## 4 Discussion
This phenomenon is a mechanism to protect the spine from LBP by increasing the stability of the spine. When LBP patients walk at a high speed, increasing the lumbar -erector spinae activity during swing phase to increase the stiffness of the lumbar spine is also effective for speed-dependent perturbations [14]. Amputees also increase lateral flexion and axial rotation of the trunk toward the amputated leg when walking, and these properties increase the activity of the global trunk muscles. This is a strategy that uses the proximal (i.e., trunk) to advance the amputated limb, and excessive muscle activity and trunk-pelvic movement increase the spinal load, increasing the risk of LBP [5; 13]. With regard to spinal load, [PERSON] et al. found that the trunk muscle force and spinal load were 10-40% and 17-95% higher, respectively, in the stance phase (i.e., heel strike and toe-off phase) to the sound side during TFA walking compared with the AS. At the same time, increases of 6-80% and 26-60% have been reported compared to those in normal controls. This occurs because the antagonist muscles contract simultaneously during the non-amputated stance phase, and at the same time as the recruitment pattern of the trunk muscles changes, the trunk is mainly used to assist the advancement of gait. The author of this paper suggested that repeated exposure to these high loads may increase the risk of LBP given the repetitive nature of walking [13]. Moreover, in this study, decrement factors other than stiffness and frequency were analyzed. The decrement refers to the elasticity of a muscle, and as stiffness or frequency increases, the decrement decreases. Maintaining adequate elasticity enables efficient muscle activity with less mechanical energy [32], whereas a decrease in elasticity causes muscle fatigue and restricts the speed of movements [40]. As indicated by the results of this study, there were significant differences in stiffness and frequency between amputees and controls, but there was no significant difference in decrement between the two groups. On the other hand, as for the average value of decrement, the elasticity of the non-amputated upper side was the lowest, and the elasticity of the contralateral side (the AS) was higher than that of the CG. In addition, the average value of the lower lumbar decrement showed a decrease in elasticity on both sides of the amputee group compared with that in the CG. In conclusion, there was no significant difference in the decrement factor, whereas the stiffness or frequency values of the upper lumbar in our study showed significant differences within or between groups. This means that it is difficult to identify potential risk factors for LBP using the decrement value. In addition, the fact that there was a significant difference between the left and right of the upper decrement within the CG makes it more difficult to use as an evaluation factor. This may be the reason why most previous studies have focused on stiffness to identify potential risk factors associated with back pain. In addition, in the results of this study, significant differences in intramuscular properties were observed in the upper region of the lumbar rather than in the lower region. In both groups, the stiffness and frequency of the upper region were significantly increased compared with those in the lower region, because the muscle arrangement or properties of the upper and lower lumbar were different. This is because the upper and lower lumbar regions are composed of muscle fibers and superficial fascia, respectively [41]. As mentioned above, the erector spinae muscle measured in this study corresponds to the inflection point of the thoracolumbar curvature [42], which is a region where high loads [33] and rotational forces [43] occur even in the upper lumbar region (T12-L1). The lower lumbar region (L4-L5) contains the multifidus muscles that play an important role in maintaining spine stability. In fact, the upper stiffness value of our study tended to be higher than those of [PERSON] et al.