Head and body centre of mass movement in horses trotting on a circular path

Abstract

Horses are often worked along a circular trajectory but the mechanics of turning have not been reported. To develop a methodology for kinematic analysis of horses during circling and to report preliminary findings describing movements of the total body centre of mass (COMbody). Seventy-five skin markers, distributed over the head, neck, trunk and limbs of 5 sound horses, were tracked in a global coordinate system at 120 Hz as the horses trotted around a small circle. Based on segmental inertial data, 3D positions of the segmental centres of mass and COMbody were calculated. Movements of COMbody were measured relative to the cylindrical vertical plane, the radius of which was defined by a marker overlying the sixteenth thoracic vertebra. During trotting, COMbody was high at the start and end of the diagonal stance phases and low in midstance. All horses leaned to the inside of the circle, with a tilt angle of mean +/- s.d. 14.8 +/- 2.8 degrees during the entire stride. In the transverse direction, COMbody swung from side to side reaching its most inward position around the time of contact of the inside forelimb and its most outward position around the time of contact of the outside forelimb. The centre of mass of the head and neck segments (COMhead/neck) had a symmetrical, double sinusoidal pattern in the vertical direction. All horses had the same motion pattern for COMhead/neck in the transverse direction but the mean position relative to the circle line varied between horses. Horses lean to the inside when turning around a small circle. COMhead/neck and COMbody showed 2 vertical oscillations in each stride, whereas there was a single mediolateral excursion that was not symmetrical to the inside and outside of the circle line. This study provides a first step toward understanding the mechanics of locomotion during turning, which has implications in the aetiology of injuries, the response to lungeing as a diagnostic technique and the incorporation of circles into rehabilitation exercises.