## 4 Discussion
However, it is not appropriate to directly compare the results of our study with that of [PERSON] et al. because our experiment was conducted once a day and the change over time was not considered. Nevertheless, it was confirmed that the average stiffness values presented by [PERSON] et al. were lower than those of the CG and amputees in our study. In addition, in the study of [PERSON] et al., the average age of male subjects was 21 years, and unlike our study, the subjects were younger, and the measurement site (L3-L4) was different. Owing to these factors, it is thought that the lumbar stiffness of the CG was higher in our study. In the future, we believe that additional research on the difference in stiffness according to age and measurement site of individuals without LBP is necessary [18]. In general, the tendency of changes in stiffness and frequency values is similar because of the characteristics of the two variables (when the stiffness value increases, the frequency value also increases). Through these factors, it is possible to evaluate the muscle tone (or tension), which is an intrinsic property of the muscle. High muscle tone means high intramuscular pressure (i.e., hypertonia), which reduces blood supply and can lead to muscle fatigue [36]. Accumulated muscle fatigue can cause back pain; hence, proper management is required. In our study, the stiffness and frequency differences between the right and left sides of amputated patients were associated with providing an asymmetric static and dynamic environment to the amputated patients. First, one of the factors contributing to static asymmetry in amputated patients is LLD [6]. Several previous studies involving LLD have reported that the leg lengths of the amputees are asymmetrical; 85% of amputees had prostheses that were too short or too long (\(n=\) 113, 29 TFAs and 84 TTAs) [4], and 14 out of 17 amputees had prostheses that were too short [7]. It has also been reported that the LLD of the amputees with LBP is greater than that of amputees without LBP, and LLDs greater than 30 mm affect the lumbo-pelvic kinematics [8]. The anterior pelvic tilt angle of TFA increased when standing or walking compared with that of the CG [37]. Intentionally shortening the length of the prosthesis to facilitate toe clearance on the prosthetic side during TFA walking causes the problem of descent of the pelvis of the prosthesis [8]. Asymmetric pelvic oblique angles result in muscle imbalance, stretching the lumbar muscles of the short leg (the AS) and shortening the lumbar muscles of the contralateral (the NAS). These characteristics can result in a prolonged distortion of the asymmetric pelvis or fixation of the imbalance in muscle condition, resulting in scoliosis [9]. In fact, functional scoliosis due to LLD occurs frequently in LLAs [10], and scoliosis has been reported in 43% of LLAs (18 out of 42) on radiological examination [11]. Second, the asymmetry of the left and right lumbar muscles in the amputees is closely related to gait. Abnormal gait patterns in amputees cause asymmetric trunk movement, which in turn alter muscle recruitment patterns. In general, during normal gait, the thoracic erector spinae muscle on the opposite side of the stance leg contracts concentrically before the lumbar erector spinae muscle is activated. This characteristic contributes to moving the upper trunk while reversing the curve of the spine toward the swing leg. Thereafter, the lumbar erector spinae muscle controls the trunk and supports the pelvis and swing leg elevation through eccentric contraction [38; 39]. When trunk neuromuscular control is compromised by LBP, the timing of activation of trunk muscle tissue is altered as above [15; 17]. For example, people with LBP had increased lumbar and thoracic erector spinae activation during the swing phase of gait, early onset and delayed offset of the lumbar erector spinae, and increased co-contraction of trunk flexors and extensors compared with those without LBP [14; 15; 17].