Frequency analysis of ankle joint quasi-stiffness during quiet unperturbed standing in Chiari Malformation

Chiari Malformation (Chiari) is a congenital condition occurring from an inferior herniation of the cerebellar tonsils into the foramen magnum. Given the role of the cerebellum in postural control, it is reasonable to expect joint motion to be affected in this patient population. In fact, joint stiffness is a common self-reported symptom of Chiari, however it has never been assessed in these individuals. This study aimed to examine if ankle joint quasi-stiffness is correlated with Chiari severity. The human body was considered as an inverted oscillating pendulum without damping. A Fast Fourier Transform was used to extract natural frequency from the center of pressure trajectories during upright standing. Ankle joint quasi-stiffness was then calculated using the relationship between natural frequency and moment of inertia. Twelve Chiari participants (Chiari), six with decompression surgery (Chiari-D) and six without (Chiari-ND), and eight control individuals (Control) participated. Participants completed three, 30-second quiet standing trials on a force plate, focused on a target three meters in front of them. Chiari, regardless of surgery, had significantly lower quasi-stiffness than controls (Chiari-D vs. Control p = 0.0011, Chiari-ND vs. Control, p < 0.001). The proposed method is advantageous as it incorporates the entire center of pressure signal, minimizes error from instantaneous muscular dynamics, and does not require motion capture.