Fractal behavior of the trajectories of the foot centers of pressure during pregnancy

Abstract

The equilibrium of pregnant women is compromised because the falling risk increases as pregnancy progresses. The trajectories of the centers of pressure are related to equilibrium, gait and posture. The purpose of this work was to investigate the fractal behavior of the trajectories of the center of pressure of both feet in standing and walking pregnant women and how they evolve during pregnancy. By using instrumented insoles (F-Scan® system working at 40 Hz), the trajectories of the center of pressure of both feet were obtained for 71 women in the three trimesters of their pregnancy. The 'detrended fluctuation analysis', a technique designed to extract the fractal properties of a signal, was used to analyze the fractal behavior of both the x and y components of the measured trajectories. Specifically, the fractal characteristic exponents corresponding to the small and large end regions of the corresponding scaling functions were obtained and compared. No differences were found between the characteristic exponents obtained for x and y components nor from the signals found for left and right feet within the statistical uncertainties. In standing conditions all the exponents obtained remained almost constant, regardless of the elapsed pregnancy time, taking values around 0.5. In walking conditions, the small end exponents did not change with respect to those found in static conditions; on the contrary, the large-end exponents reduced as pregnancy progressed, reaching values of the order of -0.6 in the third trimester. The multi-scaling character of the walking signals could be related to the anteroposterior and medio-lateral displacement observed in previous experiments and that could be on the root of the development of musculoskeletal discomfort and a falling risk that increases with the pregnancy time.