Human Postural Model That Captures Rotational Inertia

INTRODUCTION Inverted pendulum models have been very beneficial for the modeling and analysis of human gait and balance [Kuo, 2007]. These reduced models allow us to ignore the movements of the individual limbs, and instead, focus on two important points -the center of pressure (CoP) and the center of mass (CoM) -and the lean line that connects the two points. A limitation of the existing reduced models is that they represent the entire human body only as a point mass and do not characterize its moment of inertia. The rotational inertia is a property of the distributed masses of the limbs, and by ignoring it, un-natural constraints, such as zero angular momentum at the CoM and resultant ground reaction force (GRF) collinear with the lean line, are forced on to the model. The Reaction Mass Pendulum (RMP) model [Lee & Goswami, 2007] extends the existing models by replacing the point mass with an extended rigid body – the abstracted 3D reaction mass – that characterizes the aggregate rotational inertia of the subject projected at the CoM. As the person moves through different limb configurations, the centroidal moment of inertia continuously changes, which is captured by the changing shape, size and orientation of the ellipsoidal reaction mass. We postulate that analysis of the rotational inertia especially in cases of pathological gait can provide additional insight. This is demonstrated with normative gait data from four able-bodied subjects and pathological gait data from one spinal cord injured subject.