Although a large number of foot-fall sequences are possible in quadrupeds, few sequences are routinely used. The aim of this paper is to characterise, by foot-fall pattern, the gaits used by horses and develop a novel technique to classify symmetric and asymmetric gaits using one common criterion. To achieve this speed and relative foot-fall, timings of all four limbs of eight Icelandic horses were measured using accelerometers. Linear discriminant analysis (LDA) was performed to find criteria that are optimal for discriminating between the different gaits. This also allowed us to evaluate whether gaits should be considered a continuum or as discrete entities. Foot-fall timings (stance times, swing times, duty factors and stride frequencies) for walk, tolt, trot, pace, left canter, right canter, left gallop and right gallop during over-ground locomotion at a range of speeds are presented. In the gaits of walk, tolt, trot and pace, foot-fall timings were equal between left and right hindlimbs and forelimbs so these gaits can be considered as symmetrical. Differences in stance times and duty factors were observed between gaits but are unlikely to be of biological significance due to their similar magnitude and inconsistent relative trends. This implies that metabolics or peak limb forces derived from contact times are unlikely to be the principal driving factors in gait transition between walk, trot, pace, canters and gallops, although these factors may influence the use of tolt at the lower and higher speeds. Gaits did cluster in the LDA space and the running gaits (tolt, trot, pace, left and right canters and gallops) could be considered a kinematic continuum but the relative relationship with walk may be more complex. Thus, LDA analysis has enabled common criteria to be discovered to accurately classify equine gaits on the basis of foot-fall timings on a stride-by-stride basis.
