## 4 Discussion
Once the trunk was flexed beyond 45\({}^{\circ}\), FRP occurred first at LES (10.6-9.8 %MVC), followed by TES (60\({}^{\circ}\), 9.7-8.4 %MVC) and then HMS (75\({}^{\circ}\), 9.3-8.0 %MVC), as shown in Figure 4. This indicates a time sequence in the occurrence of FRP among the three muscle groups. This may be due to the roles these muscle groups play. [PERSON] et al. [38] observed that in normal trunk flexion, the lumbar vertebral bodies bend forward during the first 50\({}^{\circ}\)-60\({}^{\circ}\), and after that, the pelvis rotates anteriorly. At approximately 75-85% trunk flexion, the lumbar spine reaches its maximum range of motion, while the pelvis undergoes terminal flexion to accomplish total trunk flexion [15]. At this stage, passive soft tissues that are noncontractile, such as intervertebral discs and intervertebral ligaments, begin to provide most of the spinal support and reduce the back muscle activation to maintain the posture [12; 15]. This leads to the occurrence of FRP. It can be inferred that the FRP of the HMS would occur after the occurrence of back muscle FRP. [PERSON] examined the LES and HMS EMGs during trunk flexion, and found that the TES FRP occurred when the trunk was bent forward at about 80\({}^{\circ}\), whereas the HMS FRP occurred at almost full trunk flexion [39]. This implies that when the trunk is bent forward, in addition to the elongation of back muscles, the HMS, which is closely related to pelvic movement, is subsequently elongated, and FRP occurs. This has been confirmed by several recent studies [3; 20]. [PERSON] found that when TES FRP occurs, all participants display more active HMS contraction. Our result is consistent with that of [PERSON], in which the trunk was flexed forward from 45\({}^{\circ}\) to 75\({}^{\circ}\)[16]. [PERSON] postulated that increased HMS contraction occurs because the pelvis turns to stabilize the spine. Figure 4 shows that the TES FRP occurred after the LES and before the HMS. During trunk flexion, the contribution of lumbar spine bending is greater than that of the thoracic spine [40], which explains why TES EMG was relatively low at the first 30\({}^{\circ}\) of trunk flexion. LSA was maintained at approximately 40\({}^{\circ}\) when the trunk was upright. Once flexion began, it gradually decreased until it eventually became negative at 90\({}^{\circ}\). Thus, the lumbar spine changed from lordotic, converting into a kyphotic posture [20]. Furthermore, when LSA dropped to about 20\({}^{\circ}\), and the trunk was flexed at 45\({}^{\circ}\), LES FRP began to occur. Different flexibility levels had a significant effect on LES activation for all three measurements (all \(p<0.05\), Table 4). This verified that individual flexibility (high, middle and low) caused different degrees of contraction of LES, within which TTT significantly distinguished groups with high and low flexibility, whereas MST and SRT distinguished between high and non-high flexibility groups (high vs. middle and low), as shown in Table 6. In previous studies on flexibility and FRP, investigators have tended to employ TTT to classify flexibility levels [3; 19; 21], and have demonstrated that participants with high and low flexibility exhibit different FRP patterns; however, they rarely evaluated participants whose flexibility was within the full range. The results of this study regarding TTT and FRP conform to those of previous studies, which indicates that compared with the TTT, MST and SRT may be appropriate for discriminating between high and non-high flexibility classifications in FRP assessment. This study suggests that flexibility grouped by MST did not exert a significant effect on LSA, which was different from the results for TTT and SRT. This is because the measurements of TTT and SRT examine whole-body extensibility, and include the ability to stretch and soften joint tissues, muscles, tendons and ligaments, that is, the joint mobility of various body parts and tissues [41]. By contrast, MST primarily measures lumbar spine mobility, and the HMS is almost absent.