We provide quantitative anatomical data on the muscle-tendon units of the equine pelvic limb. Specifically, we recorded muscle mass, fascicle length, pennation angle, tendon mass and tendon rest length. Physiological cross sectional area was then determined and maximum isometric force estimated. There was proximal-to-distal reduction in muscle volume and fascicle length. Proximal limb tendons were few and, where present, were relatively short. By contrast, distal limb tendons were numerous and long in comparison to mean muscle fascicle length, increasing potential for elastic energy storage. When compared with published data on thoracic limb muscles, proximal pelvic limb muscles were larger in volume and had shorter fascicles. Distal limb muscle architecture was similar in thoracic and pelvic limbs with the exception of flexor digitorum lateralis (lateral head of the deep digital flexor), the architecture of which was similar to that of the pelvic and thoracic limb superficial digital flexors, suggesting a functional similarity.

SUMMARY 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.

A mobile system that reliably detects and quantifies hindlimb lameness in horses during unconstrained locomotion could be a valuable tool to perform an evidence-based assessment of lameness in horses in a clinical setting, e.g. before and after nerve blocks or before and after surgery.

Walking and running are two mechanisms for minimizing energy expenditure during terrestrial locomotion. Duty factor, dimensionless speed, existence of an aerial phase, percentage recovery (PR) or phase shift of mechanical energy and shape of the vertical ground reaction force profile have been used to discriminate between walking and running. Although these criteria work well for the classification of most quadrupedal gaits, they result in conflicting evidence for some gaits, such as the tölt (a symmetrical, four-beat gait used by Icelandic horses).We use established pattern recognition methods to test the hypothesis that the tölt is a running gait based on an automated and optimized decision drawn from four features (dimensionless speed, duty factor, length of aerial phase and PR for over 6000 strides from four symmetrical gaits in seven Icelandic horses) simultaneously. We compare this decision with the use of each of these features in isolation.Sensitivity and specificity values were used to determine optimal thresholds for classifying tölt strides based on each feature separately. Duty factor and dimensionless speed indicate that tölt is more similar to running, while aerial phase and PR indicate that it is more similar to walking.Then, two multidimensional pattern recognition approaches, multivariate Gaussian densities and linear discriminant analysis, were used and it was shown that, in terms of stochastic multidimensional discrimination, tölt is more similar to 'running'. The approaches presented here have potential to be extended to studies on similar 'ambling' gaits in other quadrupeds.

SUMMARY The spring-mass model is often used to describe bouncing gaits. Although at first inspection the mechanical system appears simple, the solution to the motion cannot be derived easily. An analytical solution would provide a fast and intuitive method to determine the kinetic and kinematics of the centre of mass of terrestrial animals during over-ground steady state locomotion. Here,an analytical approximation using sine wave simplifications for the motion is presented. The analytical solution was almost indistinguishable from the numerical solution across initial leg angles of 17.5–30°; percentage differences between the analytical solution and the numerical solution were less than 1% for total mechanical energy, centre of mass position, total limb compression and centre of mass velocity and less than 2% different for resultant limb force and vertical acceleration of the centre of mass. The solution matched the relationship between stance time and speed collected from a trotting racehorse and accurately characterised previously published biological data. This study has shown that a simple analytical solution can predict the kinetics and kinematics of a spring-mass system over the range of biologically relevant sweep angles and horizontal velocities, and could be used to further understanding of limb deployment and gait selection. Using this analytical solution not only the force profile but also the changes in mechanical energy can be calculated from easily observed morphological and kinematic data.