Fig. 5. Optical motion capture (blue) and inertial sensor (red) outputs for x, y and z displacements and roll, pitch and heading angles of the sensor mounted over the thoracic spinous processes of a horse during treadmill locomotion (mean ± S.D.). Data are shown for the mean gait cycle (stride) at (A,B) walk (1.4 m s^-1), (C,D) trot (3.5 m s^-1) and (E,F) left lead canter (9.0 m s^-1). For the x, y and z displacements and for the heading angle, the stride-to-stride differences, as indicated by the standard deviations, are bigger than the differences between the two methods (difference between the mean traces). For roll and pitch orientation, however, there is a small offset between the mean traces of the two methods, which increases from walk to trot and canter. The biggest inter-stride variability can be found in the y direction for both methods and across all gaits. This demonstrates the tendency of the horse to drift from side to side while performing on the treadmill.