Why some parkinson's disease patients cannot stand or walk but can ride a bicycle a control system-based analysis

Recent clinical studies report a set of striking observations that some Parkinson's disease (PD) patients, who suffer freezing of gait (FOG) and cannot maintain a stance balance and walk, are able to freely ride bicycles. Although existing some pathophysiology explanations, there is no clear understanding why such FOG and PD patients preserve bicycling capability. This paper presents some explanations from a control systems viewpoint of analyzing rider/bicycle interactions at zero moving velocity (i.e., stationary). Built on our previous work on stability analysis of rider/bicycle systems, we mainly focus on the influence of the model parameters in the human neurological balance controller on the rider/bicycle system's stability. The analysis reveals that due to the parameter sensitivity differences between quiet stance and bicycle balancing, it is possible that a human subject who cannot make a good stance but may maintain a balance of a bicycle. Indeed, the increased joint stiffness of PD or FOG patients may also help maintain the bicycle balancing. The presented analysis partially explains the preserved bicycling ability and is potentially used for bicycle-assisted rehabilitation.