Design and evaluation of an ergonomic chair for near-ground welding based on muscle activity and usability

Abstract

BACKGROUND:

Work-related musculoskeletal disorders (WMSDs) are one of the major problems in industrial societies. Awkward postures in theworkplace are considered as a main reasons for these disorders. Therefore, the study and investigation of postures to improve them (in the workplace) have a significant role in appropriate intervention.

OBJECTIVE:

This study aimed to design an ergonomic chair for near-ground welding operations and evaluate its impact on the electromyography (EMG) activity of a select group of lower limb muscles.

METHODS:

The EMG activity of lower limb muscles was measured in eight postures consisting of three postures identified via observing welders at work, and five on-chair postures suggested for chair use during welding. The usability of the designed chair was evaluated through the use of the System Usability Scale (SUS).

RESULTS:

The results showed that the suggested on-chair postures had significantly lower mean muscle activity compared with the identified postures (p < 0.001). Among the identified postures, kneeling posture had the lowest mean muscle activity (p < 0.01), still significantly higher than the mean activity of the on-chair postures (p < 0.001). Applying a 15° slope to the seat pad led to a decrease in muscle activity and an increase in usability score. The usability score of the chair was acceptable in all postures except KCC-90 posture.

CONCLUSIONS:

Using a suitable chair or support for different work postures can prevent detrimental muscle activities during work and the development of musculoskeletal disorders caused by intense muscle activity due to awkward postures.

Keywords

Chair design working posture electromyography usability

1 Introduction

Work-related Musculoskeletal Disorders (WMSDs) are a major workplace problem with significant impacts on the workers’ health and performance entailing reduced physical ability, illness, and early retirement [1 –4]. According to a 2010 report by the European Agency for Safety and Health at Work (EASHW), Musculoskeletal Disorders (MSDs), with 59% prevalence, are the most commonly reported conditions among the recognized occupational diseases [5]. MSDs have been among the major causes of disability and absenteeism in many occupational populations over the past decades, and are currently among the most important workplace challenges for both employees and organizations [6]. These disorders are multifactorial, influenced by myriad parameters including physical factors such as awkward postures, repetitive motions, excessive exertion, and whole-body vibration, which have been identified as the most significant risk factors in workplaces [7 –9]. Among the mentioned factors, awkward work postures play the greatest role as far as the incidence of MSDs among workers is concerned [10, 11].

Workers spend most of their time in the workplace, hence the importance of providing a suitable working environment for their health. Organizations and corporations are required to incorporate into their strategic plans the provision of suitable working conditions and protection against work-related injuries [12]. Studies have shown that providing a safe and healthy work environment motivates employees, increases job satisfaction, and ameliorates productivity, product quality, and the quality of work life [13 –16]. Research on the physical risk factors of work-related accidents among welders has shown that awkward postures are the leading cause of such accidents and the source of musculoskeletal disorders [17]. Accordingly, the assessment of occupational postures, development of corrective actions to reduce their negative impacts on workers, and adoption of necessary countermeasures are of particular importance if we are to surmount these challenges [18].

Many of the physical damages that occur during welding operations are due to the low level of automation and operators needing to put continuous physical effort to get the job done. The preponderance of welding tasks and methods require the operator to be in a variety of awkward postures while bending, stretching or standing for long durations [19]. In the oil and gas industry, welders who work on gas pipelines have to work on the pieces positioned near the ground surface and thus have to be in awkward postures throughout their work hours. The postures in which these welders, more often than not, work in are bending forward, half-kneeling, kneeling, and squatting, among which, squatting and half-kneeling are the most damaging ones [20]. These postures can cause fatigue and pain in the lower extremities and the entire body; fatigue is directly related to static stresses, intense muscle activity, and reduced blood circulation due to awkward postures [21], which are commonly observed among people working in shipbuilding, car assembly lines, farms, car repair shops, petrochemical industries, and oil and gas transmission pipelines.

Over the past decades, many ergonomic tools and instruments have been designed to enhance productivity and comfort since such tools can reduce working fatigue and the risk of WMSDs [22 –24]. An ergonomic tool that can compensate for the deficiencies in the workplace is a suitable chair or support, which can decrease the physical stress caused by body weight and awkward postures. Among the several methods available for the evaluation of the utility of such tools, Electromyography (EMG) is perhaps the most popular in ergonomic studies [25]. In this method, muscle performance is studied based on the analysis of electrical signals resulting from muscle contractions [26, 27]. Ergonomic studies typically use EMG data to assess the work environment design, tools, postures, and fatigue.

The objectives of the present research are, firstly, to determine the primary postures of welders in gas transmission pipelines and assess the muscle activity in the identified postures, and secondly, to investigate the effect of a designed chair on muscle activity, and evaluate chair usability in the studied postures.

2 Methods

2.1 Subjects

The subjects who participated in this study were 15 male gas welders employed in Iranian Gas Transmission Co. (age: 34.9±4.7yr. weight: 76.6±5.8 kg, height: 175.2±4.3 cm, and work experience: 7.5±1.8 years). The average time spent welding in the gas transmission pipelines was 4.3±1.6 hours per day. The participants had no history of musculoskeletal disease or disorder in the back or lower limbs. The study was carried out with the approval of the Ethics Committee of Hamadan University of Medical Sciences and participant consent.

2.2 Chair design

To design a chair tailored to the needs of the welders, their dominant postures during work on gas transmission pipelines were specified through a field survey. The welders were further interviewed for other considerations in the design process. The 3D modeling software CATIA was employed to design the chair prototype based on the identified postures, the access range required for welding on gas transmission pipelines, and the anthropometric dimensions of the Iranian population. Usable in five different postures, the body, pads and seat covers of the designed prototype were respectively made of steel, flexible foam, and a leather resistant to fire and welding sparks. The structural characteristics of the chair prototype are: Dimensions of Pad A (45cm×35 cm), diameter of Pad B (25 cm), height of chassis from the ground (10 cm), minimum height of Pad B from chassis (25 cm), the height to which Pad B can be raised using the pneumatic lift (15 cm), the angle between the front wheels (100 degrees), the angle between the rear wheels (90 degrees), and the angles to which Pad B can be sloped (0, 15 and 90 degrees). The weight of the designed chair was 7 kg (Fig. 1).

Fig. 1

Prototype chair evaluated in the study.

2.3 Studied postures

This study examined eight different postures categorized into two groups, the first consisting of three identified postures by observing welders at work, and the second comprised of five on-chair postures suggested for use during welding (Fig. 2).

Fig. 2

The 8 postures investigated in this study.

The identified postures were: Trunk bent forward (TRK-90, trunk bent forward 90 degrees from vertical), half kneeling (KNL-120, knee flexion angle of 120°), and kneeling (KNL-2 L, both knees placed on the ground and the trunk held upright). The on-chair postures were: Sitting (SC-90, hip-trunk angle of 90°), kneeling on the no-slope seat (KC, knee flexion angle of 45°), kneeling on the 15°-slope seat (KC-15, knee flexion angle of 45°), sitting on a seat with back support (SCB-110, hip-trunk angle of 110°), kneeling on a seat with front support (KCC-90, hip-trunk angle of 90°).

2.4 Procedure

The subjects were briefed as to the purpose, method and the experiment process of the study. The equipment, assessed postures, and the designed chair were further introduced. Prior to the experiment, the participants were asked to use the chair in different postures to become familiar with its features. After preparing the participants for the experiment, the electrodes were mounted on the muscles, in accordance with the protocol, to examine the muscle activities in the eight postures. In order to preclude muscle fatigue, the experiment of different postures were separated by 5-minute periods of rest. Following the experiment, the System Usability Scale (SUS) was used to appraise the usability of the designed chair in different postures.

2.5 Surface electromyography

In accordance with the previous studies on the subject, the muscles involved in the studied postures were erector spinae (ES), biceps femoris (BF), vastus medialis (VM), gastrocnemius medialis (GM), gastrocnemius lateralis (GL), tibialis anterior (TA), rectus abdominis (RA), abdominal internal oblique (IO), abdominal external oblique (EO) and semitendinosus (S) muscles. Surface electromyography signals were recorded using the 16-channel ME6000 system (made in Finland) with 14-bit A/D converter, signal to noise ratio of 110 dB, and sampling frequency of 1000 Hz. For this purpose, Ag/AgCl electrodes of 1 cm in diameter (with conductive gel/adhesive) were placed with a center-to-center spacing of 2 cm in a bipolar configuration (two sensor electrodes and one ground electrode). The data were transmitted directly from the surface electrodes to a receiver positioned near the person, and then, via a wireless connection, to a computer. To reduce the skin-electrode impedance, after preparing the skin (removing the hair and massaging the skin with alcohol-impregnated cotton), the positions with electrodes generating strong and high-quality signals were determined as recommended by SENIAM [28, 29].

Since the raw RMS (Root Mean Square) values varied with the muscle type, they were normalized in order for the muscle activities to become comparable. Normalization was done through the maximum voluntary contraction (MVC) of the muscles. In this regard, after instructing the subjects on how to contract each muscle, they were asked to slowly increase the force to reach the maximum level within 3 to 5 seconds and then hold it for 3 seconds, during which time the MVC was recorded, a process repeated 2-3 times, each followed by two minutes of rest; the best contraction observed from a number of maximal efforts was ultimately employed for normalization.

2.6 System Usability Scale

The usability of the designed chair was evaluated using the System Usability Scale (SUS) [30], a questionnaire comprised of ten statements expressing the degrees of usability based on a 5-point Likert scale ranging from 1 (strongly disagree) to 5 (strongly agree). To produce a total score for the usability of the product or the device under evaluation, the answers given to the questions have to be interpreted. In order to do so, the scores of odd-numbered questions (1, 3, 5, 7, and 9) are deducted by 1, and the scores of even-numbered questions (2, 4, 6, 8 and 10) are subtracted by 5; their sum is then multiplied by 2.5, with the final SUS score ranging from 0 to 100 [31]. A SUS score of less than 50 indicates dissatisfaction while a score of more than 70 signifies an acceptable level of satisfaction. The scores falling between 50 and 70 represent the boundary range, implying the need for improvement in the design [32, 33]. The English version of the SUS has been translated and revised into the Persian language and has established validity and reliability [34]. The Persian version of the SUS was used in this study.

2.7 Data analysis

The electromyography index used in this study was the RMS/MVC ratio. The raw data obtained from surface electrodes were normalized in Mega Win 3.0.1 using 10–450 Hz band-pass filter. Statistical analysis was performed in SPSS-22 employing descriptive and inferential statistical methods. Descriptive statistics were further used to calculate the mean and standard deviations and plot the results. Finally, the inferential statistical analysis was conducted using ANOVA. In all statistical analyses, P values of less than 0.05 were regarded as statistically significant.

3 Results

Table 1 illustrates the mean muscle activity of the participants in the evaluated postures (based on MVC). As can be seen, KNL-120 with 20.1% and SCB-110 with 3.2% have the highest and lowest mean muscle activities among the evaluated postures, respectively. The highest muscle activities among the evaluated muscles belonged to TA (55%) VM (51.6%) and BF (45.5%). Moreover, KNL-2 L had significantly lower muscle activity comparisons with the other two postures (P < 0.001).

Table 1
The mean muscles activity in the evaluated postures

RA EO IO ES VM S BF TA GM GL Avg

TRK-90 3.2 3.2 4 17.9 6.8 17.5 9.2 55 26.2 18.2 16.1

KNL-120 5.5 10.2 10.3 23.5 51.6 8.7 45.5 33.7 8.3 14.3 21.1

KNL-2L 4.7 4.3 5 9.4 15 4.8 12.4 10 2.8 2.9 7.3

SC-90 3.8 4.2 3.9 9.2 2.7 2.5 2.5 2.4 2.5 2.5 3.6

KC 4.7 3.9 4.5 8.1 2.7 5.5 8.5 4.3 2.6 2.7 4.7

KC-15 4.3 3.7 4.4 5.2 2.6 3.5 4.2 4 2.6 2.5 3.7

SCB-110 2.7 4 3.7 4.5 3.4 3.3 2.6 3 2.5 2.7 3.2

KCC-90 3.7 3.6 4.3 9.4 4.2 2.8 10.7 5.3 2.6 2.8 5

Avg 4.1 4.6 5 11 11.1 6.1 12 14.7 6.3 6.1

	RA	EO	IO	ES	VM	S	BF	TA	GM	GL	Avg
TRK-90	3.2	3.2	4	17.9	6.8	17.5	9.2	55	26.2	18.2	16.1
KNL-120	5.5	10.2	10.3	23.5	51.6	8.7	45.5	33.7	8.3	14.3	21.1
KNL-2L	4.7	4.3	5	9.4	15	4.8	12.4	10	2.8	2.9	7.3
SC-90	3.8	4.2	3.9	9.2	2.7	2.5	2.5	2.4	2.5	2.5	3.6
KC	4.7	3.9	4.5	8.1	2.7	5.5	8.5	4.3	2.6	2.7	4.7
KC-15	4.3	3.7	4.4	5.2	2.6	3.5	4.2	4	2.6	2.5	3.7
SCB-110	2.7	4	3.7	4.5	3.4	3.3	2.6	3	2.5	2.7	3.2
KCC-90	3.7	3.6	4.3	9.4	4.2	2.8	10.7	5.3	2.6	2.8	5
Avg	4.1	4.6	5	11	11.1	6.1	12	14.7	6.3	6.1

[%] based on MVC; means of 15 subjects.

For all muscles, the mean activity (in MVC %) of the on-chair postures were found to be lower compared with KNL-2 L posture. The most difference in this regard were respectively observed in VM (11.9%), TA, and BF muscles. Statistical analysis showed that the reduction in the activity of VM, TA, BF, ES and S muscles in the on-chair posture was more significant than KNL-2 L (P < 0.001) (Fig. 3).

Fig. 3

Comparison of mean muscle activity in the KNL-2 L and on-chair postures.

Figure 4 compares the mean muscle activities pertaining to identified postures (off-chair) and on-chair postures. As seen, all muscles experience lower activities in on-chair postures compared with the identified postures. The finding that the greatest improvement was observed in TA (29.1%), VM and BF muscles, because these muscles are located at the thigh region, which is supported by the ergonomic chair. Lower activities were also recorded at ES when there is lumbar support. Statistical analysis showed that the difference between the muscle activity values of these two posture groups was statistically significant (P < 0.001).

Fig. 4

Comparison of mean muscles activity between two groups (identified and on-chair postures).

The influence of sloping the seat pad (Pad B) on muscle activity is illustrated in Fig. 5. As is observed, except for GM, all muscles have lower activities in KC-15 posture (with 15° forward slope) than in KC posture (no slope). The greatest difference in this regard was seen in the activity of BF, ES, and S muscles (4.3%, 2.9%, and 2%, respectively). The difference between the activity of ES, S, BF and TA muscles in the KC-15 posture and that in the KC posture was also found to be significant (P < 0.05).

Fig. 5

Comparison of the mean muscles activity between the two postures KC-15 and KC.

Among the five on-chair postures, the highest SUS scores were obtained by SCB-110 posture (95), followed by SC-90 (88), KC-15 (85) and KC (80). The KCC-90 was given a SUS score of 62, which is noticeably lower in comparison with others (Fig. 6).

Fig. 6

Mean SUS scores of on-chair postures.

4 Discussion

This is one of the first studies to design and evaluate an ergonomic chair for welders in gas transmission pipelines. After designing an ergonomic chair prototype, the muscle activities of the welders (categorized into identified postures and suggested on-chair postures) were measured by EMG. Given the variability of welding height on the gas transmission pipelines, the welders used a combination of the three identified postures during welding. Therefore, the mean of the muscle activity was analyzed in the postures of the two groups. The results indicated that muscle activity in the suggested on-chair postures was significantly lower than in the identified postures.

Research has shown the high prevalence of WMSDs among welders because of their awkward postures during work [35]. In this study, the evaluation of the mean muscle activity of welders in three identified postures showed more activity in the TA, VM, BF, and ES muscles than in the other examined muscles. This could be due to the change in the center of gravity and the role of these muscles in maintaining balance in the studied postures along with the angle of the knee and hip joints that causing intense activity of muscles involved in these joints. Among the three identified postures, KNL-2 L was found to entail a significantly lower mean muscle activity than the other two. In this posture, the suitable angle of the hip joint and the direct transfer of body weight allow muscles to undergo less activity; however, it is to be noted that placing the knees on the ground to transfer the body weight has a damaging effect on the knees [36, 37].

Comparison of the mean muscle activity between the three identified postures and the five suggested on-chair postures showed that the activity of all studied muscles was significantly reduced when using the latter. In addition to the fact that in the identified postures, TA, VM, BF and ES muscles were more active than other muscles due to the posture type of the welders, the greatest activity reduction was observed in these muscles. Moreover, in on-chair postures, with the exception of ES and BF muscles, active at 5–10% (MVC) levels, all studied muscles had less than 5% activity. Relatively higher activity was observed in the ES and BF muscles since there was no back support in certain on-chair postures, and because of the direct transfer of body weight through knees in the KCC-90 posture. The mean muscle activity in the KNL-2 L was found to be lower than in the other two identified postures. In on-chair postures, the activity of VM, TA, BF, ES, and S muscles was significantly lower owing to the involvement of these muscles in transferring the body weight and maintaining the balance along with the lack of support in the KNL-2 L posture. The on-chair postures reduced muscle activity by providing appropriate support in the areas where the muscles were more active. Given the low activity (below 5%) of abdominal muscles (RA, EO, and IO) and gastrocnemius muscles (GL and GM) in the KNL-2 L, their activity did not decrease significantly. Reduced muscle activity in the suggested on-chair postures showed that the existence of suitable supporters at different postures can prevent intense muscle activity during welding and WMSDs, ultimately [38, 39].

This study further appraised the impact of a slope in the seat pad (Pad B) on the activity of the studied muscles. For this purpose, the muscle activity was measured when the seat pad had zero and 15° forward slope. The results showed that with 15° forward slope, all of the studied muscles were less active, a reduction which was prominent in ES, S, BF, and TA muscles. The 15° slope in the seat pad creates a suitable angle between the trunk and the thigh, thereby reducing the activity of the muscles involved in this area. Studies on sloping chairs have reported that sloping increases the activity of lower limb muscles [40, 41], which is inconsistent with our results, probably due to the absence of support for the knee area in the studied chairs, which may, in turn, result in increased muscle activity. Unfortunately, few studies have been carried out in this regard, hence the fact that ancillary studies are required to gain further insight into the effects of seat slope on muscle activity.

One of the key aspects of designing and fabricating work equipment is the usability from the consumer point of view. In this study, the usability of the designed chair was appraised in the five suggested postures through the use of SUS, which scores ranged from 62 to 95. The highest score was obtained for SCB-110, probably because of the lower activity of muscles in this posture, the presence of back support and more frequent usage of this posture in the workplace. Concerning KC and KC-15 postures, a 15° sloping of the seat pad increased the SUS score. Based on the muscle activity measured in these two postures, the increased usability of the chair can be attributed to the decreased muscle activity when using the sloped seat. The lowest SUS score was obtained for KCC-90, in which posture, the welder’s position on the chair resulted in higher muscle activity comparisons with other on-chair postures. Another cause of the low SUS score of this posture can be the position of the upper limbs, particularly the head and the neck.

The findings of this study may conduce to future efforts as regards designing and fabricating ergonomic chairs and appropriate supporters for different occupations to reduce muscle activity and further preventing musculoskeletal disorders. However, it is essential to make mention of certain limitations in this study. First, the postures examined in this study were commonly used during welding on near-ground gas pipelines, yet the observed welders used other postures during welding as well. Nevertheless, given the physical stress associated with this particular work, it is beneficial to incorporate other postures into the design of chairs or other suitable equipment. The second limitation was the focus on lower limb muscle activity, which is yet another point to be considered in future research. Finally, we employed MVC as the only measure of muscle activity and ignored muscle fatigue due to research constraints. Therefore, future studies are recommended to also consider muscle fatigue in their postural evaluations.

5 Conclusion

Muscle activity is strongly affected by posture and joints angle. The proposed on-chair postures significantly reduce muscle activity in comparison with the identified postures, implying that a suitable chair or support in different postures can preclude detrimental muscle activity during work, thereby decreasing the incidence of musculoskeletal disorders caused by intense work-related muscle activity. Comparing the muscle activity and SUS scores obtained for the KC and KC-15, it becomes evident that a 15° slope in the seat path reduces the activity of the involved muscles while augmenting chair usability. It is further to be noted that the use of support for the knee area has a great impact on the conclusion. Ultimately, the SUS scores of the on-chair postures have an inverse relationship with muscle activity.

Conflict of interests

The authors declare that they have no conflict of interests.

Footnotes

Acknowledgments

This research was part of MSc Thesis at Hamadan University of Medical Sciences. The authors would like to thank the management and welders of the Gas Transmission Company, as well as the Head of HSE Department, Mr. Sadegh Afshoun and Mr. Mehdi Alizadeh for the contribution they have made to this study. Grant number: 9409245181

References

Morken

, Riise

, Moen

, Hauge

, Holien

, Langedrag

, et al. Low back pain and widespread pain predict sickness absence among industrial workers. BMC Musculoskeletal Disorders. 2003;4(1):21.

Piedrahita

. Costs of work-related musculoskeletal disorders (MSDs) in developing countries: Colombia case. International Journal of Occupational Safety and Ergonomics. 2006;12(4):379–86.

Karimi

, Moghimbeigi

, Motamedzade

, Roshanaei

. Evaluation of related risk factors in number of musculoskeletal disorders among carpet weavers in Iran. Safety and Health at Work. 2016;7(4):322–5.

Choobineh

, Movahed

, Tabatabaie

, Kumashiro

. Perceived demands and musculoskeletal disorders in operating room nurses of Shiraz city hospitals. Industrial Health. 2010;48(1):74–84.

Chander

, Cavatorta

. An observational method for postural ergonomic risk assessment (PERA). International Journal of Industrial Ergonomics. 2017;57:32–41.

Motamedzade

, Faghih

, Golmohammadi

, Faradmal

, Mohammadi

. Effects of physical and personal risk factors on sick leave due to musculoskeletal disorders. International Journal of Occupational Safety and Ergonomics. 2013;19(4):513–21.

Baek

, Kim

, Yi

. Relationship between comorbid health problems and musculoskeletal disorders resulting in musculoskeletal complaints and musculoskeletal sickness absence among employees in Korea. Safety and Health at Work. 2015;6(2):128–33.

Moriguchi

, Carnaz

, de ALENCAR

, Junior

LCM

, Granqvist

, Hansson

G-A

, et al. Postures and movements in the most common tasks of power line workers. Industrial Health. 2011;49(4):482–91.

Ziaei

, Choobineh

, Abdoli-Eramaki

, Ghaem

. Individual, physical, and organizational risk factors for musculoskeletal disorders among municipality solid waste collectors in Shiraz, Iran. Industrial health. 2018.

10.

Widanarko

, Legg

, Stevenson

, Devereux

, Eng

, Mannetje

, et al. Gender differences in work-related risk factors associated with low back symptoms. Ergonomics. 2012;55(3):327–42.

11.

Brandt

, Madeleine

, Ajslev

JZN

, Jakobsen

, Samani

, Sundstrup

, et al. Participatory intervention with objectively measured physical risk factors for musculoskeletal disorders in the construction industry: study protocol for a cluster randomized controlled trial. BMC Musculoskeletal Disorders. 2015;16(1):302.

12.

R. R M, Vinodkumar

, Neethu

, Modeling the influence of individual and employment factors on musculoskeletal disorders in fabrication industry. Human Factors and Ergonomics in Manufacturing & Service Industries. 2017;27(2):116–25.

13.

Motamedzade

, Shahnavaz

, Kazemnejad

, Azar

, Karimi

. The impact of participatory ergonomics on working conditions, quality, and productivity. International Journal of Occupational Safety and Ergonomics. 2003;9(2):135–47.

14.

Piranveyseh

, Motamedzade

, Osatuke

, Mohammadfam

, Moghimbeigi

, Soltanzadeh

, et al. Association between psychosocial, organizational and personal factors and prevalence of musculoskeletal disorders in office workers. International Journal of Occupational Safety and Ergonomics. 2016;22(2):267–73.

15.

Baek

, Yang

, Lee

, Chung

. The Association of Workplace Psychosocial Factors and Musculoskeletal Pain Among Korean Emotional Laborers. Safety and Health at Work. 2017.

16.

, Tamrin

SBM

, Yik

, Yusoff

ISM

, Mori

. The prevalence of musculoskeletal disorder and association with productivity loss: a preliminary study among labour intensive manual harvesting activities in oil palm plantation. Industrial Health. 2014;52(1):78–85.

17.

Vieira

, Kumar

. Occupational risks factors identified and interventions suggested by welders and computer numeric control workers to control low back disorders in two steel companies. International Journal of Industrial Ergonomics. 2007;37(6):553–61.

18.

Brandl

, Mertens

, Schlick

. Effect of sampling interval on the reliability of ergonomic analysis using the Ovako working posture analysing system (OWAS). International Journal of Industrial Ergonomics. 2017;57:68–73.

19.

Francisco

, Edwin

. Implementation of an ergonomics program for the welding department inside a car assembly company. Work. 2012;41(Supplement 1):1618–21.

20.

Roffey

, Wai

, Bishop

, Kwon

, Dagenais

. Causal assessment of awkward occupational postures and low back pain: results of a systematic review. The Spine Journal. 2010;10(1):89–99.

21.

Alavinia

, van Duivenbooden

, Burdorf

. Influence of work-related factors and individual characteristics on work ability among Dutch construction workers. Scandinavian Journal of Work, Environment & Health. 2007:351-7.

22.

Koningsveld

, Dul

, Van Rhijn

, Vink

. Enhancing the impact of ergonomics interventions. Ergonomics. 2005;48(5):559–80.

23.

Carlisle

, Parker

. Psychological distress and pain reporting in Australian coal miners. Safety and Health at Work. 2014;5(4):203–9.

24.

Vyas

, Das

, Mehta

. Occupational injuries in automobile repair workers. Industrial Health. 2011;49(5):642–51.

25.

Hoozemans

, Van Dieen

. Prediction of handgrip forces using surface EMG of forearm muscles. Journal of Electromyography and Kinesiology. 2005;15(4):358–66.

26.

Cifrek

, Medved

, Tonković

, Ostojić

. Surface EMG based muscle fatigue evaluation in biomechanics. Clinical Biomechanics. 2009;24(4):327–40.

27.

Wright

, Lyons

, Navalta

. Effects of exercise-induced fatigue on postural balance: a comparison of treadmill versus cycle fatiguing protocols. European Journal of Applied Physiology. 2013;113(5):1303–9.

28.

Hermens

, Freriks

, Disselhorst-Klug

, Rau

. Development of recommendations for SEMG sensors and sensor placement procedures. Journal of Electromyography and Kinesiology. 2000;10(5):361–74.

29.

Perotto

. Anatomical guide for the electromyographer: the limbs and trunk: Charles C Thomas Publisher; 2011.

30.

Brooke

. SUS-A quick and dirty usability scale. Usability Evaluation in Industry. 1996;189(194):4–7.

31.

Alavinia

, van Duivenbooden

, Burdorf

. Influence of work-related factors and individual characteristics on work ability among Dutch construction workers. Scandinavian Journal of Work Environment and Health. 2007;33(5):351–7.

32.

Bangor

, Kortum

, Miller

. Determining what individual SUS scores mean: Adding an adjective rating scale. Journal of Usability Studies. 2009;4(3):114–23.

33.

Lewis

, Sauro

, editors. The factor structure of the system usability scale. International conference on human centered design; 2009: Springer.

34.

Dianat

, Ghanbari

, AsghariJafarabadi

. Psychometric properties of the persian language version of the system usability scale. Health Promotion Perspectives. 2014;4(1):82.

35.

Chung

, Lee

, Kee

. Effect of stool height and holding time on postural load of squatting postures. International Journal of Industrial Ergonomics. 2003;32(5):309–17.

36.

Porter

, Mayton

, Moore

. Pressure distribution on the anatomic landmarks of the knee and the effect of kneepads. Applied Ergonomics. 2010;42(1):106–13.

37.

Reid

, Bush

, Cummings

, McMullin

, Durrani

. A review of occupational knee disorders. Journal of Occupational Rehabilitation. 2010;20(4):489–501.

38.

Haddad

, Sanjari

, Amirfazli

, Narimani

, Parnianpour

. Trapezius muscle activity in using ordinary and ergonomically designed dentistry chairs. The International Journal of Occupational and Environmental Medicine. 2012;3(2 April).

39.

Shin

, Zhu

. User discomfort, work posture and muscle activity while using a touchscreen in a desktop PC setting. Ergonomics. 2011;54(8):733–44.

40.

Curran

, Dankaerts

, O’Sullivan

. The effect of a backrest and seatpan inclination on sitting discomfort and trunk muscle activation in subjects with extension-related low back pain. Ergonomics. 2014;57(5):733–43.

41.

Hamaoui

, Hassaïne

, Watier

, Zanone

P-G

. Effect of seat and table top slope on the biomechanical stress sustained by the musculo-skeletal system. Gait & Posture. 2016;43:48–53.