## 1 Introduction  
Lower-limb length discrepancy (LLD) is defined as a discrepancy between the lengths of the two lower limbs, occurring when the natural symmetry of the body is affected. LLD, when exceeding the limits accepted by clinicians, can affect locomotion, balance, posture foot pathologies, and low back pain [1, 2]. LLD can be categorized as either structural (anatomical), where limb inequality is caused by bone structure, or functional (apparent), where biomechanical changes such as knee or foot deformities (varus or valgus) and spinal abnormalities (especially functional scoliosis in children [1]) are present. LLD leads to a shift in the normal position of the center of mass (CoM), thereby disrupting the proper functioning of the human body, including changes in gait kinematics [2, 3, 4]. There are several methods and devices available for measuring limb length discrepancy (LLD). Traditional devices, typically operated by medical specialists, offer limited accuracy;these include methods such as direct limb measurement and bubble level devices. Modern methods, on the other hand, rely on advanced technologies such as imaging, ultrasound, and large-scale devices designed for precise center-of-mass determination. However, these advanced systems are often inaccessible to most patients due to their size, cost, and the specialized environments in which they are typically housed [4; 5; 6; 7; 8; 9; 10]. Accurate measurement of LLD is crucial in the study of both functional and anatomical LLD. Subjects are often unaware of an LLD smaller than 20 mm, and a discrepancy of up to 20 mm can typically be tolerated [2; 11]. Additionally, the literature indicates that an LLD greater than 20 mm causes pathological changes in gait and induces fatigue in the quadriceps muscle of the longer limb [12]. It has been reported that subjects can perceive a limb length discrepancy greater than 20 mm [13]. Furthermore, in cases of structural LLD, a greater vertical ground reaction force is observed on the shorter limb compared to the longer limb. For example, when LLD is 3.5 cm, the load on the shorter limb reaches 64% of the subject's body mass [3; 14; 15]. Both categories of LLD (anatomical and functional) are associated with hip or knee osteoarthritis and other mechanical pathologies resulting from improper distribution of load across the limbs [16]. Changes in spinal kinematics were studied in 20 subjects with unilateral transtibial amputation (prosthesis wearers for at least 1 year) by simulating LLD through the placement of 30 mm wooden blocks under the foot. Limb inequality resulted in lateral pelvic tilt, which in turn caused lateral bending of the subjects' spines. The changes induced by a shorter prosthetic limb appeared to be more substantial than those induced by a longer prosthetic limb [11]. It is well known that prosthetic users must make two visits per year (depending on age and growth stage) to a specialist for monitoring the alignment of the spine, pelvic girdle, and limb length discrepancy. As the child grows, adjustments to the prosthesis parameters or regular replacement of the prosthesis are necessary to ensure proper fit that keeps pace with the patient's growth. Typically, the conventional prosthetic device is replaced once a year [17]. Conventional prostheses designed for patients in the growth period require length adjustments as long as the prosthetic structure allows (at which point replacement is necessary). Their replacement occurs at least once a year, and specialist consultations are typically conducted about twice a year [17]. Some prostheses allow for adjustment by removing the artificial foot and inserting elements, such as wooden blocks, between the foot and the prosthetic rod. This process compensates for the limb length discrepancy until it matches the contralateral foot. Adjustable rods with metal inserts, which are fixed at a specific length, are also used to achieve the same effect [18]. The Ring concept, which features passive adjustment for length modification, is well known.