Analysis of Lower Limb Key Point Displacement in the Frontal Plane

Abstract

This paper presents the results of an experimental study of the trajectories of lower limb key points in the frontal plane at different walking speeds on a treadmill. The aim of the study was to identify patterns of lateral oscillations and deviations necessary for maintaining walking stability, which is critically important for the design of biomechanically correct rehabilitation systems. The experiment was conducted with three subjects of different anthropometric profiles at three speed modes (0.14 m/s, 0.28 m/s, and 0.42 m/s). The methodology included motion video capture, trajectory digitization using OpenSourcePhysicsTracker software, and subsequent mathematical data processing. To transition from discrete coordinates to continuous analytical models, the approximation method using Fourier trigonometric series was applied. The results, presented as a series of graphs, demonstrate characteristic changes in the trajectories of the hip, knee, and ankle joints depending on speed: reduction of vertical deviation, enhancement of figure-eight pattern motion, rounding of trajectories, and symmetric multidirectionality of paired joints. The hierarchical principle of coordination was confirmed: in-phase movement of the proximal pelvic joints is combined with antiphase activity of paired distal joints, both between limbs and within a single limb. The obtained data and their analytical description can serve as a foundation for creating more accurate biomechanical models and improving control algorithms for rehabilitation exoskeletons and mechanotherapy systems.