Effects of trunk deformation on trunk center of mass mechanical energy estimates in the moving horse, Equus caballus

The estimation of the position of the center of mass (CM) is essential in a wide range of biomechanical analyses. In horses, the majority of the body mass is contained in the trunk and in most studies, the trunk is assumed to be rigid. However, this rigidity assumption has not been tested. We quantified changes in the position of the trunk CM due to external shape changes by measuring the kinematics of a mesh encompassing the trunk. Using a frame of reference fixed to the horse's spine, we described the shape deformation of the trunk during walking. In addition, we tested for speed and individual differences. The significance of any trunk deformation was illustrated by calculating mechanical energy profiles. Errors in the estimation of the trunk CM due to a rigid body approach were always small in the vertical direction, but can be significant in the transverse direction and in the longitudinal direction at high walking speeds. This is enough to change the mechanical energy expenditure estimates up to 25%. When extrapolating the position of the trunk CM from cadaver data, one should be aware of this extra source of error, separated from the measurement error of the cadaver CM. We also found considerable inter-individual variation, which complicates theoretical correction routines. We suggest using extra markers on the trunk during gait analysis to correct this CM shift experimentally.