  Although ankle injuries resulting from postural instability are frequently
observed during high-speed and intense physical activities, most current
research has been limited to static or quasi-static models of the lower limb,
or has focused solely on the ankle joint itself. In this study, to explain the
kinetic mechanism underlying postural instability and ankle twists, we present
a nonlinear dynamical model for the human lower limb that takes into account
transient behavior and large rotations. The landing of the lower limb on the
ground is modeled as a mechanical system with two degrees of freedom (DoFs).
The configuration of the lower limb mechanism can be completely defined by the
vertical displacement and the inclination angle. We quantify the postural
stability of the lower limb using a dimensionless energy threshold. Numerical
examples verify the effectiveness of this stability indicator and demonstrate
that individuals with a normal foot have better postural stability compared to
those with flat and high-arched feet, and are less likely to experience ankle
injuries in jumping and landing scenarios.
