# System for Evaluation and Compensation of Leg Length Discrepancy for Human Body Balancing
###### Abstract
This method is more reliable and accurate than the tape measure method [8; 9]. Clinical methods for LLD measuring are safe and simple to use, and the measuring equipment is inexpensive. Imaging methods are considered to be much more precise than clinical methods. Professional medical equipment for LLD measuring by using imaging methods is costly. The most precise LLD measuring methods are radiography and CT. However, these methods involve subjecting patients to radiation [3; 10]. The aforementioned LLD measuring methods are based on measuring the length of extremities between anatomical reference points, and such LLD measuring is frequently unreliable. Furthermore, the aforementioned LLD measuring methods do not point to the change in the body's CoM, which means that they cannot offer a recommendation on what should be done from the non-surgical aspect of the treatment for the purpose of improving the posture and achieving more stable, more natural and more energy-efficient walking. Research conducted on healthy subjects showed that a simulated LLD of more than 20 mm makes the subjects feel uneasy, with a sense of difference in their legs' length [11]. The results of research done with persons with LLD showed a significant asymmetry in how load is distributed per leg as opposed to the healthy population [1]. Studies have shown that structural LLD features a higher vertical ground reaction force on the shorter leg than on the longer [12; 13]. The Authors of [14] showed that, by using a pedobarograph, the difference in load distribution on the left leg can be measured by simulating a longer or shorter right leg. If the right leg were simulated to be 3.5 cm longer, the load on the left leg would amount to 64% of the patient's mass. An in-shoe pedobarograph system was used to measure the load on the left and right leg of the patients in [15]. These studies have shown that there is an asymmetry in load distribution with LLD patients, but they did not offer a solution to how they can evaluate and compensate the size of the LLD based on the difference in load distribution with the goal of achieving symmetrical load distribution on both legs. This paper proposes a new methodology for LLD evaluation and compensation with the aim of balancing the human body based on a shift in the CoM. A preliminary study was conducted with two equal commercial digital scales, placed next to one another on a flat surface. If a healthy person, with an upright posture, stands on the scales so that the position of both the feet is in the middle of their respective scale, the scales will show approximately equal values (Figure 1). By lifting one of the scales from the surface, the values shown by the other scale are increased due to a shift in the CoM. The same result is achieved if a patient with varying leg lengths stands on the scales. By lifting the height of the scale underneath the shorter leg, the load on the two scales can be equalized. Manual height correction of the commercial scales is exceptionally time consuming, complex and in practice difficult to apply. Thus, it is necessary to develop a system which will make LLD evaluation and compensation faster, simpler and possibly even automatic. For this purpose, a mechatronic system was created with two 3-RPS (Revolute-Prismatic-Spherical) parallel manipulators comprising force plates with adjustable angles and height. LLD is evaluated and compensated in the following manner: A patient with LLD places their left and right leg on the force plates, which are used to measure the Center of Pressure (CoP) of each leg. If the patient maintains a static posture, a projection of the human body's CoM can be calculated in the transverse plane based on each leg's CoP. The patient's shorter leg will produce a higher load on the force plate due to the human body leaning toward the shorter leg. This is why the body's CoM will be displaced toward the shorter leg.