## 4 Discussion
Although our subjects were older, the lumbar measurement level was higher than that of [PERSON] et al. (L3-L4), which is thought to be because this area is the inflection point of the thoracolumbar curve and stress is more concentrated in this area [18]. In addition, in both groups, all three factors were significantly higher in the upper than in the lower lumbar region. The higher muscle tone and lower elasticity in the upper region than in the lower region means that muscle fatigue is more likely to occur in the upper lumbar region. In general, relaxation of the erector spinae muscle occurs during the double limb support phase and swing phase during walking [16]. However, in patients with CLBP, the activity of the erector spinae muscle increases throughout the gait cycle [15; 17]. Insufficient muscle relaxation and sustained activity of motor units contribute to muscle damage and degeneration [44]. The pattern of muscle activation during walking in LLAs is similar to that in LBP patients. The initial activity of the lumbar erector spinae prepares the trunk for movement towards the amputated leg by providing stability to the lumbar spine before the heel of the amputated leg contacts the floor. This unique neuromuscular control strategy of LLAs translates into a functional strategy for locomotion than that of LBP patients. These characteristics support the increase in the non-amputated upper stiffness in our findings. Gait strategies that require greater activation of the trunk muscles of the supporting leg to provide stability to the lumbar region for propulsion on the AS are thought to increase muscle stiffness and frequency, especially in the upper part on the NAS [12]. Continuous activation of muscles can cause musculoskeletal problems in the long term, so it should be prevented. Nevertheless, the imperative question is which strategy to apply to prevent these musculoskeletal problems. If increased lumbar muscle activity is associated with a maladaptive coping method, treatment strategy such as myofeedback may help reduce the increased lumbar muscle activity. As the asymmetry of the left and right muscle stiffness of the amputees shown in our study is more likely to develop into LBP if it persists for a long time, it is of utmost importance to eliminate risk factors that cause asymmetric characteristic in amputated patients. As mentioned above, LLD identified as a static risk factor should be aligned as symmetrically as possible, and pelvic descent due to leg length should be avoided. In addition, asymmetric movements of the trunk and pelvis caused by abnormal gait patterns should be improved through prosthetic gait training. In particular, proper muscle strengthening and education are required to prevent excessive trunk rotation, extension, and lateral flexion during the training process. Therefore, it is necessary to establish a strategy to relieve LBP by regularly evaluating the lumbar muscle condition of LLAs. Furthermore, we believe that myotonometry can be useful as a simple non-invasive measurement tool for clinical evaluation. If the stiffness of the muscle is high, the muscles should be relaxed through stretching or massage. It is also necessary to improve the exercise methods and lifestyle to strengthen the back muscles through patient education. This study has several limitations. Although voluntary muscle contraction was restricted by taking sufficient rest before the experiment, complete restriction of muscle contraction could not be confirmed by quantitatively measuring the electrical activity of the muscles. Future studies should consider concurrent electromyography to ensure the reliability of the results. In addition, it is necessary to analyze the muscle properties according to the level of amputation or the presence or absence of LBP and the increase in the number of subjects.