Stride to stride variability in joint angle profiles during transitions from trot to canter in horses

Spontaneous transitions from anti-phase to in-phase manual coordination are explained in the Haken model that describes the two preferred states as stable regions that work as attractors in a stability landscape. Switching between states coincides with a temporary loss of stability. Coordination variability is believed to be indicative of such a loss of stability. In this study, the hypothesis was tested that an increase in variability in the angle profiles of the joints responsible for the transition will precede the transition. A full gait analysis of four miniature horses transitioning from trot to canter was performed. Joint angle profiles were determined for the joints of all four limbs and were time-normalised to stride duration. Per horse and per stride, the coefficient of variance was calculated as the mean standard deviation of the joint profile over all trials divided by the mean joint angle × 100. As hypothesised, the most proximal limb joints (hip, scapulothoracic, shoulder) followed the predictions to a large extent. The variability of the hip joint angle of the trailing hind limb showed a peak of variability at stride 0; this was quickly reduced after the transition was completed. The detection of this brief perturbation in the hip joint indicates the importance of this joint in the transition process. The hip joint is related to the movements of the limb, pelvis and back, which is one of the main differences between symmetrical and asymmetrical gaits.