Standardised Terminology for the Description and Analysis of Equine Locomotion

1. What they did

Researchers studying how horses move had been using many different words for the same things, and the same word for different things. An earlier survey of published work on equine locomotion and biomechanics had shown this confusion. In this paper the authors review the terms in use and propose one standard set for researchers, with particular reference to the racing gaits of Standardbreds and Thoroughbreds (the trot, pace and gallop).

For each part of the movement, they list the names earlier authors used, choose the term they recommend, and explain why. They favour terms that can be applied to any gait and any combination of limbs, rather than terms that fit only one gait. They cover:

• General terms: names for the limbs, limb pairs and the lead leg.
• Phases of the limb cycle: when a hoof is on the ground and when it is not.
• Phases of the stride: how a full stride is divided in different gaits.
• What a gait is.
• Timing measurements: how long phases last and how the limbs relate in time.
• Distance measurements: stride length, where the hooves land, and the number of tracks a horse makes.

2. What they found

Limbs and limb pairs

• Limbs should be called the right or left frontlimbs and hindlimbs. The old terms “near” and “off” for left and right should not be used.
• A diagonal limb pair is a frontlimb and the hindlimb of the opposite side; an ipsilateral pair is a frontlimb and hindlimb of the same side; a contralateral pair is the two frontlimbs or the two hindlimbs. Diagonal and ipsilateral pairs are named left or right after the hindlimb of the pair.
• In asymmetrical gaits (canter, gallop), the pattern of one side does not repeat that of the other. In symmetrical gaits (trot, pace), one side repeats the other half a stride later.
• The lead leg is the last leg of a front or hind pair to leave the ground; the non-lead leg is the first. Both must be given for the frontlimbs and the hindlimbs. The authors point out an earlier book that named the wrong lead leg.

The limb cycle

• Each limb alternates between a stance phase (hoof on the ground) and a swing phase (hoof off the ground). Both terms are recommended over older ones such as “support phase” or “weightbearing phase”, because they describe both the limb’s position and its function.
• The stance phase is split at the mid-stance position into a deceleration phase and a propulsion phase. Force-plate work had shown that the push between hoof and ground changes from braking to driving forward at mid-stance, although the authors stress that much is still to be learned. The mid-stance position describes where the limb is, not the halfway point in time.

The stride

• A stride is one full cycle of the repeated limb pattern. Its start and end must be fixed before stride length, stride duration and stride frequency can be measured. The gallop stride starts when the non-lead hindlimb touches down; trot and pace strides start when the first limb of the left pair touches down.
• In the gallop, the stride has one stride stance phase (one or more limbs on the ground) and one suspension phase (no limbs on the ground).
• In the trot and pace, there are two suspension phases in each stride, so the authors use left and right stance phases and left and right suspension phases.
• Rotary and transverse gallops are explained by the order in which the four hooves land (Fig 1).

Gaits

• A gait is a limb coordination pattern used in moving. Citing Hildebrand, the authors note that horses’ gaits form a continuum rather than separate types, and that over 167 symmetrical gaits had been defined for the horse. Names like walk, trot, canter and gallop are an oversimplification.
• They recommend that research papers report speed, limb timing and distance and stride measurements, so readers can tell exactly which gait was studied.

Timing terms recommended

• Overlap: the time when two limbs are on the ground together (preferred over “bipedal support” and “double support”).
• Single support: the time when a limb is on the ground alone.
• Advanced placement and advanced lift off: the time between one limb landing (or lifting off) and another, for any pair of limbs in any gait.

Distance terms recommended

• Line of motion: the path of the horse’s centre of gravity over the ground. Stride length is the distance covered in one stride.
• Line gait and overcrossing: whether the hooves land on or near the line of motion, or cross over it to the other side. Diagonal width is the side-to-side distance between the two limbs of a diagonal pair.
• Tracks: a track is the line along which one limb is placed. Each limb makes its own track, but tracks can fall on top of each other, so a horse may move on one, two, three or four tracks (Fig 6). Riding books usually say a straight horse, whose hind feet follow in the tracks of the front feet on the same side, moves on a single track; the authors point out that this horse is in fact moving on two tracks. Sideways movements often called “two track” are actually done on three or four tracks.

Terms to drop

• Step (and forestep, hindstep, step cycle): earlier authors used it for the distance between two prints of the same hoof (the same as a stride), the distance between prints of the two front or two hind feet (about half a stride), and other things. Because it is so poorly understood, the authors suggest dropping every term with “step” in it.
• Overreach: in a clinical setting it implies abnormal hindlimb placement. For the distance by which a hind hoof lands ahead of the front hoof’s print, “overtracking” (a dressage term) is a possible alternative, though it also suggests excessive movement; the authors say the distance can simply be measured without a special name.
• Cadence: it means different things in music, dressage and human gait research, so it should not be used.
• Gait-specific terms such as “impure trot”, “line trot”, “line pace” and “overstep”, and loose terms such as “hasty hindlimb”.

3. What they concluded

• Terms for horse locomotion should be precise and should apply to every gait and every limb combination, so that different gaits can be compared and redundant terms avoided.
• The recommended core set is: stance phase and swing phase; mid-stance position, deceleration phase and propulsion phase; stride, stride stance phase and suspension phase (gallop); left and right stance and suspension phases (trot and pace); stride length, stride duration and stride frequency; overlap and single support; advanced placement and advanced lift off; line of motion, line gait, overcrossing and diagonal width; and one track to four track.
• Ambiguous terms (near and off, step, overreach, cadence and gait-specific terms) should be dropped.
• Because named gaits blend into each other, research papers should report the actual timing and distance measurements.

6. Other technologies, skills or innovations

• A three-axis frame (X, Y and Z axes) for naming the planes in which limbs move and hooves are placed (Fig 4).
• Diagrams showing the order of hoof placement in rotary and transverse gallops (Fig 1), the phases of the stride in the gallop, pace and trot (Fig 2), overlap and advanced placement as bars of stance time (Fig 3), hoof placement relative to the line of motion (Fig 5), and a horse moving on two, three and four tracks (Fig 6).