I am examining the stability of a normal gait through a kinematic parameter (Displacement) analysis.I am getting negative Lyapunov exponent in Z-direction (vertical) but positive in other directions for the same body position (calcaneus) data.For displacement parameter, the vertical direction is considered in the literature.I am using treadmill data which is captured by Vicon camera.

Does negative Lyapunov exponent in vertical direction means gait signal is non-chaotic? If it is, then what about other direction?