Balance and stability can be investigated by studying the interaction between the centre of pressure (COP) and the centre of mass (COM). In order to study balance and stability in quadrupeds it is necessary to determine the COM accurately, as incorrect COM location can markedly change the moments due to vertical ground reactions forces. This study investigated the influence of COM location on pitching moments and determined an appropriate method for defining the COM location.
The diagonal limb support pattern at trot provides pitch and roll stability, but little is known about the control of moments about the centre of mass (COM) in horses. Correct COM location is critical in the calculation of pitching moments.
The objectives were to determine the effect of COM location on pitching moments in trotting horses and explore how COM location could influence balance.
Kinematic (120 Hz) and GRF (4 force plates, 960 Hz) data were collected at trot from three trials of eight horses. The position of the COM was determined from the weighted summation of the segmental COMs and this was then manipulated cranially and caudally to test the model. Sagittal-plane moments around the COM were calculated for each manipulation of the model and their relationship determined using reduced major axis regression.
Over the stride, the moments must sum to zero to prevent accumulation of rotational motion. This was found when the weight on the forelimbs in standing was 58.7%±3% (mean±95% C.I.), which corresponded closely to the COP ratio in standing. Moments were typically nose-up at foot strike changing to nose-down prior to midstance, and then reversing to nose-up in late stance. Mean moments were larger in the hindlimbs and more sensitive to COM location changes.
Divergence of the COM from the COP creating a vertical force moment arm prior to midstance may assist the hindlimb in relation to propulsive effort. A similar effect is seen in the forelimb during single limb support.