Introduction
The mechanics of circular locomotion are relevant to equine training and lameness evaluation. Circular trotting is associated with asymmetries in upper body movement. This study investigated whether changes in foot placement explain upper body asymmetry and whether this asymmetry is dependent on centripetal force.
Methods
Six functionally sound horses walked and trotted on a 6 m diameter circle and straight line at their preferred speed. Optical motion capture (Motion Analysis Corp.) recorded foot position in 3D space. For each limb, horizontal foot placement was calculated in a body-based reference system. Vertical displacement asymmetry (Vector Sum, VS) was calculated from inertial sensors (Xsens, The Netherlands) attached above poll, withers, sacrum and tubera coxae. Variables were compared across movement directions and gaits using repeated measures ANOVA (parametric datasets) or Friedman Test (non-parametric datasets).
Results
Changes in foot placement relative to body position and orientation were small for both gaits in cranio-caudal and medio-lateral direction (maximum effect size at walk/trot: 66/102 mm). The amount of systematic movement asymmetry on the circle was comparable between walk (mean VS head/sacrum: 36/15 mm) and trot (30/15 mm, respectively), despite a 3-fold higher centripetal force during trot.
Conclusions
Foot placement relative to body position and line of travel does not differ markedly between straight line and circle and cannot solely account for the observed upper body movement asymmetry. Since movement asymmetry was comparable at walk and trot (despite differences in adopted movement pattern), centripetal force may not be the primary variable responsible for movement asymmetry on the circle.
