Meeting Abstract
14.1 Thursday, Jan. 3 Significance of trunk deformation in estimating trunk center of mass during locomotion in the horse, Equus caballus NAUWELAERTS, S*; KAISER, L; MALINOWSKI, R; CLAYTON, H; Michigan State University nauwelae@msu.edu
Inverse dynamics analysis is a technique that uses the kinematic representation of movement to derive the kinetics responsible for that movement. One of the underlying assumptions is that the segments are rigid with mass concentrated at their centers of mass (CM). In horses, about 66% of their mass is concentrated in the trunk segment. Although the rigidity assumption is most likely a good estimate for the limb segments, a large part of the trunk consists of soft tissue, making it deformable. The aim of this study is to assess the significance and magnitude of error created by the deformability of the trunk segment. We attached 40 skin markers in a grid pattern on the trunks of five adult horses. In addition, we used four hoof markers, one sternum marker and one marker on the head, using a motion analysis system to record their positions. We calculated the CM of the trunk twice: (1) under the assumption of a rigid body and (2) from the volume comprised by the 40 markers. Using a floating frame of reference that described the deformation of the body with respect to its coordinate system, we tested for differences between the two calculation methods. Preliminary results show a small volume change during locomotion that increases with speed within a gait. The CM of the trunk calculated from the markers is considerably different from the rigid body approach (up to six cm) in all three directions. The CM of the trunk moves up, backwards and from left to right during locomotion. This pattern can be linked to movements of the forelimbs, which in turn cause shape changes of the rib cage. The error on the vertical position is largest during fast walks and smallest during fast trots. The latter is due to the increasing importance of the suspension phase during which errors are minimal. For future modeling, changes in trunk shape will have to be taken into account.
