Electromyographic analysis of bilateral upper trapezius muscles at different levels of work-pace among sewing machine operators

Abstract

Workers are driven to work faster in the industrial work environment to meet high productivity targets. An increased work pace leads to increased muscle activation. However, the effect of work pace on bilateral upper trapezius muscles during sewing machine operation in an industrial work environment has not been thoroughly investigated in experimental studies. Therefore, this research aims to conduct an experimental study to analyze the bilateral upper trapezius muscle activity of industrial sewing machine operators at different levels of work pace. Thirty subjects (15 males, and 15 females) continuously performed the sewing operation for two hours in an industrial work environment. Experiments were conducted for two levels of work pace i.e. low pace (100% of standard cycle time) and high pace (120% of standard cycle time). Electromyographic signals were recorded from the bilateral upper trapezius muscles. The EMG amplitude (RMS) among the muscles was computed. A statistically significant (p < 0.05) increase in muscle activity was observed with an increased work pace. In this study, right upper trapezius muscle activity increased by 30.4% during high work pace tasks compared to low pace, while the left upper trapezius showed a 24.12% increase. The right upper trapezius showed a mean difference of 0.696 (%MVC), and the left upper trapezius showed 0.399 (%MVC), both indicating greater activity during high-pace tasks. The increase in muscle activity with time indicated the presence of muscle fatigue among sewing machine operators. Furthermore, higher muscular activity was observed among females than males. This research highlights the critical need to balance productivity goals with the health and safety of workers, reducing the risk of muscle fatigue and associated work-related musculoskeletal disorders.

Introduction

Work-related activities carry inherent risks to workers’ health and safety [1]. The need to improve safety records and reduce the incidence of illnesses and injuries across several industries is becoming recognized worldwide [2]. The recent paradigm shift of the industrial sector from the technology-driven approach (Industry 4.0) toward a human-centered approach (Industry 5.0) [3], has also encouraged stakeholders and organizations to prioritize the health and well-being of their workforce [4]. Industry 4.0 emphasizes automation and smart technologies while Industry 5.0 aims to integrate human well-being into these advancements. This shift is reflected in the implementation of proactive measures designed to prevent injuries and illnesses and in performing detailed assessments within industries, assembly lines, and workplaces [5]. This organizational emphasis on worker health and well-being is considered a key step toward optimizing overall working conditions and reducing the prevalence of work-related musculoskeletal disorders (WMSDs) among workers [1]. The in-depth evaluations of the body parts that are mostly used to perform work activities are an integral part of this commitment. Hence, by performing rigorous experimental investigations of muscular activities, motor performance, and fatigue levels, the aim of Industry 5.0 in creating a safe and healthy work environment can be achieved. This proactive approach is in line with the mission of developing a sustainable and human-centric industrial work environment. However, such experimental assessments are usually performed in laboratories and are rarely performed in an on-field work environment, which creates a gap between laboratory-based work environments and real on-field work environments.

Despite efforts over the past decade to transform factories into smart factories under the Industry 4.0 framework, many sectors, such as apparel, construction, and electronics, remain labor-intensive and have not yet fully or partially automated their operations. Works in these sectors often adopt poor working posture by bending their backs, stiffening their shoulders, and extending their arms far from their bodies, and remain in these positions for extended periods without being aware of the potential consequences [2, 6, 7]. All these work-related risk factors lead to muscle fatigue and musculoskeletal disorders, stressing their muscles, ligaments, tendons, and nerves. The literature indicates that sewing machine operators in garment industries are exposed to back, neck, and shoulder muscle fatigue [8,9,10,11]. Sewing machine operations in garment industries, officially classified as “light work”, actually demands significant exertion in a restrained position [12]. This work involves maintaining a seated posture with limited movement, often requiring workers to lean forward, extend their arms, and keep their shoulders elevated, all of which contribute to muscle strain and fatigue over time [8].

Sewing machine operators are involved in repetitive, high precision, and monotonous tasks that induce muscle fatigue [7, 8, 13, 14]. To achieve high work productivity targets, sewing machine operators work at a high pace [6], whereas higher work pace results in high muscular activity [15,16,17]. Basmajian and De Luca [18] reported that an increase in muscle activity results in the prognosis of muscle fatigue. Muscular fatigue is a continuous process [19], that causes a decline in performance [20]. The impact of fatigue imposes a significant annual cost, amounting to $136 billion on U.S. employers due to health-related lost productive time, which is $101 billion more than the costs associated with non-fatigued workers [21]. Similarly, Japanese industries face an economic burden of approximately 30 trillion yen as a result of fatigue [22]. The accumulation of muscle fatigue not only contributes to these economic losses but also leads to musculoskeletal disorders over time, resulting in functional disability among workers [23, 24]. Furthermore, research by Swaen, Van Amelsvoort [25] also identified fatigue as a risk factor for workplace accidents. Given these impacts, it is essential to investigate the variation in muscle activity at different work paces to better understand and mitigate these risks.

Multiple subjective assessments have been conducted to assess the risk of musculoskeletal disorders among sewing machine operators. However, technological solutions in diverse applications especially in industrial work environments must be based on strong theoretical grounds, supported by rigorous experimental assessments. Sundelin and Hagberg [26] investigated muscle fatigue of the shoulder and neck muscles among only six female sewing machine operators involved in Method-Time-Measurement (MTM) paced work. David [27] reported that a group of 15–25 participants is adequate to obtain a reliable estimate of exposure. In addition, this research was conducted in a controlled laboratory setting. The laboratory-based studies have multiple limitations. However, the experiments in real work environments yield more accurate and applicable results. While workplace-based studies mostly consider questionnaires, scales, surveys, or checklists to investigate the results. Few workplace-based studies use instrumental measures to investigate work fatigue among workers [28]. Therefore, the current study was conducted in a real industrial environment to overcome this research gap.

Previous epidemiological studies have focused primarily on female sewing machine operators [8, 10, 29, 30]. However, recent research by Kee [31] emphasized the need to address WMSDs among male workers as well. Literature also indicates that a substantial number of men are also employed in garment industries [6, 13]. This body of research underscores the importance of considering gender in evaluating muscle activity.

Hence, there is a need to evaluate the muscle activity across both genders at varying work pace. To date, there are no established references available identifying the effect of work pace on bilateral upper trapezius muscle activity among industrial sewing machine operators. Therefore, this research aims to analyze bilateral upper trapezius muscle activity at different levels of work pace while performing sewing operations in real industrial work environments. Henceforth, this knowledge can be used as a guideline to design tasks according to the worker’s capacity and minimize the risks of WMSDs.

Materials and methods

Participants

For this study, 30 healthy participants, including 15 males and 15 females, were recruited to perform the experimental tasks. The average age of male participants was 25.90 ± 7.46 years, while female participants had an average age of 23.77 ± 4.84 years. The overall mean age of all participants was 24.83 ± 6.28 years. The participants with any neurological or musculoskeletal disorders were excluded from this study.

Apparatus

The muscle activity of the upper trapezius muscles was recorded using an EMG system (iWorx B3G with iWorx TA Recording Module (IX-TA-220)) with pregelled adhesive surface electrodes Ag/AgCl/Solid. Bipolar Ag/AgCI surface electrodes with a 20 mm distance between them were positioned at the belly of the selected muscles. Additionally, a reference electrode was positioned on the pisiform bone. Hermens, Freriks [32] instructions were followed to accurately place the electrodes. Initially, a waterproof pencil was used to mark the muscle positions to ensure precise electrode placement under both experimental conditions. Alcohol was used to clean the subject’s skin before the electrodes were placed. Muscle activity recording was started using an iWorx B3G system as soon as the interelectrode resistance was less than 10k.

Muscles selection

The electromyographic signals from the upper trapezius muscles on both the right and left sides were recorded by placing electrodes at the sensor location. These muscles were selected because of their frequent use, contribution, and involvement in the sewing machine operation. The trapezius muscles are critical for clinical application since they exhibit frequent pain symptoms in repetitive work environments. Kazemi Kheiri, Vahedi [33] also reported that shoulder muscles are more fatigued than other upper limb muscles. A consultation session with an anatomist helped to accurately identify the muscle’ location. Furthermore, the EMG signal measurement was performed under the guidelines of the International Society of Electrophysiology and Kinesiology (ISEK) [34] and the European Recommendations for Surface Electromyography (SENIAM) [32].

Work pace

The experimental tasks were designed with respect to work: low pace (LP) and high pace (HP). The cycle time of each work cycle was calculated using methods-time measurement (MTM). The standard task cycle time derived from the MTM was 7.95 s. Therefore, for this study, a cycle time of 8 s for the low-pace task (100% of the standard cycle time) and a cycle time of 6.40 s was used for the high-pace task (120% of the standard cycle time). The subjects were asked to carry out a repetitive sewing task on a 12*12-inch piece of fabric. The fabric was 100% cotton. The subjects were asked to pick the fabric from the right-hand side and place it on the left-hand side after sewing as shown in Fig. 1.

Fig. 1

Workstation design

Trial duration and details

Each participant completed two full trials, one for each work pace condition. The trials were conducted on separate days to minimize the influence of one pace on the other. On each experimental day, the session lasted approximately 3 h, including setup, maximum voluntary contraction (MVC) tasks, the sewing operation, and equipment removal. The order of the work paces was randomized for each participant to control for any order effects. Participants were randomly assigned to start with either the low-pace (LP) or high-pace (HP) condition on their first day. This randomization ensured that any observed effects could be attributed to the work pace conditions rather than the sequence of task execution.

Experimental procedure

The experimental procedure consists of two stages: [1] preparation and [2] performing a work task (sewing operation). Before data collection, the subjects were asked to attend an orientation session. This session helped the subjects familiarize themselves with the data collection procedures and apparatus. The approval for the experimental procedure was also obtained from the University Malays Research Ethics Committee (UMREC). In the first phase of the experiment, an information sheet was distributed to the participants to outline the possible risks attached to the study. A written consent form was signed by each participant indicating that they completely agreed to take part in the study. The participants’ descriptive details such as gender, age, weight, and height were subsequently recorded. The participants’ skin was thoroughly cleaned with alcohol swabs before electrode placement. Once the experimental tools were set for data collection and the signals from all the electrodes were stable, the participants were requested to perform maximum voluntary contraction (MVC) tasks. During the MVC tasks, participants were seated comfortably with their backs supported and feet flat on the floor. The sitting height was adjusted to achieve a knee angle of 90 degrees, and the table surface was positioned 5 cm below the wrist with the elbow flexed at 90 degrees. This setup was designed to maintain a standard working height and posture. Participants were instructed to exert their maximum effort to contract the target muscles (upper trapezius) during isometric contractions, where muscles were activated without changing their length. The muscle activity was recorded using EMG during these contractions to establish baseline measures of muscle strength and activation. After completing the MVC tasks, the participants were asked to adjust their sitting posture for optimal ergonomics, based on their dominant hand. This adjustment ensured that the workspace was ergonomically suitable for the dominant hand, enhancing comfort and accuracy in the sewing task. During the second phase of the experiment, the subjects performed the sewing operation for two hours continuously without rest or break. The muscle activity was recorded using EMG throughout this phase.

Data processing / data acquisition

The EMG data, processed using LabScribe software, was sampled at 1500 Hz and bandpass filtered between 20 and 400 Hz to capture relevant muscle activity while removing ECG artifacts [35]. The analysis primarily used the root mean square (RMS) amplitude method, preferred for its accuracy, consistency, and validity in measuring noisy signals [36]. RMS values, representing the square root of the raw EMG signal’s average power, were normalized to the highest maximal voluntary contraction (MVC) value and reported as %MVC. These normalized %MVC values indicate muscle activity and were used in statistical analysis.

Statistical analysis

The statistical analysis of the processed EMG data was conducted using IBM SPSS (version 27). Initially, the Shapiro-Wilk test was performed to assess the normality of the data. Based on these results, appropriate parametric or non-parametric tests were chosen. Paired sample t-tests were used to investigate differences in muscle activity between the bilateral upper trapezius muscles at high work pace and low work pace. The independent sample t-tests was applied to examine differences in muscle activity with respect to gender at both levels of work pace. Additionally, repeated measures ANOVA was applied to evaluate the effect of work pace and gender on bilateral upper trapezius muscle activity at different levels of work pace. A p-value of less than 0.05 was considered statistically significant, indicating meaningful differences in the observed muscle activity.

Results

Muscle activity for work pace tasks

The bilateral upper trapezius muscle activity of the participants while they performed the sewing operation at a high pace and low pace is shown in Table 1. Muscle activity, represented as mean normalized EMG RMS (%MVC), was analyzed using paired sample t-tests. The results indicate significant differences in muscle activity between high and low-pace tasks. For the right upper trapezius muscle, the mean difference in muscle activity was 0.696 (%MVC), with a standard deviation of 1.548 and a p-value of 0.020, indicating significantly higher muscular activity during the high-pace task. For the left upper trapezius muscle, the mean difference was 0.399 (%MVC), with a standard deviation of 0.746 and the p-value was 0.006, also indicating significantly greater muscular activity at a high pace. These findings demonstrate a significant increase in muscle activity during high pace tasks for both muscles, with the right upper trapezius showing higher levels of activity compared to the left.

Table 1 Muscle activity (%MVC) for work pace tasks

Table 2 presents the mean muscle activity (%MVC) of male and female participants across low-pace and high-pace tasks, focusing on the right and left upper trapezius muscles. During the low-paced task, male participants exhibited a mean muscle activity of 1.903 (SD = 0.752) in the right upper trapezius muscle, while female participants showed a significantly higher mean activity of 2.680 (SD = 0.779), with the difference being statistically significant (mean difference = -0.777, p = 0.010). In contrast, the left upper trapezius muscle activity did not differ significantly between males and females, with males having a mean activity of 1.694 (SD = 0.679) and females 1.614 (SD = 0.494) (p = 0.714).

Table 2 Muscle activity (%MVC) for work pace tasks in male and female subjects

During the high-pace task, males had a mean muscle activity of 2.504 (SD = 1.601) for the right upper trapezius, while females showed a higher mean activity of 3.467 (SD = 1.632); however, this difference was not statistically significant (p = 0.113). For the left upper trapezius, females again demonstrated significantly higher muscle activity than males, with a mean activity of 2.506 (SD = 0.740) compared to 1.600 (SD = 0.863) in males (p = 0.005). This data indicates that female participants generally had higher muscle activity than males, particularly in the right upper trapezius during low-paced tasks and in both upper trapezius muscles during high-paced tasks.

The results of the repeated measures ANOVA, examining the effects of work pace and gender on muscle activity for both the left and right upper trapezius muscles are shown in Table 3. The results show that work pace significantly affects muscle activity in both the left and right upper trapezius muscles. For the left upper trapezius muscle, work pace had a strong effect (F = 15.194, p < 0.001, η² = 0.352), and there was a significant interaction between work pace and gender (F = 23.160, p < 0.001, η² = 0.453). However, gender alone did not significantly influence left upper trapezius muscle activity (F = 3.041, p = 0.092, η² = 0.098).

Table 3 Results of repeated measures ANOVA on muscle activity of bilateral upper trapezius muscles at different levels of work pace

For the right upper trapezius muscle, work pace also had a significant effect (F = 7.263, p = 0.022, η² = 0.173), but the interaction with gender was not significant (F = 0.108, p = 0.745, η² = 0.004). Gender alone significantly affected the right upper trapezius muscle activity (F = 5.806, p = 0.023, η² = 0.172).

Variation in muscle activity with time for work pace tasks

The variation in muscle activity with respect to time for the bilateral upper trapezius muscles during low-paced and high-paced tasks is shown in Figs. 2 and 3. The slopes of the graphs were positive for all muscles and work pace tasks, indicating an increase in muscle activity throughout the experiment. This consistent upward variation indicates the development of muscle fatigue over time. These findings highlight the physiological response to sustained or repetitive sewing tasks, emphasizing the need to manage the work pace to prevent excessive strain and fatigue in workers.

Fig. 2

Variation in muscle activity over time for the right upper trapezius muscles at different levels of work pace

Fig. 3

Variation in muscle activity over time for the left upper trapezius muscles at different levels of work pace

Discussion

In this study, the bilateral upper trapezius muscle activities of industrial sewing machine operators at two different levels of work pace (low pace and high pace) were recorded, analyzed, and compared with respect to gender. The muscle activity of the bilateral upper trapezius muscles (shoulder muscles) was measured using surface Electromyography (sEMG) in a real industrial work environment. The muscle activity of the right upper trapezius (RUT) and left upper trapezius (LUT), expressed as the RMS value (%MVC), was significantly greater in the high work pace task than in the low work pace task. As the subjects sped up the task, the muscle activity tended to increase. These results are also in accordance with previous studies that reported higher muscle activity with higher work pace levels while performing repetitive tasks [15,16,17]. This finding indicates that as the task accelerates, a pattern of increased sustained muscle activity can be observed. Another study by Laursen, Jensen [37], supports the results that shoulder muscle activity increases when the speed demand increases through hand movement. The gradual increase in muscle activity of bilateral upper trapezius muscles over time indicates the occurrence of muscle fatigue [38]. This finding is consistent with earlier research showing that increased muscle activity (RMS value) is accompanied by increased discomfort [39], and higher muscle fatigue [26, 40]. Muscle fatigue is an initiating factor of WMSDs [41]. Work pace is an important factor that can be utilized to prevent discomfort and muscle fatigue [26].

In this study, the right upper trapezius muscle exhibited a significantly higher level of activity during high work pace tasks, with muscle activity increasing by 30.4% compared to the low work pace task. Similarly, the left upper trapezius muscle showed a 24.12% increase in muscle activity during high work pace tasks, highlighting the overall impact of increased work pace on muscle fatigue. The reason might be that the sewing task was not symmetric and that most of the subjects were right-handed. The subjects performed more repetitive tasks with their right hand. Another reason might be that workers must extend their right hand farther from their body to pick up the fabric than their left hand for every cycle. Mohd Nur and Dawal [42] reported that muscle activity significantly increases while individuals perform work at the far boundary area. The work fatigue of the right upper trapezius muscle during the sewing operation was consistent with the findings of the previous studies [8, 10, 43]. This bilateral upper trapezius muscle fatigue was also consistent with other studies in which other repetitive tasks were performed [26, 44–47].

The analysis of bilateral upper trapezius muscle activity revealed that females exhibited higher muscle activity compared to male subjects during both the high-paced and low-paced tasks. This increase in muscle activity among females could be attributed to multiple factors. The differences in muscle endurance and strength between males and females can be a contributing factor, which may influence muscle contraction when performing repetitive activities. Variations in muscular mass and distribution can also have an impact. Additionally, ergonomic factors, such as workstation design and task requirements might also contribute. It is crucial to understand these differences as higher muscular activity in females can increase fatigue, potentially leading to the risk of work-related musculoskeletal disorders. By addressing these factors with appropriate ergonomic adjustments and targeted interventions can help mitigate the risk and improve overall workplace comfort and safety. This finding is consistent with numerous epidemiological studies that have reported high muscle activity among female sewing machine operators [8, 10, 29]. However, some studies did not find significant differences in muscle activity with respect to gender [48, 49]. While a few studies have indicated higher muscle activity among male subjects than females [31, 50]. These contrasting results suggest that the relationship between muscle activity and gender may vary and highlight the need for further research to better understand these differences.

Hence, this study concludes that work pace has a significant impact on bilateral upper trapezius muscle activity. A higher work pace leads to higher muscular activity that results in the form of muscular fatigue, discomfort, decreased efficiency, and work-related musculoskeletal disorders among sewing machine operators. Therefore, it is essential to manage work pace effectively to achieve high productivity goals in industrial settings as well as to ensure the health and safety of the workers.

In line with Industry 5.0’s human-centered approach, industries should adopt technological advancements and ergonomic interventions, such as adjustable workstations, improved layouts, and supportive seating, to reduce fatigue in bilateral upper trapezius muscles. Training programs on the importance of proper working posture and safe work practices can further enhance worker’s ability to perform work tasks efficiently without compromising their health and well-being.

In summary, this research highlights the critical need to balance work pace and worker health in industrial work environments. By understanding the impact of work pace on bilateral upper trapezius muscles, industries can implement strategies that protect workers from fatigue and long-term musculoskeletal issues while maintaining high levels of productivity.

Limitations

This study has several limitations. First, this study focused only on the bilateral upper trapezius muscle activity, while muscle activity of other muscle groups was out of the scope of this research, which limits the understanding of overall muscle fatigue of sewing machine operator. Additionally, this research findings are specific to sewing machine operations and may not be directly applicable to other industrial tasks involving different repetitive movements or body postures. Moreover, the major emphasis of this study was on muscle activity of sewing machine operators, while the mental activity of workers was not assessed, which can also have an impact on worker health and performance. Future research should address these limitations by examining a broader range of muscle groups and incorporating assessments of mental fatigue in similar industrial contexts.

Conclusion

This study concludes that a higher work pace leads to higher upper trapezius muscle activity among sewing machine operators. The findings also indicate that gender has a significant impact on upper trapezius muscle activity. While females experience higher muscle activity compared to males. These results emphasize the importance of managing work pace to mitigate the risks of muscle fatigue and work-related musculoskeletal disorders (WMSDs). This research has significant implications for both worker’s health and industrial productivity. Hence, it is critical to integrate ergonomic solutions, enforce regular breaks, and provide training on working postures, and safe work practices. Such measures are vital for creating a safer and more sustainable industrial work environment.