Walking is a complicated business on four legs, even more so when stepping up to a run.
Horses are well known for their different gaits, with some even carrying the so-called gait-keeper gene mutation that allows the use of ambling gaits at intermediate speeds.
In horses, the transition from a walk to a run has been taken up a step or two by dressage.
Researchers Hilary Clayton and Sarah Jane Hobbs, in a review just published in the open-access journal Animals, set out to review the biomechanical gait classification of the collected trot, passage and piaffe in dressage horses.
Horses move naturally through a range of speeds by transitioning between different gaits when it is energetically beneficial, as a means of reducing limb forces or to preserve gait stability, the pair note.
Horses typically walk at slow speeds, trot at intermediate speeds, and canter or gallop at fast speeds.
In ponies, the transition speeds are around 2 meters per second from walk to trot, and 4–5 meters per second from trot to gallop.
However, the DMRT3 nonsense gene mutation allows the use of ambling gaits at intermediate speeds, and has a favorable effect on speed capacity in trot or pace, allowing these gaits to be retained at high speeds rather than transitioning to a gallop.
Dressage, in which horses perform a predetermined arena test consisting of different gaits and patterns, adds fresh layers of complexity to gaits and the changes between them.
Dressage horses are taught to perform their natural gaits through a wide speed range, thus over‐riding the natural trigger to make a transition.
As training progresses, dressage horses learn to move in an uphill posture with the forehand raised and the haunches lowered.
Highly trained dressage horses may learn to perform passage and piaffe, which are regarded as artificial gaits.
In nature, horses sometimes perform a display gait similar to passage but this can be distinguished from the competition gait by the fact that the horse’s back is hollow and the body is held stiffly without the roundness and relaxation required in dressage.
Clayton and Hobbs said biomechanical analysis of different aspects of the dressage horse’s performance of enhanced natural gaits and artificial gaits offers a chance to study the range of biomechanical abilities of the athletic horse.
Equine gaits are recognized according to stride kinematics (movements).
The walk has a four‐beat rhythm with each footfall occurring separately in a lateral sequence; the footfalls may be equally spaced in time or they may occur as lateral or diagonal couplets.
Dressage horses typically walk at speeds in the range of 1.4–1.8 m/s. In trot and pace, the movements of a fore and hind limb are synchronized by pairing diagonally or laterally, giving a two‐beat rhythm.
The speed range for trotting dressage horses has been reported to be 3.2 to 4.9 m/s.
The ambling gaits, performed at intermediate locomotor speeds by horses with the DMRT3 mutation, are lateral sequence gaits that may be coordinated with lateral couplets, diagonal couplets or equal intervals between footfalls.
The canter and gallop are asymmetrical gaits in which the hindlimb pair and the forelimb pair move as couplets; the first limb of the couplet to contact the ground is the trailing limb, the second is the leading limb.
Horses typically canter and gallop with the leading limb on the same side in the fore and hind limb pairs. The range of cantering speeds for dressage horses has been reported to be 3.3 to 6.0 m/s.
As the pair point out, regardless of which gait is being performed, locomotion results from the application of forces that are generated when the hooves press against the ground.
Legged locomotion is based on protraction and retraction of the limbs relative to the body. During the retraction phase, the hooves press against the ground to generate ground reaction forces (GRFs).
These GRFs have gait-specific patterns.
Biomechanists classify gaits into two broad categories, walking gaits or running gaits, based on kinematics, the nature of the GRFs, and energy‐conservation mechanics.
Some gaits fit within the same category of walk or run according to all three classifications, whereas others are assigned differently.
For example, the walk has no aerial phase. The trot has aerial phases, a distinctive GRF curve, and it conserves energy using spring‐mass mechanics. It is classified as a running gait by all three methods.
The Icelandic tölt, which lacks an aerial phase, combines walking kinematics with a similarly distinct GRF curve and spring‐mass mechanics. It is classified as a running gait with walking kinematics.
So how do we classify the diagonally coordinated gaits of dressage horses, specifically, collected trot, passage and piaffe?
Each gait is classified as a walking gait or a running gait based on three criteria: limb kinematics, ground reaction forces and center of mass mechanics.
Clayton and Hobbs found that the data for trot and passage were quite similar and both were classified as running gaits according to all three criteria.
“In piaffe, the limbs have relatively long stance durations and there are no aerial phases, so kinematically it was classified as a walking gait.
“However, the shape of the vertical ground reaction force curve and the strategies used to control movements of the center of mass were more similar to those of a running gait.
“The hind limbs act as springs with limb compression increasing progressively from collected trot to passage to piaffe, whereas the forelimbs show less compression in passage and piaffe and behave more like struts.”
The collected trot conforms to the general pattern of a running gait, they said. “The kinematic, GRF, and mechanical energy exchange patterns are consistent with the classification of collected trot as a running gait by all criteria,” they said.
“Passage retains the kinematic, GRF and energy exchange characteristics of collected trot, with some differences in magnitude associated with the slower speed of progression and higher swing phase elevation of the center of mass. Therefore, passage is also classified as a running gait.”
In the absence of forward motion, piaffe had some major differences from passage and trot. The movements of the diagonal limb pairs are less tightly synchronized than in trot or passage and there tends to be greater variability in piaffe steps due to the difficulty of maintaining balance in the absence of forward motion.
The authors said the absence or presence of aerial phases was formerly regarded as distinguishing between walking and running gaits.
However, it is now acknowledged that aerial phases alone do not represent functional mechanics.
Management of the movements of the center of mass and the associated exchanges between different types of mechanical energy are recognized as the most important criteria for distinguishing between walking and running gaits.
“It is suggested that future research be directed toward comparing the behavior of the forelimbs and hind limbs in relation to the movements and mechanics of the center of mass, especially in piaffe.
“Measurements of mechanical variables describing the relationships between potential energy and kinetic energy, such as relative phase, percent congruity and percent recovery will provide further insight into the underlying mechanisms of the gaits and movements of dressage horses.”
Clayton is with Sport Horse Science in Mason, Missouri, and the College of Veterinary Medicine at Michigan State University; Hobbs is with the Centre for Applied Sport and Exercise Sciences at the University of Central Lancashire, in England.
Clayton, H.M.; Hobbs, S.J. A Review of Biomechanical Gait Classification with Reference to Collected Trot, Passage and Piaffe in Dressage Horses. Animals 2019, 9, 763.