To measure the forces produced by all four limbs when travelling over ground is only possible when multiple force platforms are available. This study was undertaken using multiple force platforms and synchronized kinematic data to investigate the motion of the centre of mass (COM) of the horse in relation to the forces it produced at the ground. Typical values from sound horses are included.
Studies of equine ground reaction forces (GRF) have been limited by the difficulty of measuring concurrent GRFs from all loaded limbs during full strides of over-ground locomotion.
The aims of this study were to measure GRFs of concurrently loaded limbs and to evaluate spatial relationships between the centre of pressure (COP) and centre of mass (COM) in trotting horses.
Kinematic (120Hz) and GRF data were collected at trot from three trials of eight horses using four force plates (960Hz). Forelimb and hind limb GRFs were measured, COP was calculated from the resultant vertical GRF vector and COM was calculated by summation of weighted segmental COMs.
Peak total vertical force (19.3±1.3N/kg at 45.1±2.3% diagonal stance) coincided with zero total longitudinal force (45.1±2.4% stance). Initially, COP position corresponded with the fore or hind hoof of the diagonal that contacted the ground earlier. During diagonal overlap, COP position reflected forelimb contribution to total vertical GRF; it maintained a fairly constant position relative to the base of support through the middle part of stance, then moved cranially in the last third of stance towards the fore hoof, which was always the last hoof to leave the ground. The COM moved forward continuously; its longitudinal velocity decreased with the net braking force in early stance and increased with the net propulsive force in late stance. The COM was caudal to the COP in early stance, coincident at 35.8±4.4% stance and was maximally ahead of the COP at 67.6±4.8% stance.
Changes in the spatial relationship between COP and COM affect the moment arms of the forelimb and hind limb vertical GRFs and their effect in generating a pitching torque around the COM.