that side may occur. A similar phenomenon was observed by Albert et al. [19] in bus drivers, who tended to continuously shift their position towards one side of the seat as the task progressed. It appears that workers have a sort of asymmetry in terms of load distribution when they are forced to remain seated for a long time. At present, though, we are unable to clarify if this is a need caused by the particular tasks they have to perform with both upper and lower limbs, or by a specific distribution of the controls on their consoles. On the other hand, asymmetric postures have frequently been observed in car drivers due to different tasks and postural requirements placed on each lower extremity[12, 22].

Finally, the lower pressure values observed in the backrest are likely to be due to the particular position assumed by the operators during their activity; this position requires a forward flexion of the neck and trunk as operators visually check the carried load position through the glass floor of the cabin and thus allows only limited and occasional contacts with the backrest. In this case, comparisons with previous studies are not possible, as most of them focus on drivers, and in the usual driving position almost the entire back is in contact with the backrest, thus generating contact pressures that appear very similar to those measured in the seat pan [13].

Some limitations of the study must be acknowledged. First, given the nature of the study (which was mainly a pilot study to assess the feasibility of long-term monitoring of body-seat interface pressure), and due to the limited size of the tested sample, only descriptive statistics were reported. Additional analysis, carried out on the basis of previous similar studies, suggests that 16–26 participants are needed to achieve power of 80% and thus understand whether the body-interface pressure changes here observed are significantly altered over time.

Second, our sample was composed of operators who, even though all are assigned to the same task, had substantial differences in experience (range = 5–13 years). In future work, it would be of value to determine whether experience plays a role in the adoption of ‘smart’ postural adjustments and, correspondingly, if different trends in body-seat interface contact pressure exist. Third, the effect of different anthropometric characteristics was not taken into account, though it is known that stature, for example, affects neck, shoulder, buttock and thigh comfort, and thus may be responsible for differences in body positioning on the seat [11, 12].

5 Conclusions

Analysis of body-seat interface contact pressures is a useful tool in investigating changes in posture during working tasks that require prolonged sitting. Data presented here from quay crane operators revealed that during the work shift operators tended to transfer part of the body load from the buttocks region to the thighs, probably to reduce stress concentrations due to the presence of the ischial tuberosities, while contact of the back with the backseat was very limited. However, it remains to be clarified whether such postural changes are associated with possible degradations in working performance. To this end, future developments of the research will focus on compiling postural data along with other biomedical parameters (muscular activity, heart rate, eye movements) and analyses productivity (e.g. number of transferred containers per hour), performance (deviation from ideal trajectory of the container during the movement of the crane), and adverse events (collisions).

Conflict of interest

The authors report no conflict of interest.