Walking horses commonly use a lateral sequence single-foot gait.1 This gait is defined by a limb phase centering on 0.25 (range, 0.19 to 0.31); limb phase (also known as lateral advanced placement) is the elapsed time between the footfalls of a hind limb and its ipsilateral forelimb normalized to stride duration.1,2 At higher speeds, most horse breeds transition to a trot (limb phase of 0.5).
Many gaited horses, however, use a symmetric 4-beat stepping gait consistent with the lateral sequence single-foot footfall pattern.3 These include familiar breeds such as the Paso Fino horse (classic fino, paso largo) and Icelandic horse (tölt). The popularity of ambling horses is on the rise among recreational riders, yet our understanding of the locomotor mechanics of gaited horses is limited. Further research is required to assess whether these gaits should be best considered walking gaits4 or running gaits. This distinction may influence susceptibility to lameness and rehabilitation protocols.
In the literature, mechanical descriptions are typically used to classify a walk from a run and numerous approaches exist.5 One approach uses stride kinematics described by duty factor (ratio of stance duration to stride duration) and gait. A duty factor of ≥ 0.5 and < 0.5 distinguishes a walk from a run, respectively.1 Furthermore, the walk-run transition in most horses is distinguished by an abrupt shift in gait from a 4-beat gait to a 2-beat gait (usually a trot),6 and running typically includes a period of suspension.7 A second approach evaluates the movements of the body's COM to determine whether a horse is moving with inverted pendulum mechanics (walking) or spring-mass or bouncing mechanics (running).8 When walking, the limbs function as semirigid struts so that the COM is lifted to its highest position near midstance, while forward velocity of the COM is highest at hoof touchdown and liftoff and is lowest at midstance. Consequently, Ep cycles out of phase with Ek-tot, and the phase shift between minima of Ep and Ek-tot is near 180°. This pendulumlike exchange of energies provides an opportunity to recover external mechanical energy with every step (up to 70% in quadrupeds8–11), thereby reducing muscular effort during locomotion at slow speeds. At faster speeds, the limbs function with greater compliance during stance phase so that the COM no longer rises during the first half of stance but rather drops to its lowest position near midstance. Accordingly, Ep and Ek-tot fluctuate in phase with each other (phase shift near 0°). As much as 40% of the muscular work of trotting is recovered via bouncing mechanics by the storage and return of elastic strain energy in ligaments, tendons, and muscles of the limbs with every step.12 A third approach for distinguishing a walk from a run relies on Fr, a parameter that is related to the ratio of kinetic energy to Ep. Horses moving with similar dynamics are expected to move at comparable Frs, with cursorial mammals switching from a walk to a trot at an Fr of approximately 0.5 and from a trot to a gallop at an Fr of 2 to 3.13
The purpose of the study reported here was to evaluate the locomotor mechanics of the tölt in Icelandic horses to assess whether the gait conforms more closely to walking or running. This breed and gait were selected because stride characteristics are well established3,14,15 and individual limb GRFs have been analyzed.16 Understanding the mechanics of the tölt has implications for performance, injury, and treatment of gaited horses.
Center of mass
Gravitational potential energy
Total kinetic energy
Ground reaction force
Whole-body vertical force
Whole-body craniocaudal force
Whole-body mediolateral force
Total external mechanical energy
Relative leg-spring stiffness
Model LG6-4, AMTI, Watertown, Mass.
Motion Analysis Corp, Santa Rosa, Calif.
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