Application of Nonlinear Dynamics to Human Knee Movement on Plane and Inclined Treadmill

The objective of this study is to quantify and investigate nonlinear motion of the human knee joint for a sample of 7 healthy subjects on plane treadmill and inclined treadmill with an angle of 10° and for both knees of a sample of 3 patients who suffer of osteoarthritis, only on the plane treadmill, using nonlinear dynamics stability analysis. The largest Lyapunov exponent (LLE) and correlation dimensions are calculated as chaotic measures from the experimental time series of the flexion-extension angle of human knee joint. Larger values of LLEs obtained for patients group suffering by osteoarthritis are associated with more divergence and increase of knee variability, while smaller values obtained for healthy subjects reflect increase of local stability, less divergence and variability, less sensitivity to perturbations and higher resistance to stride-to-stride variability. The use of nonlinear tools may provide additional insight on the nature of the step-to-step fluctuations present in human and robotic locomotion.