Motion analysis: accuracy of joint centre estimation

Central to all areas of (human) motion analysis is the objective to reconstruct the bone motion in the laboratory reference system, i.e., the complexity of the captured data is reduced and mapped to a simplified model allowing for the computation of joint kinetics and kinematics. In clinical gait analysis, the calculation of hip joint moments may form the basis of the therapy or for the determination of the lower limb alignment axes during surgical intervention. Therefore the exact location of the hip joint centre is of primary importance. Data acquisition suffers from several errors arising from palpation, movement between markers and underlying bone, misplaced alignment devices, marker location, anthropometric regression equations, and optimization algorithms.
Clinical movement analysis requires the most accurate knowledge of joint parameters. Mathematically the problem that is dealt with can be formulated as improving the accuracy and precision in the transformation between technical (defined by surface mounted markers) and anatomical (defined by the underlying anatomy) coordinate systems. In order to assign a local coordinate system to a segment in three dimensions a cluster consisting of at least three markers is required.
The aim of the project is to investigate existing and to develop more accurate methods for estimating joint centres from marker points. A substantial part of the studies is devoted to find recommendations or a standardized protocol for the placement of the marker sets in relation to specific movement types, e.g., in gait analysis, in cycling or in running.
In order to account for errors in the centre estimation induced by different sources, the problem will be assessed in two steps.
Errors which are not caused by biological properties (such as marker shifting with respect to the bone of the very segment due to soft-tissue movement) are the topic of the first examination. A representative joint to be considered will be the hip joint. A mechanical analogue will be constructed. Investigations regarding the precision and the accuracy of the (optical) measurement system will be performed. These measurements will be based on different protocols.
The second step will focus on errors originating from biological sources. Instead of using the mechanical analogue, the markers will now be attached directly to the skin of human subjects. Again the measurements will be based on different protocols. Special emphasis will be put on marker placement. Redundant markers will also be applied.