Ergonomics Study of Musculoskeletal Disorders Among Tram Drivers

Abstract

Work-related musculoskeletal disorders (WMSDs) are among the most prevalent occupational health issues, particularly affecting public transport drivers due to prolonged sitting, constrained postures, and poorly adaptable cabins. This study addresses the ergonomic risks associated with tram driving, aiming to evaluate biomechanical load and postural stress in relation to drivers’ anthropometric characteristics. A combined methodological approach was applied, integrating two standardized observational tools—RULA and REBA—with anthropometric modeling based on three representatives European morphotypes (SmallW, MidM, and TallM). ErgoFellow 3.0 software was used for digital posture evaluation, and lumbar moments at the L4/L5 vertebral level were calculated to estimate lumbar loading. The analysis was simulation-based, using digital human models, and no real subjects were involved. The results revealed uniform REBA (Rapid Entire Body Assessment) and RULA (Rapid Upper Limb Assessment) scores of 6 across all morphotypes, indicating moderate to high risk and a need for ergonomic intervention. Lumbar moments ranged from 51.35 Nm (SmallW) to 101.67 Nm (TallM), with the tallest model slightly exceeding the recommended ergonomic thresholds. These findings highlight a systemic mismatch between cabin design and user variability. In conclusion, ergonomic improvements such as adjustable seating, better control layout, and driver education are essential to reduce the risk of WMSDs. The study proposes a replicable methodology combining anthropometric, observational, and biomechanical tools for evaluating and improving transport workstation design.

1. Introduction

Ergonomics, also called human factors, is an interdisciplinary scientific discipline focused on the interaction between people and their environment. In the study of ergonomics, a scientific approach is applied including theory, principles, data, and design methods to optimize human well-being and overall system performance [1]. Therefore, ergonomics can be defined as a discipline that offers both the skills and approaches necessary for analyzing work and professional tasks. The scientific literature shows that ergonomics can provide new perspectives on workplace well-being, which are connected to the social dimension of sustainability [2,3,4,5]. The main goal of ergonomics is to adapt the job to the individual, rather than forcing the individual to adapt to the job [6].

The work environment, job demands, and level of control all influence the mental and physical condition of the worker. An ergonomically inadequate workplace can contribute to both physical and emotional stress. Ultimately, both types of stress are associated with reduced productivity and lower quality of work [6]. Several theoretical approaches within the work pressure framework emphasize the importance of designing healthy working conditions [7]. Ignoring ergonomic principles—guidelines for designing work environments, tools, and tasks to enhance comfort and efficiency—can result in pain and, eventually, injury. Most of this pain is related to musculoskeletal disorders (MSDs), which include a set of injuries and symptoms affecting the osteomuscular system and associated structures [8]. Symptoms of work-related MSDs (WMSDs) are typically expressed as pain in one or more regions of the body. Accumulated microtraumas caused by prolonged and repetitive work-related loading are considered a key cause of WMSDs [6]. Thus, working conditions, including the environment, tools used, and immediate surroundings, contribute to the type and frequency of musculoskeletal disorders. However, organizational factors and health outcomes are still insufficiently investigated [9,10,11].

Work-related musculoskeletal disorders (WMSDs) are injuries or conditions related to job demands that place the whole body or specific body parts in positions that lead to pain in spinal disks, as well as surrounding nerves, joints, muscles, tendons, and the circulatory system [12]. WMSDs are further aggravated by psychosocial factors [13,14], although the mechanisms are more complex [13]. These disorders can lead to pain, stiffness, and reduced mobility, all of which affect daily activities and quality of life. The most influential causes and risk factors include repetitive motion, poor posture, age, obesity, and genetic predisposition. Almost half of the causes stem from the type of work and the work environment itself. Many WMSDs are directly linked to job performance in physically demanding roles [12]. By taking a proactive approach to health and well-being, individuals can better manage symptoms and maintain an active, fulfilling lifestyle.

Globally, low back pain caused by WMSDs is a leading cause of disability, with a rising prevalence in low- and middle-income countries in recent years [15,16]. Workers who spend prolonged hours sitting, such as drivers, often suffer from low back pain due to constrained positions, along with vibrations and jolts experienced while driving [15,16]. It is important to emphasize that WMSDs rank among the most common occupational disorders across various industries [13]. Biomechanically, prolonged seated posture increases compressive and shear forces at the lumbosacral junction, particularly at the L4/L5 and L5/S1 segments. Reduced spinal mobility, combined with static muscle activation, leads to higher intradiscal pressure and accelerates degenerative changes. In response to pain, drivers often adopt asymmetrical or guarded postures, which further disrupt spinal alignment and increase muscular fatigue. Whole-body vibration and limited postural variation contribute to cumulative spinal stress, making professional drivers particularly vulnerable to chronic biomechanical overload [10,17,18,19]. In 2014, Bhattacharya accounted for approximately 67% of work-related injuries and illnesses in Korea, while in the U.S., they made up 29–35% of such cases from 1992 to 2010 [13,20]. Globally, WMSDs are responsible for nearly 40% of the compensation costs related to occupational injuries and diseases, highlighting their significant economic and health impact [20].

Drivers frequently experience a range of medical issues including cramping, muscle fatigue, reduced circulation, back pain, stiffness, and even chronic conditions. Symptoms like depression and spinal numbness are also reported. While a comfortable seat may provide relief in static conditions, it often lacks proper support during dynamic movements, resulting in discomfort while driving [21]. One study examined the connection between lower back pain, manual material handling, and whole-body vibration in various public transport bus models—insights that can also apply to other public vehicle drivers. The results indicated that many drivers face an increased risk of chronic lower back pain [21,22]. WMSDs and related injuries are becoming a growing concern in the transportation sector, affecting areas such as the shoulders, wrists, forearms, back, and neck [13]. These conditions result in significant discomfort, including nonspecific low back pain, neck pain, and carpal tunnel syndrome [13]. Tram drivers are particularly affected due to repetitive movements, exposure to vibration, muscular strain, and a challenging psychosocial work environment [13]. Additionally, long periods of sitting make ergonomic seat design crucial to avoid slouched postures that contribute to WMSDs [13,23].

Despite their major impact on worker health and public safety, WMSDs are often underestimated. They are usually seen as individual medical problems, yet their consequences extend to the broader system. This includes issues such as shift scheduling, rostering difficulties, and increased pressure on the remaining workforce [13,16,23]. WMSDs are frequently linked to physical ergonomics, with design-related causes and solutions partly responsible. In the literature, there are cases where tram organizations blamed the design of the Master Controller—the primary speed and acceleration interface—as the root cause of WMSDs among drivers [13,16,23]. However, WMSDs are symptoms of wider systemic issues that include equipment design but are also shaped by complex, long-term factors [24].

To manage and prevent WMSDs effectively, it is necessary to assess workers’ exposure to risk factors and implement interventions to reduce physical strain to safe levels. This helps minimize the likelihood of developing musculoskeletal issues at work [25,26]. To quantify and qualify the link between low back pain and WMSDs caused by awkward or static postures, excessive force, repetitive actions, and environmental factors, observational techniques have been widely used [25,26].

Although the use of direct measurement methods has grown only slightly, observational techniques saw a significant rise in adoption among U.S. ergonomists in 2017 compared to 2005 [13,25,26]. These techniques are widely favored due to their affordability, simplicity, flexibility, and ability to evaluate workers without interrupting their tasks [8,27].

According to the literature, musculoskeletal load—defined as a function of posture, force, and duration—correlates with WMSD frequency [5]. A well-balanced workload can reduce the risk of developing WMSDs, which otherwise threaten worker safety. To ensure the workplace is ergonomically suitable for most workers in various conditions, conducting an ergonomic assessment is essential. Numerous tools are available for such evaluations, and choosing the right one for a specific task is crucial. The most used noninvasive analytical tools in ergonomics are the Rapid Upper Limb Assessment (RULA) and the Rapid Entire Body Assessment (REBA). Their key feature is the direct observation of work processes. These tools differ in assessment phases, targeted body regions, and applicable work tasks. Thus, it is important to select the appropriate tool before beginning an assessment [21]. Their primary advantages include non-interference with workflow, the lack of a need for expensive equipment, and consistent usability across diverse conditions [8,27]. The recent literature has investigated the relationship between WMSD-causing low back pain and RULA scores, with findings revealing a direct link between neck pain, age, gender, and working posture [28,29].

In this study, we investigated the ergonomic challenges associated with tram driving, with a particular focus on the selection and application of appropriate assessment tools to evaluate musculoskeletal strain caused by prolonged sitting. By analyzing different body morphotypes in simulated seated postures within a tram cabin, the study highlights biomechanical differences and potential risk factors. The objective of this study was to evaluate the ergonomic risks of tram driving by analyzing musculoskeletal strain and lumbar spinal loading across three representative digital body models (SmallW, MidM, and TallM) in seated postures. Using RULA, REBA, and biomechanical modeling, the study aimed to identify design mismatches in the tram cabin and propose a replicable methodology for ergonomic workstation assessment.

2. Materials and Methods

2.1. Sample

This study was conducted as a simulation-based ergonomic assessment using standardized digital human models. The methodology integrates three components: digital anthropometric modeling, postural risk assessment, and lumbar moment calculation. Ergonomic risk assessment was conducted using two widely recognized observational methods: the Rapid Upper Limb Assessment (RULA) and the Rapid Entire Body Assessment (REBA). The assessments were carried out with the aid of ErgoFellow 3.0 software(Belo Horizonte, State of Minas Gerais, Brazil), which facilitated systematic data entry, scoring, and interpretation of the ergonomic risks according to the RULA and REBA methodologies.

No human participants were involved in the study. Instead, three representative digital morphotypes—SmallW (Small Woman), MidM (Mid-sized Man), and TallM (Tall Man)—were selected to approximate the 5th percentile female, 50th percentile male, and 95th percentile male of the European adult population [30]. As such, no formal inclusion or exclusion criteria applied, and sample size calculation was not relevant.

The analysis was performed in the following steps:

• Anthropometric parameters were obtained from existing datasets and adjusted using Muftić’s harmonic method.
• Postural risk levels were assessed using RULA and REBA tools within the ErgoFellow 3.0 software environment.
• Lumbar loading was estimated by calculating the moment at the L4/L5 spinal segment based on segmental mass and body posture.

2.2. Anthropometry in Tram Driver Workspace

Anthropometry, the systematic measurement of human body dimensions, plays a key role in ergonomic design, particularly when it comes to seated work environments such as tram driver cabins [31]. Accurate anthropometric data enables the design of tools, workstations, and vehicle interiors that are aligned with the body proportions of users, reducing the risk of musculoskeletal disorders and improving safety and comfort [32]. In driver-oriented workplaces, improper alignment between body dimensions and cabin geometry can result in awkward, static postures, repetitive strain, and insufficient support. Research shows that professional drivers frequently suffer from lower back pain (56%), knee pain (43.75%), and neck pain (39%) as a result of such mismatches [20].

In this study, anthropometric data were applied to analyze the ergonomic compatibility of the tram driver’s cabin. The data source was the study by Bertrand et al. (2006) [30], which offers detailed external and internal body geometry of 64 European adults. The subjects were grouped into three morphotypes—Small Women (SmallW), Mid-sized Men (MidM), and Tall Men (TallM)—representing a wide range of body dimensions relevant to European driver populations. According to international ergonomics standards, ergonomic workplace design should accommodate at least 90% of the user population, which is typically achieved by designing for the range between the 5th and 95th percentiles [33]. The selected morphotypes correspond approximately to key anthropometric percentiles: SmallW represents the 5th percentile female, MidM the 50th percentile male, and TallM the 95th percentile male, thus covering a broad range of the target population. The study includes comprehensive data on height, limb length, body mass, segment circumferences, and internal skeletal geometry based on 3D stereoradiographic reconstructions.

This classification enabled the calculation of body segment lengths and masses through a biomechanical method developed by Donskoy and Zatsiorsky. Their model is based on a statistical analysis of 200 individuals and uses regression equations derived from radioisotope volume measurements to estimate segmental masses [34,35]. These values were further refined using Muftić’s harmonic circle method [34,35], which relates body segment lengths to total body height through a canonical system of proportions based on eight head heights. This approach allows for the estimation of static anthropometric dimensions corresponding to each of the three morphotypes (SmallW, MidM, and TallM), providing a consistent foundation for ergonomic evaluation.

The application of these methods allowed precise estimation of segmental lengths and postural parameters for seated drivers. These values served as a reference for analyzing spatial congruence between the driver’s body and cabin layout, particularly the positioning of the seat, pedals, armrests, and control interfaces. Figure 1 illustrates the wireframe representations of all three morphotypes seated in the tram cabin. The corresponding body segment lengths for each morphotype are presented in

Table 1. Anthropometric measurements of morphotypes TallM, MidM, and SmallW.

	TallM		MidM		SmallW
Height, cm	184.8		179.6		159.1
Mass, kg	100.2		76.1		50.4
	Weight force, N	Length, cm	Weight force, N	Length, cm	Weight force, N	Length, cm
Foot	12.7	23.1	10.5	22.5	5.7	19.9
Lower leg	37.4	40.4	29.3	39.3	27.4	34.8
Thigh	140.0	52.0	105.4	50.5	75.8	44.7
Hand	5.6	20.2	4.6	19.6	2.8	17.4
Forearm	15.3	23.1	11.9	22,5	7.8	19.9
Upper arm	27.2	28.9	20.2	28.1	14.9	24.9
Head and neck	55.2	32.2	50.4	31.3	46.3	29.6
Upper trunk	158.5	21.0	117.4	20.5	61.7	15.4
Mid-trunk	169.8	19.7	120.4	18,4	75.8	16.5
Lower trunk	111.2	13.7	85.6	14.0	55.6	13.0

. These visual and numerical representations form the basis for the subsequent biomechanical analyses presented in the study.

2.3. RULA

The Rapid Upper Limb Assessment (RULA) is a standardized observational method developed to assess postural and biomechanical risks related to the neck, trunk, and upper limbs during sedentary or repetitive tasks [8,27]. The method assigns risk scores based on the angles of specific body parts, muscle use, and external loads, producing a final value that indicates the urgency of ergonomic intervention. RULA is particularly suitable for evaluating workstations that involve static postures, limited movement, or constrained environments, such as seated driving. It does not require invasive measurements and can be effectively applied using digital simulations [8,27]. The evaluation process is based on established scoring charts and decision trees, as shown in Figure 2.

In this study, RULA was applied to assess the seated working posture of three digital morphotypes (SmallW, MidM, and TallM) during tram operation. Each model was analyzed in a standardized seated position, reflecting typical hand placement on the driving lever and body alignment relative to the control panel and seat. The assessment was conducted using the ErgoFellow 3.0 software, which integrates the RULA scoring algorithm and allows precise posture modeling based on input joint angles [27,28]. The final scores were generated for each morphotype, enabling a comparative analysis of ergonomic risk across body types [8,27,36].

2.4. REBA

The Rapid Entire Body Assessment (REBA) is an observational ergonomic tool designed to evaluate the risk of musculoskeletal disorders associated with whole-body postures during work tasks. It assesses key body segments—neck, trunk, legs, upper arms, lower arms, and wrists—while accounting for coupling quality, task type, and external load [2,24]. REBA is especially useful for identifying risk in dynamic and spatially constrained environments, such as vehicle cabins, where workers must maintain fixed postures for extended periods. Like RULA, it produces a numerical score that corresponds to an action level, indicating whether immediate intervention is required. The structured step-by-step evaluation procedure is presented at Figure 3.

In this study, REBA was used to assess the seated posture of tram drivers represented by three digital morphotypes (SmallW, MidM, and TallM). The analysis focused on spinal alignment, lower limb positioning, and trunk deviation, with special attention given to the operator’s interaction with fixed control interfaces. Postural data were processed using ErgoFellow 3.0 software, which allowed for standardized risk scoring and comparison across different body types. The results provided insight into how workstation geometry influences ergonomic risk at the whole-body level, complementing the upper body-focused RULA analysis [2,24].

2.5. Ergofellow 3.0 Software

For the ergonomic evaluation, the ErgoFellow 3.0 software was used to analyze selected postures. This application integrates 17 ergonomic assessment methods aimed at minimizing occupational risks, enhancing workplace quality, and supporting productivity. In this study, one representative frame of a tram driver in a seated position—capturing the moment of greatest deviation from the neutral (N) posture—was extracted and assessed using the REBA and RULA tools within ErgoFellow [34,35]. The selected frame served to highlight the most critical ergonomic risks associated with the driving task [37].

2.6. Driver’s Working Posture in the Tram Cabin

In addition to the assessment of working posture, specific characteristics of the tram driver’s cabin were considered, as they directly influence body alignment, range of motion, and overall ergonomic load. The cabin is generally compact, with limited space for repositioning during prolonged sitting. The driver’s seat is adjustable in height and longitudinal direction; however, the degree of adjustability is often insufficient to accommodate drivers of varying anthropometric profiles. The backrest is slightly reclined but lacks dynamic lumbar support, which can lead to passive spinal loading over time.

For example, the tram driver’s control panel is typically divided into left, central, and right sections. The central section includes a monitoring display, while the left side houses are functions less frequently used. The most frequently used controls—such as door buttons and emergency braking—are positioned on the right side. The driving lever (T-handle), located on the left, plays a central role in operational control. The ergonomic analysis considered the relative reach, visibility, and adjustability of these components for each morphotype, ensuring that the driver’s posture remains within safe biomechanical limits throughout the workday (Figure 4).

The control interface, including the T-handle lever and dashboard, is fixed in position, typically located at a horizontal distance of approximately 32 cm from the seat reference point, and at a vertical height of 73 cm relative to the floor. This requires the operator to maintain a forward-reaching posture. The left armrest, where the T-handle is located, is positioned at a height of 70 cm, which may or may not align optimally with the driver’s elbow height, depending on body dimensions. The right armrest is positioned at a height of 60 cm, particularly with the dominant arm moving forward when needed to reach the right control panel. Legroom is restricted laterally, often causing drivers to keep their knees and feet close together throughout the shift.

Overall, the spatial configuration and fixed elements of the cabin contribute to reduced postural variability and limited ergonomic adaptation, increasing the potential for static muscular loading, particularly in the lumbar spine, shoulders, and neck regions.

2.7. Lumbar Moment Calculation at L4/L5

The lumbar moment at the L4/L5 intervertebral level is a critical biomechanical parameter that reflects the mechanical load on the lower spine. Elevated moments at this level have been strongly associated with the development of low back pain and other musculoskeletal disorders, particularly in occupations involving prolonged sitting or awkward postures [10,17,38].

In this study, the lumbar moment around the L4/L5 joint was calculated based on anthropometric data and segmental modeling. The procedure involved several key steps. The first step was calculating segmental mass and length estimation. Segment masses were calculated using the method of Donskoy and Zatsiorsky, which relies on regression equations derived from a study of 100 male and 100 female participants, using radioisotope-based volume estimation. Segment lengths for the three representative anthropometric models, SmallW, MidM, and TallM, were computed from the same dataset [34,35]. The harmonic circle method proposed by Muftić [34,35] was then applied to refine segment length estimation, using proportional relationships based on eight canonical head heights. The next step was, for each body segment, determining the position of the center of mass according to the data provided by Kromer and Muftić, expressed as the distance from the proximal joint of the segment. Then, the segmental weight force was calculated, and the force was positioned in center of mass. Using a graphical method based on sagittal plane postural analysis, the lumbar moment at L4/L5 was calculated for segmental weight force (M = F × d). The total lumbar moment at L4/L5 was obtained by summing the individual moments of all body segments, under the assumption of static equilibrium. All calculated values, including segment lengths, masses, segmental weight forces, and distances to L4/L5, are provided in Table 1. The position of the segmental weight forces is shown in Figure 5. The forces are put in the center of mass, as defined by the literature [34,35].

3. Results

3.1. RULA and REBA

Working postures were analyzed using the REBA and RULA methods to assess the risk of work-related musculoskeletal disorders (WMSDs). A single representative posture was evaluated for three anthropometric models (SmallW, MidM, and TallM). REBA and RULA evaluations were conducted for all three morphotypes. The results yielded consistent scores across models. Each driver received a REBA score of 6, placing them in the moderate risk category (action level 2) [37]. The RULA analysis also returned a score of 6 for each model, placing the posture in the high-risk category (action level 3) [8,27]. These results indicate that ergonomic interventions are needed for all morphotypes. RULA and REBA results are shown in Figure 6 and summarized in

Table 2. Results for RULA, REBA, and for lumbar moment at L4/L5 for SmallW, MidM, and TallM.

	TallM	MidM	SmallW
RULA	6	6	6
REBA	6	6	6
L4/L5, Nm	101.67	71.12	51.35

.

In this study, working postures were analyzed using the REBA and RULA methods to assess the risk of work-related musculoskeletal disorders (WMSDs). A single representative posture was evaluated for three anthropometric models (SmallW, MidM, and TallM). The analyzed position was captured in the sagittal plane, with the right arm resting on the armrest and the left arm elevated, holding the driving lever positioned on a higher surface. The driver’s back was in contact with the backrest at an angle of 100°, and the legs were positioned closely together in a typical seated posture. The wireframe representations of all three models in this posture are shown in Figure 5.

3.2. Lumbar Moment at L4/L5 and the Influence of Anthropometry

The lumbar moments around the L4/L5 joint were calculated based on the sum of gravitational forces acting on each body segment. These moments reflect the mechanical load exerted on the lower spine and were determined by three different anthropometric models: SmallW, MidM, and TallM. The obtained values indicate that the lumbar moment was 51.35 Nm for SmallW, 71.12 Nm for MidM, and 101.67 Nm for TallM. These results demonstrate a linear increase in spinal load with increasing body height. The summary results for RULA, REBA, and lumbar moments are presented in Table 2.

4. Discussion

4.1. Interpretation of Postural Risk Scores

REBA and RULA evaluations were conducted for all three anthropometric types. The results yielded consistent scores across models. Each driver received a REBA score of 6, placing them in the moderate risk category (action level 2), indicating the need for further investigation and suggesting that the posture and work environment should be modified in the near future [37]. This score reflects a work scenario where ergonomic interventions are necessary to reduce the likelihood of musculoskeletal disorders.

The RULA analysis also returned to a score of 6 for each model, placing the posture in the high-risk category with an action level of 3. This score signals the need for prompt ergonomic assessment and corrective action [8,27]. According to RULA interpretation guidelines, scores of 5–6 indicate a posture that requires investigation and imminent change [36]. The RULA and REBA results are shown in Figure 6.

Prolonged sitting is recognized as one of the primary risk factors for developing musculoskeletal disorders among public transport drivers [27,39]. Sitting continuously for more than 30 to 60 min is considered detrimental to musculoskeletal health [23,29], and tram drivers often remain seated for over two hours without interruption [23,29].

REBA and RULA scores of 6 reflect problematic static postures that substantially increase the risk of WMSDs [5]. Awkward or sustained positions—such as elevated arms, a twisted neck, or unsupported trunk—result in mechanical stress, including tendon and nerve compression [24]. In tram driving, the combination of constrained movement and long static periods introduces complex ergonomic challenges [17,40,41].

The consistency of REBA and RULA scores across SmallW, MidM, and TallM profiles suggests a systemic design limitation in the tram cabin [16,37]. The current configuration does not adequately accommodate anthropometric variability among drivers, which contributes to elevated ergonomic risk [40]. These findings are consistent with prior studies reporting that professional drivers often experience WMSDs, particularly in the shoulder, neck, and lower back regions [23,42].

Numerous studies confirm that professional drivers (bus, tram, and truck) are exposed to high postural risks. For example, Yasobant et al. (2015) reported that 46% of bus drivers fell into the high-risk category based on REBA scores (8–10), while 46% also required ergonomic intervention based on RULA score 6 [38,43,44]. Similar results were observed in a study on intercity drivers in Indonesia, where the average REBA score was 6, with 57% of drivers classified as moderate risk and 29% as high risk [42]. That study also found a significant correlation between poor posture (REBA ≥ 4) and the frequency of WMSD symptoms. Das et al. (2022) confirmed through RULA analysis that the lower back, neck, and shoulders are especially vulnerable to overload in long-duration driving tasks [44].

In summary, both REBA and RULA methods consistently identify seated driving postures as high-risk, demanding ergonomic modifications [44]. For example, Das et al. reported a RULA grand score of 6 (the highest possible), indicating a critical need for intervention in bus driving environments [43,45]. These findings align with data from the Nordic Musculoskeletal Questionnaire (NMQ), which show that 24% to 35% of drivers report lower back pain in various studies [40,42]. The literature repeatedly highlights a strong correlation between poor sitting posture (e.g., trunk leaning, elevated arms, or neck torsion) and elevated REBA/RULA scores, alongside a higher prevalence of WMSD symptoms [43,45].

Based on these insights, a reference framework can be proposed: an average driver with a REBA score of 6, RULA score 6, and an estimated lumbar moment falls into a moderate to high-risk category, reinforcing the need for targeted ergonomic adjustments in tram cabin design. Results for RULA and REBA as well as for values of lumbar moment at L4/L5 are provided at Table 2.

4.2. Interpretation of Lumbar Moment Results

The lumbar moment results further illustrate biomechanical variation across different body types. These values are significant biomechanical indicators that reveal the increasing spinal load in correlation with body size. The L4/L5 segment is of particular interest because it is responsible for approximately 95% of lumbar flexion and extension, making it the most exposed area of the lower spine during sitting [10,17]. It is also the second most common site of intervertebral disk herniation after the L5/S1 segment, and its intervertebral disk is especially susceptible to degenerative changes due to continuous loading [10,17]. Research has shown that sitting without back support can increase the load on the lumbar disks by up to 40% compared to standing [10,17], clearly indicating that seated posture places the spine under significant mechanical stress.

The lumbar moment at the L4/L5 level thus provides a direct biomechanical measurement of the load imposed on the spine during sitting [18]. Results obtained in this study demonstrate a clear and consistent increase in lumbar moment with increasing driver height. The lowest moment was recorded for the SmallW model, followed by a moderate increase for the MidM model, and a considerably higher value for the TallM model. This progression is consistent with fundamental biomechanical principles: individuals with greater body height have longer segmental lever arms, which increases the moment acting on the lumbar region under the influence of body weight and gravity [34,35].

The observed value of 101.67 Nm for the TallM model is particularly important, as it indicates a heightened risk for the development of work-related musculoskeletal disorders (WMSDs). This elevated moment, combined with prolonged static sitting, creates conditions that increase the likelihood of intervertebral disk herniation, contribute to the development of degenerative spinal changes, and impose greater ergonomic risks on taller drivers [19].

To assess biomechanical risk, the lumbar moments at the L4/L5 spinal segment obtained for the three morphotypes (SmallW, MidM, and TallM) were compared against the recommended thresholds provided by several ergonomic standards and biomechanical models. According to the guidelines from NIOSH (1991) [46] and ISO 11228-1 [47], the maximum recommended lumbar moment under favorable working conditions is approximately 100 Nm. Biomechanical models (Chaffin et al., 2006) similarly propose acceptable limits in the range of 60 to 100 Nm for prolonged or repetitive loading scenarios [48]. The results of the comparison show that the lumbar moment for the SmallW morphotype (51.35 Nm) is well within safe limits. For the MidM morphotype (71.12 Nm), the moment exceeds the average values typically observed in common occupational tasks yet remains within accepted ergonomic thresholds. In contrast, the TallM morphotype demonstrates a lumbar moment of 101.62 Nm, which slightly exceeds the upper limit proposed by certain standards. This may indicate an increased biomechanical risk for individuals of taller stature. These findings highlight the variability of spinal loads across different body types and point to the necessity of personalized ergonomic interventions in the design of tram cabins. Tall drivers may require additional adjustments to minimize the risk of lumbar overload during prolonged seated work.

Modern ergonomic research increasingly emphasizes the importance of measuring and mitigating such spinal loads. For example, Gao et al. (2023) conducted biomechanical modeling of seated posture and found that ergonomic modifications—such as adjusting the backrest angle and adding lumbar support—can significantly reduce forces and moments in the lumbar region [17]. Their findings showed that the use of a 4 cm lumbar support pad and a seat pan tilted at 10 degrees reduced L4–S1 loading by up to 11–26%. Moreover, they identified that a backrest angle between 29 and 33 degrees, in combination with a slight seat tilt, produced the most favorable results in terms of spinal unloading [17].

These findings underscore the importance of applying ergonomic principles to tram driver cabin design. Adjustability in seat configuration, backrest angle, and lumbar support can play a decisive role in reducing lumbar load and, consequently, in lowering the incidence of WMSDs. The results of this study support the notion that biomechanical load is directly influenced by anthropometric characteristics and point to the necessity of adapting cabin geometry to a range of body types in order to preserve driver health and long-term occupational performance.

4.3. Study Limitations

This study was conducted using three digital morphotypes to approximate anthropometric variability, without involving real human participants. Although the morphotypes—SmallW, MidM, and TallM—were selected to approximate the 5th percentile female, 50th percentile male, and 95th percentile male, thereby representing approximately 90% of the European tram driver population, it is acknowledged that this selection may not fully capture underrepresented user groups, such as shorter men or taller women. Therefore, it is recommended that future studies include a more diverse range of body types to support more inclusive ergonomic design. For the assessment of anthropometric compatibility between tram driver body types and the workspace, the existing anthropometric data for SmallW, MidM, and TallM were evaluated using Muftić’s harmonic analysis method [34,35]. This method allows a structured comparison of human body measurements with the spatial characteristics of the workplace, supporting ergonomic adaptation across diverse body sizes. The analysis was further supported by design principles from [48] and aligned with relevant ergonomic standards such as ISO 7250-1 and DIN 33402 [49,50].

4.4. Practical Implications and Design Recommendations

The study highlights the importance of integrating adjustable components into tram cabin design, such as seat height, lumbar support, and control positioning. Procurement specifications for public transportation vehicles should explicitly require ergonomically adaptable workspaces. By aligning vehicle design with anthropometric diversity, operators can reduce the risk of WMSDs and improve driver comfort and performance. These findings reinforce the need for future tram designs to incorporate adjustability features as a standard requirement. Designing solutions that allow adaptation for the seat, pedal, and control panel can significantly improve ergonomic fit and accommodate a broader user base. Such criteria should be integrated into rolling stock procurement to ensure compatibility with anthropometric diversity.

The findings of this study emphasize the critical role of anthropometric compatibility in the ergonomic design of tram driver cabins. Through the application of RULA and REBA tools, as well as the biomechanical modeling of lumbar loading, it was demonstrated that current driver workstations impose moderate to high musculoskeletal risk across all three representative morphotypes (SmallW, MidM, and TallM). Identical REBA and RULA scores (score 6) indicate that key body regions—particularly the lumbar spine, neck, and shoulders—are under considerable static and postural strain during typical tram operation. This study represents a preliminary ergonomic analysis based on three representative morphotypes (SmallW, MidM, and TallM), selected to illustrate potential mismatches at the lower, middle, and upper ends of the anthropometric spectrum. Although these models do not cover the full diversity of the population, they provide a foundation for methodological validation. Future research will include a broader range of body types and measurements from actual participants to improve the representativeness and applicability of the results. In line with ergonomic design principles, it is widely recommended that workplaces be designed to accommodate at least 90% of users—typically ranging from the 5th to the 95th percentile—in order to ensure inclusivity, safety, and comfort across the working population. This approach helps minimize the exclusion of both smaller and larger individuals and reduces the risk of work-related musculoskeletal disorders [3,47,51]. While this study included three morphotypes corresponding approximately to key percentiles (SmallW ≈ 5th percentile female, MidM ≈ 50th percentile male, TallM ≈ 95th percentile male), future research will expand to a more detailed percentile-based design analysis to support broader design applicability.

Additionally, the calculated lumbar moments at the L4/L5 intervertebral level (51.35 Nm for SmallW, 71.12 Nm for MidM, and 101.67 Nm for TallM) confirm that taller and heavier drivers experience significantly greater mechanical loading. This loading increases the risk of disk herniation and long-term spinal degeneration, especially in professions involving prolonged sitting without adequate support. As previous studies have shown, unsupported or improperly flexed postures considerably elevate the pressure on intervertebral disks, particularly at the L4/L5 level, which is already anatomically predisposed to biomechanical stress.

The focus of this study was on evaluating the current tram driver workspace and exploring how minimal adjustments, especially those under the driver’s control, could contribute to reducing the risk of work-related musculoskeletal disorders (WMSDs). While general recommendations for system-level changes were beyond the scope of this preliminary analysis, future work will address procurement-level guidelines and provide more comprehensive design recommendations aligned with anthropometric standards such as ISO 3411, ISO 7250-1, and DIN 33402-2 [47,50,52]. In procurement processes for new tram rolling stock, it is essential that technical specifications explicitly require adjustable driver workspace components—including seat height and depth, lumbar support, pedal reach, and steering/control interfaces—to accommodate the anthropometric diversity of operators. Incorporating ergonomic criteria based on international standards (e.g., ISO 7250-1, ISO 3411) into procurement guidelines ensures that newly acquired vehicles are designed for inclusive use, reducing the risk of injury and improving long-term driver’s comfort and performance.

Based on these results, several recommendations are proposed to mitigate ergonomic risks and improve workplace health outcomes. First, adjustable seating systems with dynamic lumbar support are essential. Seat height, backrest angle, seat depth, and lumbar pad thickness must be adjustable to accommodate drivers of various body sizes and reduce the degree of spinal flexion during driving. Second, the control layout should be optimized to minimize hand reach distances and shoulder elevation. Placing frequently used controls within the driver’s primary reach zone can prevent overuse injuries and reduce shoulder fatigue.

Third, regular movement and micro-breaks should be encouraged throughout shifts. Even brief stretching exercises, particularly targeting the lower back, shoulders, and neck, have been shown to reduce musculoskeletal discomfort. Finally, training programs focused on posture awareness and workstation adjustment are vital. Educating drivers on optimal sitting posture, proper mirror positioning, and the risk of prolonged static load promotes behavioral changes that support long-term health.

These conclusions align with recent ergonomic research emphasizing the need for systemic design improvements and health education programs in the transport sectors. Contemporary design guidelines highlight the importance of continuously updating workstation geometry based on current anthropometric data, including gender-specific and population-wide variations. Future tram cabin designs should integrate virtual fit modeling and diverse user profiles during the early development phases to ensure safe, inclusive, and sustainable working conditions for all drivers.

5. Conclusions

This study identified moderate to high ergonomic risk across three representative tram driver morphotypes (SmallW, MidM, and TallM), based on consistent RULA and REBA scores and progressively increasing lumbar moments. The highest spinal load was observed in the TallM model, slightly exceeding recommended ergonomic thresholds, indicating that taller drivers are particularly at risk. These findings point to a systemic mismatch between cabin design and the anthropometric diversity of the driver population. The results underscore the importance of incorporating adjustable seating, optimized control layouts, and inclusive design standards into tram procurement processes to reduce the risk of WMSDs and improve long-term driver well-being.