Computer simulation of arm and leg kinematic structures

To obtain a visual and quantitative verification of the appropriateness of 1-, 2- and 3-degree-of-freedom (DOF) models for motion, we have developed an interactive system for the independent adjustment and definition of multiple-DOF linkage systems representative of human leg and arm motion. Once the kinematic structure is defined, control points for interactive definition of tendon and muscle parameters may also be defined and adjusted, leading the way to interactive musculoskeletal modeling and simulation. All kinematic transformation nodes are built as linkages within an OpenGL hierarchical structure. The structure for the independent adjustment of each axis of motion required tracking the inverse of all transformations applied to the axis during visualization and adjustment. The inverse is applied to all structures in the hierarchy below the axis of interest so that only the axis is affected during 3D adjustment. The result is a kinematic structure definition program with which the user interactively builds the kinematic model. The program has the capacity for unlimited rotational DOFs. The system allows for the visual adjustment and verification of the placement of axes. This interactive task is carried out through control of the observer viewpoint and the position and orientation of the view and of each axis. These dynamic commands are carried out simultaneously with rotational control of distal joint segments about their defined axes. With such flexibility, the user is able to rapidly iterate upon appropriate axis placement based on a 3D visual verification of joint congruence throughout the joint range of motion.