Using a simplified marker configuration to determine the cardan angles of shoulder orientation

Cardan angles are commonly used to represent clinically significant joint motions for simple movements such as those observed during gait. For motions involving large, three-dimensional, angular displacements, Gimbal locks occur and can produce erroneous Cardan angle results. This paper presents the results of an investigation to determine if Cardan angles can adequately predict complex shoulder orientations during manual hammering using a simplified opto-electronic marker configuration. A mechanical simulation of the simplified marker configuration was also constructed to test for known static and dynamic orientations and validate Cardan angle calculations. Results indicate that the simplified configuration is effective in tracking shoulder orientations in complex measurement scenarios involving limited marker placement choices. None of the rotation sequences were able to accurately and totally describe the entire anatomical range of shoulder motion during hammer usage. Results for the mechanical simulations were observed to describe shoulder orientations for simple, single-axis rotations less than or equal to 30". Single-axis rotation angles taken from different sequences were also observed to closely resemble actual motions.