Reducing Errors Due to Soft Tissue Artifact (STA)
STA is considered a leading source of error in motion analysis studies. STA arises because skin-mounted markers move along with soft tissues (skin, muscle, fat) relative to underlying bones (e.g. femur and tibia) yet classical methods used to determine joint motion assume that skin markers move rigidly with the bones.
The nonlinear continuum mechanics theory for a Triangular Cosserat Point Element (TCPE) has been implemented to more accurately determine the kinematics of knee motion by reducing soft tissue artifact-induced errors. An enhanced Helen Hayes marker set with 32 markers and 2 clusters of 7 randomly spaced markers on the thigh and shank are placed on the participants. Kinematic data are captured using a motion analysis system with digital cameras and Cortex software.
The 7-marker clusters are used to obtain STA corrected body segment orientations. For the TCPE method, each 7-marker cluster is partitioned into sub-clusters of 3 markers. Sub-clusters are modeled as Triangular Cosserat Point Elements (TCPEs). Using a director vector approach, a deformation gradient tensor, a Finite Lagrangian strain tensor, and a translation vector are calculated. Using the right polar decomposition, a TCPE’s rotation tensor is calculated from the deformation gradient tensor. TCPEs whose strain tensors and translation vectors have the lowest magnitude and whose rotation tensors are most consistent with other TCPEs are then selected using a filtering algorithm. From the selected TCPEs (and, thus, from a subset of the 7-marker clusters), thigh and shank rotation tensors are determined. With the thigh and shank rotation tensors obtained as described above, a floating axis coordinate system is used to define knee angles: flexion/extension (FE), internal/external (IE) rotation, and adduction/abduction (AA).
Currently, we are extending this work to more accurately calculate knee joint angles for children with the long term aim of better understanding the relationship between overweight/obesity and adverse knee joint biomechanics for overweight children - a critical aim due to their status as a high risk population for early knee joint osteoarthritis.
Current and past students that have worked on this project include Jake Larson, Kaila Lawson, Sam Tucker, Scott Weinhardt, Valentina Profiti, Alejandro Gonzalez-Smith, Nina Yadlowsky, and Jake Deschamps.