## 4 Discussion  
The aim of this study was to determine which flexibility measurement was best able to predict the corresponding FRP. The results indicated that TTT can significantly discriminate differences in LES FRP between high and low flexibility groups, and a positive correlation with the FRP onset was also found. This suggests that the higher the TTT, the slower the occurrence of FRP. The TTT could be considered as an appropriate tool for on-site worker recruitment and flexibility training when work requires a deeper trunk flexion. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tests** & **Muscles** & **r** & **Significance** \\ \hline TTT & TES & 0.365 & \(p\) < 0.05 \\  & LES & 0.553 & \(p\) < 0.01 \\ \hline SRT & TES & 0.388 & \(p\) < 0.05 \\  & LES & 0.260 & \(p\) = 0.097 \\ \hline MST & TES & 0.399 & \(p\) < 0.05 \\  & LES & 0.253 & \(p\) = 0.104 \\ \hline \hline \end{tabular} Notes: TTT—toe-touch test, SRT—sit-and-reach test, MST—modified Schober’s test, TES—thoracic erector spinae, LES—lumbar erector spinae.  
\end{table} 
Table 7: Correlation of FRP onset index with three flexibility measurements. Figure 5: LES EMG alterations at varying trunk flexions for three flexibility grouping methods (%MVC, percentage of MVC data). The correlations between MST and TTT, and MST and SRT, were relatively low, which may be because MST primarily measures lumbar spine flexibility. Regardless of whether the original Schober's test or MST used in this study is employed, it has been developed to detect patients with LBP such as ankylosing spondylitis (AS) [32]. Typical early symptoms include chronic low back pain and spinal stiffness, which cause difficulty in trunk bending [33]. The Schober's test is commonly used and is useful for screening the status of AS disease, and also for determining the progression and therapeutic effects of AS [34; 35]. In a validation study utilizing a Taiwanese sample, [PERSON] et al. [26] further clarified that the original [PERSON]'s test strongly correlated with MST measurements. Compared with TTT and SRT, which involve different extensibilities of muscle groups throughout the body, especially the HMS, MST only evaluates lumbar spine mobility, which results in a lower correlation with TTT and SRT. To assess the flexibility of the HMS muscle, field tests such as TTT and SRT are commonly used in sports training and physical fitness evaluation. The procedures of these measurements are simple, easy to administer, and have minimal skill requirements [36]. However, [PERSON] and [PERSON] [22] have identified significant differences in thoracic and lumbar spine postures; this may have affected the validity of the measurement and resulted in a correlation coefficient of 0.55-0.61 for TTT and SRT. [PERSON] et al. [37] also found that the corresponding correlation coefficient for young Spanish men and women (average 21 years) was 0.829 (\(p<\) 0.001), which was similar to the results of this study. Although the body postures between the two measurements appeared to be alike, it is difficult for them to replace each other. One of the possible reasons for this is that the sitting posture of the SRT anteriorly limits the rotation of the pelvis during measurement, and causes differences in postures between the thoracic and lumbar spine [28]. [PERSON] et al. compared lumbar and thoracic spine angles during TTT and SRT measurements, and found that although the lumbar spine angles of the two were not significantly different, the difference in thoracic spine angle was approximately 8\({}^{\circ}\), which can be attributed to the effect of the pelvis being restricted [28]. In the current study, when participants flexed their trunk from upright to 45\({}^{\circ}\), all muscle EMG values gradually increased as the trunk angle increased (TES: 5.1-8.3 %MVC, LES: 3.5-10.6 %MVC, HMS: 2.4-8.5 %MVC).