A quasi-experimental study was designed to evaluate the mean recommended holding time in common postures of different body parts in office jobs. This cross-sectional study was performed in the laboratory during the first quarter of 2021.
orty student volunteers (20 male and 20 female) participated in the study. Inclusion criteria included having a normal BMI in the range of (18.5 to 24.9 kg/m2 ) (25) and no apparent physical and mental problem (self-reported). Before the study, participants signed an informed consent form approved by the ethics committee of Shiraz University of Medical Sciences.
2.2. Data gathering tools
- Demographic questionnaire:
The questions asked in this survey covered personal details such as age, gender and level of education.
- Borg CR10 scale for recording body discomfort:
The 10-point rating scale for recording body discomfort is commonly used in ergonomics studies for rating the perceived discomfort. This study used this scale to assess the perceived discomfort in different body parts (trunk, neck, and legs) from 0 (no discomfort) to 10 (extreme discomfort, almost maximum) (21).
Volunteers were invited to attend the laboratory to complete a demographic questionnaire and then their weight and height were measured for body mass index (BMI). The musculoskeletal health of the individuals was assessed based on their description of any pain and movement limitations during physical activities at work, home or leisure time and based on examining the movement of their body parts.
After selecting the participants, they were informed about the objectives of experiments, postures to be studied, duration of experiments, experiment settings, and the Borg CR10 scale.
The workstation used for the experiment was equipped with an office chair with adjustable height and footrest. The chair's backrest was at an angle of about 95 degrees concerning the chair seat (if the trunk reclined against the backrest, the angle would increase up to 110 degrees). All participants were adjusted to an upright position, which means that the upper arms were almost vertical, the lower arms were close to the body and were horizontally laid on the table, the thighs were approximately horizontal, the lower legs were vertical, and the knees were at approximately 90° of flexion (13).
After setting up the workstation, the Borg CR10 scale was used to assess the discomfort in different body parts before starting the experiments as a reference.
The experimented postures were common static sitting postures of the trunk (seven postures), neck (six postures), and legs (four postures) during office work and working at an office computer in an ergonomically adjusted workstation presented in Table 1.
This study used a within-subject design in which the subjects participated in all posture experiments (17 postures). Consequently, counterbalancing was conducted to eliminate the order and carry-over effects, which can act as disturbing external variables and make it difficult to interpret the results of within-subject experiments. In this process, the order of presenting different postures to the participants changed. Therefore, each participant's order and carry-over effects were counterbalanced by the order and carry-over effects on the other participants. In this way, each of the 17 postures was selected randomly based on drawing.
After drawing and determining a posture for the experiment, participant was asked to maintain the posture at the workstation for 4 minutes suggested by Grandjean, as a maximum holding time of a comfortable static posture(22). Although in most studies, the duration of discomfort tests is considered one minute or less (26, 27),given that most of the postures studied are in the comfortable category, this study used the 4 minutes limit prescribed by Grandjean. Also, the short test duration caused the subject to feel very minimal discomfort, which increased the error in expressing the discomfort score by the subject. Excessive test duration could also cause harm to the participant or cause an error in defining the score due to the boredom.
A goniometer was used to adjust the angles of the trunk, neck, and legs. The proximal (fixed) arm and the center of the goniometer were placed on a joint, while the distal (moving) arm was aligned with the target limb (23) ( The adjustment of the angles of different body parts will be explained in the next section (2-4)).
The participants were asked to look straight ahead during the experiment (4 minutes) and place their hands on the desk to mimic working with a computer with no task to prevent the cognitive workload from distracting their focus on the perceived discomfort.
In each experiment, a specific posture had to be maintained for a body part, and the participant was asked to maintain the other body parts in a neutral position. However, keeping the other body parts in a neutral position was not obligatory, and the participant was allowed to change the positions.
The Borg CR10 scale was given to the participant after 4 minutes to specify the discomfort in different body parts (trunk, neck, and legs) due to the discomfort in the target part after the experiment.
Each experiment was followed by a one-minute walk to relieve the potential fatigue and boredom. The experiments were performed for all postures (17) in two morning and afternoon sessions.
The difference between the degrees of discomfort in the target part before and after each experiment was used to obtain the net discomfort caused by the maintained posture.
Previous studies have shown that body part discomfort measured by the Borg CR10 scale [1982, 1990] at group level increases linearly with increasing holding time and is independent of the MHT magnitude (21, 28).
Miedema et al. have recommended that the mean discomfort of a group of participants working in static postures must not exceed a score of 2 based on the Borg CR10 scale (low discomfort) (21). In the present study, based on the same criteria, the recommended holding time of a posture was estimated for each body part based on the linear relationship of discomfort and the duration of maintaining the posture.
By obtaining the difference between the discomfort scores before and after the experiment, the time (T) for a discomfort score of 2 was estimated assuming the linear increase of discomfort score with time, as follows:
T = (4×2)/(Net Discomfort Score) (21). The time was an estimate of the recommended holding time for trunk, neck, and legs in static posture experiment.
A flow diagram of the study process is shown in (Fig.2).
2.4 Adjusting angles of different body parts of subjects
For neck postures (except for neck rotation), the goniometer center was approximately placed on the seventh cervical vertebra (C7). With the neck in an upright position, the proximal arm was aligned with the imaginary line that connects the nasion and the inion. The distal arm was rotated simultaneously with the head to the desired angles. For the rotated neck posture, the center of the goniometer was placed over the center of the cranial aspect of the head, which was considered the middle of the imaginary line connecting nasion and inion (29, 30). The examined neck postures are shown in Fig.3.
To adjust the trunk angles, the participant was first asked to maintain his/her lower back (lumbar spine) in a neutral position so that the angle between the thigh and the trunk was approximately 90 °. Then, a point was marked on the iliac crest extending from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS). The proximal arm was placed along the imaginary line from the marked point, perpendicular to the chair seat. The distal arm was then rotated to the desired angles, and the participant was asked to bend so that the marked point was below the distal arm. To adjust the trunk angle in lateral flexion position, the proximal arm was placed between the spinous process of the twelfth thoracic vertebra (T12) and the beginning of the sacral curve perpendicular to the chair seat. The distal arm was then rotated to the desired angle and the subject was asked to bend so that the marked point was under the distal arm (29, 30). The examined trunk postures are shown in Fig.3.
To simulate lumbar curvatures, the subject was first asked to maintain a neutral lumbar posture. The angle between the thigh and the trunk was approximately 90° (forward hip rotation to maintain neutral lordotic posture). For the second posture, the subject was asked not to exert any muscular force by the back and abdominal muscles so that the trunk was flexed and the hip rotated backward (kyphotic or slumped posture). The studied lumbar spine curves are shown in Fig.4.
To adjust the angles of the knee, the center of the goniometer was placed over the lateral epicondyle of femur and the distal arm was almost over the lateral midline of fibula (23, 25).
2.6. Statistical analysis
IBM SPSS, version 22, was used to calculate the mean, standard deviation, maximum and minimum of discomfort scores for each posture. In addition, MS Excel was used to calculate MHT.