Simbios Talk by Ajay Seth, Stanford University, November 7, 2007

Title:  Biologically inspired joints using Simbody

Accurately describing the kinematics of biological systems is critical for understanding the morphology and function of biomechanisms.  To capture the complex geometrical relationships present in biological joints, like the human knee, many degrees-of-freedom (DOFs) are used to describe the motion and then constraints are applied to restrict motion to be consistent with experimental data. Unfortunately, adding DOFs and constraints proportionally increases the computational costs of generating simulations.  Whether simulating the molecular dynamics of thousands of interconnected atoms or controlling complex skeletal models, minimal formulations of system dynamics are sought to bolster computing efficiency and increase the practicality of simulation.  The aim of this work was to develop a methodology to exploit specified joint geometry to obtain a minimum set of motion coordinates and eliminate constraints.  We will see that the concept of the "joint mapping matrix" as implemented in Simbody provides new found flexibility in the design of custom joints by directly embedding the geometrical relationships between bodies without the use of constraints.  This approach offers a simplicity and elegance in design of joints that easily incorporates biological/experimental data to improve modeling accuracy while introducing virtually no additional cost.