Decomposition of a Parallel Cascade Model For Ankle Dynamics Using Subspace Methods

The dynamic behavior of the human ankle during posture and movement can be described by joint stiffness, which is defined as the relation between the angular position of a joint and the torque acting about it. Joint stiffness can be separated into intrinsic stiffness and reflex stiffness, which are modeled as a linear system and a Hammerstein system, respectively. A two-pathway parallel cascade model, with the intrinsic stiffness forming one pathway and the reflex stiffness the other, can be used to describe joint stiffness. In this paper, we present a new method for decomposing the torques associated with each pathway from the total torque measurement. A single-step, subspace based system identification method is used to estimate the dynamics of each pathway directly from measured data. Experimental studies demonstrate that the method produces accurate results.