Relationship between morphological and stabilographic variables in standing horses

A stabilogram plots movements of the centre of pressure (COP) in the horizontal plane. Derived stabilographic variables quantify postural balance, but it is not known if these variables are size dependent. The aims of this study were to determine which morphological variable was most representative of size, which stabilographic variables were most representative of balance and whether size normalisation improved estimates of postural performance. Croup height (0.93-1.77 m), mass (117-666 kg), base of support (BOS) length (0.74-1.18 m) and BOS width (0.22-0.45 m) were measured in 24 horses. Stabilographic variables describing craniocaudal (CC), mediolateral (ML) and resultant amplitudes, velocities and frequencies of COP motion were measured as the horses stood stationary for 15s with fore and hind hooves on separate force plates (960 Hz). Principal component analysis identified morphological and stabilographic components. Morphological variables were consolidated into a single size component that was represented by body mass. Five stabilographic components explained 91% of the variation in sway patterns and five representative stabilographic variables were identified: CC amplitude, CC velocity, CC frequency, ML amplitude and ML frequency. Mass was correlated with CC velocity and ML frequency, with larger horses having smaller CC velocities and slower ML sway frequencies. When horses were grouped by mass (small horses <400 kg; large horses ≥ 400 kg), the within-group values for CC velocity and ML frequency were no longer correlated with mass.