An Overview of the Biomechanics and Function of the Trot
The trot is a two-beat, medium speed leaping gait. The horse’s legs move in pairs which coordinate on a diagonal, making the trot a symmetrical gait. Unlike the walk, the trot has a period of suspension in which all four hooves are off of the ground. The sequence of footfalls in the trot starting with the left hind is: LH/RF, RH/LF. (Pilliner et. al., 2002, p. 99) The trot is the gait most commonly used for assessing lameness due to its symmetry. Because the trot is a symmetrical gait, most of the features which quantify its correctness are based in symmetry as well as athleticism.
Riding the trot
The trot places the most demand on the rider because of its moment of suspension, which can knock a novice rider off balance. (Pilliner et. al., 2002, p. 118). The “bounce” that can knock a rider off balance is the powerful elastic recoil that propels a horse into the air, a natural biomechanical mechanism to save energy by using the recoil of soft tissue (tendons and ligaments) rather than muscular effort to move forwards. (Blignault,2009, p. 179) Rising on a particular diagonal of the trot during the moment of suspension allows the rider to more easily maintain balance, and also allows more freedom of the back for a developing horse. Experienced riders can remain seated while trotting and continue to positively influence the horse. (Pilliner et. al., 2002, p. 118). While the combination of diagonal stances and suspension may be difficult for the rider, it gives the horse a very solid base of support on which to improve balance. The better the horses balance, the longer the suspension phase will be. (Blignault, 2009, p. 177)
Biomechanics of trot
The biomechanics of the trot are not necessarily more complicated that the four-beat, asymmetrical walk, but it does requires different coordination and the use of muscles to combat gravity in the moment of suspension between strides. While the thrust up and forwards is accomplished mainly through the muscles of the hindquarters and back, all the muscles of a horses “topline” and “bottomline” must also engage for a superior trot.
Although a horse moves both up and forwards while trotting, its overall height will usually be lower than its standing height (with the possible exception of very well developed extended trots). This is because the serratus ventralis and the flexion of joints combine uses to damped concussion and improve propulsion and elastic recoil. This is likely an energy saving mechanism. The serratus ventralis relaxes and the trunk of the body sinks down as the forelimbs extend to mediate concussive forces. (Blignault, 2009, p. 178).
The trot begins with the horse lifting a forelimb. In order to do this, the horse must move it’s center of gravity back off of the limb by contracting the serratus ventralis, which raises the thorax on the side of the active muscles. The anterior deep pectoral muscles must contract at the same time, also contributing to raising the thorax. An almost imperceptible flexion in the hocks happens nearly simultaneously, dropping the hindquarters just enough to assist the center of gravity moving backwards to release the forehand. (Pilliner et. al., 2002, p. 102).
The suspension phase occurs when the horse has released weight from the forehand and has pushed off of a diagonal hind leg from the raised foreleg (right front/left hind, etc.) This lifts the horse into the air and carries it across the ground. Young, poorly trained, or unfit horses often show less suspension. Suspension is a movement both up and forwards. (Pilliner et. al., 2002, p. 104) As the trot becomes more collected, suspension must be maintained so that the forward momentum of the trot is not lost. (Ibid. p. 106)
When the horse is in the beginning of the suspension phase, the foreleg is extended via the radial carpal extensor over the front of the leg and the common digital extensor muscle on the back of the leg. The digital extensor tendon extends below the knee from the digital extensor muscle and joins with the lateral digital extensor tendon and attaches to the pedal bone in the foot. (Ibid p. 104)
The hindquarters provide the lift-off for the suspension. The four main extensors, or the hamstrings, are the biceps femoris from the sacral vertebrae to the femur, semitendinosus from the pelvis to tibia, and the semimembranosus from the pelvis to the femur, and the deep medial gluteal muscle from the top of the pelvis to the femur. The biceps femoris is the most prominent of the hamstring group. The hip joint must also flex to pull the hind leg up off the ground, via use of the superficial gluteal muscle, the four-part quadriceps femoris, and the tensor facia latae. (Pilliner et. al., 2002, p. 110) the gluteal muscles and the biceps femoris extend the femur and flex the stifle while the hind leg is pushing off the ground. The triggers hock flexion as well.
As the suspension period ends, the horse’s body absorbs the shock of hitting the ground. The fetlock joint sinks. This is controlled by the suspensory ligament, the superficial flexor tendon, the deep flexor tendon, and the check ligament. The action of the soft tissue is to absorb the concussion of the landing. The tissue and fetlock joint recover after the leg has passed vertical. The result of this is that the body stays at the same level since the leg is functionally shortened and lengthened during this biomechanical action. When the leg has passed vertical, this same soft tissue/joint combination, along with the deep flexor muscles pull the pedal bone back with enough force that is helps to create enough propulsion to fling the horse into the next suspension phase. This is assisted by the elasticity of the suspensory ligament, and is part of process of elastic recoil. (Pilliner et. al., 2002, p. 109).
At the end of a suspension phase, the hind leg will often contact the ground a millisecond before the front leg. The hind lower leg extends under the horses body, ideally tracking up to the foreleg on the same side. The muscles responsible for this are the long digital extensor muscle at the front, and the lateral digital extensor muscle behind it. Both attach to the extensor tendons as they do in the forelegs. (Pilliner et al., 2002, p. 112) Lower hind limb flexion is caused by the superficial digital flexor muscles and the deep digital flexor muscles. Assisting them is the gastrocnemius, ending in the Achilles tendon running over the hock. The flexor pattern muscles terminate in the flexor tendons attached to the pastern and pedal bones. (Pilliner et. al., 2002, p. 112).
Dissociation
Visually, horses appear to trot by placing diagonal pairs of limbs on the ground simultaneously. However, there is usually a very slight dissociation. This is called either positive or negative dissociation. Positive dissociation, also referred to as advanced diagonal hoof placement or “over tracking,” happens when the hind feet land first. The inverse happens when the horse lands its front feet first, which leads to “forging” and the horse being heavy on the forehand. Advanced diagonal placement results in a higher quality, more balanced trot as the horse is able to better engage its hindquarters. (Blignault, 2009, p. 179-180). Very good training can increase advanced diagonal placement. For this reason, it is advantageous for sport horses to have some natural positive dissociation. (Clayton, 2004, p. 178)
Dissociation is often seen in the racing trot of Standardbred racehorses and is certainly desirable, but unlike sport horses, Standardbreds show a very specific type of negative dissociation wherein the front hoof will impact and leave the ground before the hind hoof makes impact. The greater the speed, the larger the dissociation is. After the front limb has hit the ground, the hind limb will continue to move until it makes contact with the ground. This affects the timing of the stride, creating a temporal dissociation as well as a biomechanical one. The longer the temporal dissociation, the shorter the diagonal phases of the stride will be. This ideally gives the racing trot enormous forward drive, increasing the over-track of the trot (since the front hoof is out of the way, the hind foot can move farther under the horse’s body) and increases the aerial phase of the gait. The longer the aerial phase, the more efficiently the horse can cover ground. Unlike the aerial phase seen in sport horses, Standardbred racehorses more primarily forwards during the aerial phase, since the elevation sought after in dressage is not needed (although it is created as a byproduct of the enormous impulsion generated by the speed of a racing trot). (Clayton, 2004, p. 178)
Transitions
Upward transitions to trot call for more engagement than transitions to walk. In nature, the horse has the option to move through the gaits sequentially to gain impulsion via momentum. The ridden horse is often asked to skip gaits.
When moving from halt to trot, the ridden horse is asked not to use his head and neck to initiate the movement, but to remain round and begin the motion from his hindquarters. This is a learned biomechanical action. The horse must move from no impulsion to superior impulsion almost instantaneously. It must make use of the power elastic recoil mechanism to spring up and forwards into the trot by ground a diagonal pair of legs, lifting the other two and pushing down through the fetlock joints to move forward. It is the elastic recoil that allows the ridden horses head and neck to remain steady and soft in the rider’s hand. (Blignault, 2009, p. 181).
Moving from a walk to a trot requires the horse to organize its body to transition from an asymmetrical to a symmetrical gait. When ridden, the horse will organize so that a hind leg steps into place to form a diagonal pair with a front leg. This diagonal pair will then be used to propel the horse into trot. A rider must therefore ask for trot with extreme tact to avoid excess shuffling steps while a horse gets organized to respond to the aid for trot. If the timing of the aids is not exact, the transition will not be “clean.” (Blignault, 2009, p. 182).
Downward transitions from trot simply require less impulsion. The horse will ground one diagonal for breaking, and then step into the walk as the diagonal association breaks up. To skip walk and move to halt from trot, the breaking must be more powerful and both diagonals will ground rather the second breaking into the first two beats of the walk. (Blignault, 2009 p. 185). If a horse is not properly prepared for a downward transition by the rider, it will lift its head and neck in a balance reaction against the riders hand and “fall” into a transition. The horse appears to hollow, but it trying to use its natural balance reactions to move its center of gravity back to assist the breaking mechanism. The ridden horse learns to use the engagement of the hindquarters to assist with slowing forward momentum. (Blignault, 2009, p. 186).
The trot in Dressage
The trot in dressage calls for four variations: collected, working, medium, and extended. Ideally, the tempo of footfalls and suspension remains the same in all variations of the trot. To maintain the rhythm in spite of differing stride lengths, the horse must lift its legs higher. This is referred to as cadence. (Pilliner et. al., 2002, p. 114).
The collected trot is developed as the abdominals and hip flexors become stronger and the hindquarters are drawn more underneath the horse’s body. This moves the center of gravity back, lightening the load on the forehand and allowing the head and neck to be carried in a more upright frame with the classical look of the nasal plane on the vertical. The hindquarters are engaged in more strenuous muscle use and the steps become higher and more energized, which gives the trot more cadence. The hind feet often step just short of fully tracking up in the collected trot. (Pilliner et. al., 2002, p. 114).
The working trot is a schooled version of the horses natural gait. With young or novice dressage horses it is used to teach balance, acceptance of the riders aids, build strength, and establish rhythm and impulsion. The hind feet must track up to the front feet or even reach slightly beyond. (Pilliner et. al., 2002, p. 114). Working trot in novice horses is often ridden with the rider rising out of the saddle on the outside diagonal. This puts less weight on the horses developing back muscles. When the rider is able to activate the inside hind leg and “catch” the impulsion in the outside rein (without losing the inside rein) the trot will gain more energy and impulsion. (Pilliner et. al., 2002, p. 116).
Medium trot is a precursor to extended trot. The stride is longer and more energized than in working trot. The moment of suspension should be clearly visible, and the hooves should track up. The horse should lengthen its frame slightly to accommodate more forward motion. The medium trot can teach the horse to move forward with added impulsion without becoming “strung out” in its body, or letting the movement become disconnected and rushed and lacking rhythm. (Pilliner et. al., 2002, p. 114)
Maximum stride length is reached in extended trot. In a horses natural state, a lengthened trot is a default way of moving. The horse moves its shoulders freely while the head and neck stretch forwards. Horses do not usually move from medium to extended trot naturally; when the balance shift occurs they simply begin to canter instead. The greater engagement of the hindquarters that results in variation of speed and engagement within a gait are not commonly seen in nature. (Blignault, 2009, p. 186). If a horse under saddle is not properly physically developed for medium and extended trots, it will lose balance and break into canter as natural biomechanical reflexes overtake the rider’s aids. (Ibid. p. 187).
Under saddle, tracking up should remain the same, with the hindquarters taking more body weight to free the shoulders and generating more power and impulsion to send the horse forward and the forehand up. The speed of the extended trot is faster than medium trot due to lengthened strides and bigger suspension but the rhythm and stride frequency should remain the same. During the suspension period, the horse is propelled higher and farther across the ground. A good extended trot is a spectacular movement. (Pilliner et. al., 2002, p. 115).
Medium and extended trots depend on freedom of the shoulder to be performed correctly. The biceps and brachiocephalicus muscles bring the shoulder and forelimb forwards when they contract, while at the same time the triceps must relax and stretch to allow the scapula to move. A tight triceps is prohibitive for medium and extended trots. A hollowed or overly lifted head and neck is caused by contraction of the trapezius, serratus ventralis and rhomboideus attached to the top of the scapula. This also prevents the scapula from moving freely which severely inhibits stride length. The inhibited scapula can be a factor in a tight triceps. When a horses neck is properly flexed instead of hollowed, these muscles can function properly and stride length will increase correctly. (Pilliner et. al., 2002, p. 115).
Lengthened forms of trot also require good elastic recoil. The more extended the gait, the more the fetlock joint sinks or extends, which causes the elastic recoil mechanism of the tendons and ligaments to be magnified. This propels the horse higher and farther than in the collected and working trots. (Blignault, 2009, p. 187).
When training collected and lengthened forms of trot, it is advantageous to school them at the same time. This develops both the necessary muscles and the capacity for elastic recoil without overtaxing any one muscle group. If a horse is pushed beyond its physical capacity, it may break into canter or show signs of faults in the trot. (Blignault, 2009, p. 188).
Faults and Lameness
The three most common faults in the trot are forging, pacing, and a wide hind stance. Forging occurs when the forefeet hit the ground before the corresponding diagonal hind leg, causing the hind hoof to hit the front leg. This is often caused by an unfit or poorly trained horse carrying too much weight on the forehand. Forging is corrected through conditioning and increased engagement of the hindquarters so that the front feet can move out of the way to avoid interference. (Blignault, 2009, p. 189)
Pacing is seen in some breeds of gaited horses, but is usually considered a fault. In a pace, lateral pairs of legs move together and the diagonal pattern of the trot is diluted or disrupted. The stance and suspension phases are shorter in the pace, so the stride rate usually increases. The base of support is also smaller, and the horse must carry its head and neck high to assist with balance, resulting in very tight topline muscles. Horses can start to pace due to muscle imbalances or if they are a breed that is naturally inclined to pace (such as Tennessee Walkers). To correct a pace at the trot, an underlying muscular imbalance must often be addressed. (Blignault, 2009, p. 188). The horse must be asked to lower the base of the neck, relax the topline and slow the movement. (Ibid. p. 190).
Horses that have a wide hind limb stance often do so for any combination of three reasons: conformation, loss of balance, or tightness in the topline. Conformation that causes a wide stance behind in the trot is usually a short backed horse with long legs; the horse will move wide behind to prevent forging. Loss of balance can cause a horse to move wide behind if they are pushed forwards while too much body weight is on the forehand before the hindquarters are correctly developed. If the topline is tight, the horse is moving in a hollow backed extension pattern (even if it is not visible to the eye). The horse may also show a lack of suspension. This is corrected by slowing the tempo, lengthening the neck and teaching the horse to maintain a contact with the rider’s hands through the rein. (Blignault, 2009, p. 190-191).
The trot is the gait most commonly used to determine lameness. The higher ground reaction forces in the trot highlights lameness in a way that the walk cannot since it has no aerial phase. (Clayton, 2004, p. 180). Since the trot should have equal weight bearing strides and it is a symmetrical gait, unsoundness is more obvious. If a horse is lame on front, it’s head will nod as the sound leg impacts the ground as the horse protects the injured leg by placing more weight on the sound leg. In hind limb lameness, the horse will exhibit asymmetry in the movement of the hindquarters, with the injured side rising and falling more than the sound side. (Pilliner et. al., 2002, p. 120).
Variations of the trot
The four most common variations of the trot are the jog trot, passage, piaffe, and racing trot (described above).
The jog trot is a very slow gait shown by western pleasure horses. It has no suspension, and a lateral sequence of footfalls due to negative dissociation. It is a movement that falls between a walk and a trot, with the diagonal movement of a trot that also maintains the four-beat lateral sequence of the walk. The horse’s body should have a very smooth trajectory of motion and a high degree of collection. The jog trot is extremely easy to ride. (Clayton, 2004, p. 180).
Passage is a symmetrical leaping movement performed by dressage horses. It must have positive diagonal dissociation to be a true passage. The aerial phase should be very distinct. The limbs pause momentarily in their most elevated positions, giving the passage a very regal appearance. Passage is distinct from trot by its slower tempo, a long diagonal dissociation, and a pause in the forward movement of the limbs at their more elevated position (cadence). Passage requires enormous strength and coordination on the part of the horse, and any discrepancy between the right and left sides of the horse’s body will lead to an uneven gait. (Clayton, 2004, p. 180).
The piaffe is considered one of the most difficult movement to train a horse to perform correctly. Piaffe retains all of the characteristics of the trot, but the horse does not travel forwards (or only minimally). It is the movement with the highest degree of collection in dressage. The limbs have the same characteristic pause during their elevation as they do in passage, but piaffe does not have an aerial phase. Some horse may leap from one hoof to another, but the appearance of an aerial phase is caused by highly coordinated dissociations and one hoof always remains on the ground. Very good piaffes make use of positive diagonal dissociation. Since the temporal characteristics of piaffe vary greatly between horses, there is the potential for enormous variety in limb support sequences. (Clayton, 2004, p. 182).
Conclusion
The trot is an enormously useful gait. Because of its symmetry, it is the gait most commonly used to assess a horses soundness as well as potential for more strenuous work. This is because any asymmetries or injuries will cause an uneven stride. Trot has the potential for variation, and is the most easily improved of all the gaits due to its two-beat rhythm. It is often taxing for the novice rider, but advanced riders make use of its regularity to train out muscular imbalances and asymmetries. The trot is an important gait in all disciplines of sport horses, which means that a quality trot gives a horse the potential to be useful in multiple areas. The ideal trot is symmetrical in motion and moves powerfully forwards with a good balance of ground cover and suspension.
Bibliography
1. Blignault, Karin. Equine Biomechanics For Riders: The Key to Balanced Riding. J. A. Allen, 2009. Print.
2. Clayton, H.M. The Dynamic Horse: A Biomechanical Guide to Equine Movement and Performance. Sport Horse Publications, 2004. Web.
3. Pilliner, Sarah, Samantha Elmhurst, and Zoe Davies. The Horse in Motion: The Anatomy and Physiology of Equine Locomotion. 1st ed. Wiley-Blackwell, 2002. Print.