###### Abstract  
Lower limb amputees (LLAs) have a high incidence of low back pain (LBP), and identifying the potential risk factors in this group is key for LBP prevention. This study analyzed the intramuscular properties of the resting lumbar muscle in thirteen unilateral LLAs and age-matched controls to predict the onset of LBP. To measure the lumbar intramuscular properties, resting erect spine muscles located in the upper and lower lumbar regions were examined using a handheld myotonometer. The dynamic stiffness, oscillation frequency, and logarithmic decrement were measured. In our results, the stiffness and frequency of the upper lumbar region were greater in the amputee group than in the control, whereas the decrement did not differ between the two groups. Additionally, the measured values in the lower lumbar region showed no significant difference. Within each group, all three factors increased at the upper lumbar region. In the LLAs, the frequency and stiffness values of the upper lumbar on the non-amputated side were significantly higher than those on the amputated side. These results indicate that the upper lumbar muscles of the amputees were less flexible than that of the control. This study can help in providing therapeutic strategies treating LBP in amputees. l 2021  
the lateral flexion and axial rotation of the trunk towards the prosthetic leg and increase the activity of the global trunk muscle [5]. Excessive muscle activity and trunk-pelvis motion during walking increase the spinal load, and exposure to repetitive walking increases the risk of LBP [12]. A previous study reported that the trunk muscle force and spinal load in the non-amputated side (NAS) increased by 10-40% and 17-95%, respectively, compared with those in the amputated side (AS), which changed the recruitment pattern of trunk muscles [13]. Lumbar muscle activity patterns during LLA's walking are similar to those in patients with LBP [14; 15]. In healthy individuals, the erect spinae muscle relaxes during the double limb support and swing phases, but in patients with LBP, the activity of the erector spinae muscle is increased throughout the entire gait cycle [15; 16; 17]. The activity of erector spinae muscle in patients with chronic low back pain (CLBP) is 8 to 48% higher than in healthy individuals [14]. This is closely related to the guarding mechanisms that protect the spine from pain. However, LLAs increase the activity of the erector spinae as a functional strategy for gait propulsion, not due to LBP [12]. In preparing the trunk for movement of the prosthetic leg, it is necessary to provide stability to the lumbar region, which increases the muscle activity of the trunk. As a result, the activity of the erector spinae muscle increases throughout the gait cycle, and the recruitment pattern of the trunk muscles may change, similar to that in patients with LBP. As mentioned above, the asymmetry of pelvic inclination caused by LLD or trunk-pelvis asymmetry movement due to abnormal gait changes the onset or recruitment pattern of the back muscles, causing overall lumbar muscle imbalance. Abnormal patterns  
Figure 1: Schematic of static and dynamic contributing factors associated with lumbar muscle imbalance in patients with unilateral transfemoral amputation. or excessive activation of muscles can lead to musculoskeletal problems in the long term. Therefore, it is of utmost importance to identify potential risk factors to prevent them. Many studies have been conducted to analyze the properties of muscles in relation to LBP. In particular, analyzing the properties of resting lumbar muscles is useful to identify potential risk factors for back pain [18], and analyzing the stiffness of resting lumbar muscles enables the analysis of inherent skeletal muscle properties in a state where central nervous system (CNS) activation is excluded [19]. To identify the lumbar muscle properties, the stiffness is mainly analyzed.