Achieving kinematic identity across shape diversity in musculoskeletal modeling

Musculoskeletal modeling is emerging as a powerful approach to investigate the locomotor biomechanics of extinct taxa. These models rely on bony morphology to define joint locations and muscle geometry. Using different motion profiles to drive models of extinct and extant taxa complicates comparisons because results reflect both morphological and kinematic differences. Here we report on an approach that permits changes in shape while maintaining model kinematics. Using a human musculoskeletal model, we carried out walking simulations of ten humans. Next, we morphed the model pelvis and proximal femur to match a reconstructed australopithecine pelvis and proximal femur. Using the kinematic results from the human walking simulations, we created new motion files based on the positions of joint centers and tracked anatomical locations. These files were used to drive simulations of the same walking trials with the australopithecine model. Joint centers and axes of the human and australopithecine models were compared for each walking trial simulation. We found that joint centers in the australopithecine simulations were typically within similar to 1 mu m of their locations in human simulations, and joint axes differed by less than 0.005 degrees. Such small differences have negligible effects on external joint moments calculated during inverse dynamics analyses. This conservative comparison will serve as a baseline for more complex simulations. Although this work focuses on one taxon, the approach outlined is applicable to a wide variety of extinct animals.