# Ergonomics Study of Musculoskeletal Disorders Among Tram Drivers
## 4 Discussion
### 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 ([PERSON] 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, [PERSON] 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%.