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Contents: Traditionally, the selection of horses has been based on subjective evaluation of several factors including bloodlines, conformation and movement. In judging the quality of movement, riders and trainers seek to identify characteristics that satisfy one or more of the following criteria:
Equestrian sports cover a wide range of disciplines, and different sports
require specific athletic abilities and techniques. This article will describe
some of the locomotor characteristics of the equine athlete, using two
very different sports, racing and dressage, to illustrate the diversity
of talents required of the equine athlete. Since the work surface has a
profound effect on performance and soundness, consideration will also be
given to the characteristics of different types of footing.
Horses races encompass the whole spectrum from sprinting to marathon distances. Regardless of the length of the race, however, the goal is to complete the distance in the shortest time, in other words at the fastest average speed.
Stride length depends on the distances between hoof placements. Greater extension implies that the horse is reaching further between successive limb contacts. At slow and moderate speeds, alterations in stride length are the primary means of changing speed. Most of the increase in stride length occurs during the suspension phase that follows lift off of the leading fore limb. One of the factors that ultimately limits stride length is the need to retract the limb before it contacts the ground to reduce its forward velocity at impact. Without this retraction, the limb would plow forward into the track with the effect of decelerating the forward motion. This type of contact occurs during over-striding, which is sometimes used by human athletes to slow down after a race; it makes deliberate use of the propping action to reduce speed. Stride Rate depends on how quickly the limbs can be cycled back and forth. A faster stride rate implies the ability to protract the limbs rapidly during the swing phase and the ability to push forcefully against the ground to create sufficient impulse during a shorter stance time (impulse is the sum of the forces exerted over a period of time). It has been suggested that the swing time is almost constant, regardless of gait or speed. If this is true, then swing time cannot make a significant contribution to changes in stride rate in response to the need to increase speed. Consequently, changes in stance duration are likely to make the greatest contribution to changes in stride rate. This implies that the horse needs sufficient muscular strength to generate higher forces over a shorter period of time to maintain the impulse (impulse is the total force summed over a period of time). When racing over short distances, rapid acceleration and the attainment of a high maximal speed are the most important considerations, whereas in races over longer distances the ability to maintain a moderate speed over a long period of time is the prime consideration. For the purposes of this discussion, sprint racing encompasses Quarter Horse (QH), Thoroughbred (TB) and Standardbred (SB) racing though the longer TB and SB races might be regarded as middle distance races. The racing gaits of the SB differ from the TB/QH in that the trot and pace are symmetrical gaits, whereas the gallop is asymmetrical. In a symmetrical gait the footfalls of the left and right limbs are separated by equal periods of time, whereas in an asymmetrical gait they occur as couplets for the hind and/or fore limb pair. Gait symmetry affects the rotation point of the limbs, which influences the effective limb length. In symmetrical gaits, the fore limbs rotate around the proximal scapula and the hind limbs rotate around the hip joint. In asymmetrical gaits, in which the two hind limbs are protracted simultaneously, the point of rotation moves proximally to the lumbosacral joint. This increases the effective length of the hind limbs, and allows a corresponding increase in stride length. In SB trotting at racing speed, there is a diagonal dissociation such that the fore limb contacts and leaves the ground before the diagonal hind limb with the magnitude of the dissociation increasing with speed (Drevemo et al., 1980). After the fore limb contacts the ground the hind limb continues moving forward until it makes ground contact. Therefore, the longer the diagonal dissociation with a fore limb making contact first, the shorter the diagonal distance. Over-tracking, which is the distance by which the hind hoof steps ahead of the fore hoof, makes a major contribution to stride length. Since over-tracking measures the distance covered during the suspension, the most effective way to increase over-tracking at a given speed is to prolong the suspension by lifting off with a higher vertical velocity. It then takes longer for gravity to overcome the vertical velocity and reverse the direction of motion, so the horse stays airborne longer and covers more ground at a given speed. However, at racing speeds there is a compromise between the different stride components; a longer suspension means a shorter stance time during which the horse is generating impulsion by pushing against the ground. The most successful horses are able to combine these variables in an optimal manner. In the pace the dissociation between the lateral pair of limbs is such that the hind hoof contacts the ground before the fore hoof, and a longer dissociation allows an increase in the lateral distance. As speed increases, the lateral dissociation also increases, producing what is effectively a four-beat gait. It has been suggested that this is a mechanism for increasing the effective stance time (during which the horse is pushing against the ground) without reducing the overall stride rate (Wilson et al., 1988). The gallop is an asymmetrical gait that normally has a single suspension in each stride; this is a gathered suspension that occurs between lift off of the fore limbs and contact of the hind limbs. Each limb makes contact with the ground separately and distinctly. As speed increases there are reductions in stance durations of the individual limbs and in the overlaps between limbs. At extreme speeds there may be a short suspension between lift off of the leading hind limb and contact of the trailing fore limb (as normally occurs in galloping dogs), and there may even be a suspension between stance phases of the trailing and leading fore limbs (Seder et al., 1993). The frequency and duration of these multiple suspensions increases with speed. In endurance racing, the achievement of maximal speed is not the objective,
but rather the ability to maintain a moderate speed for a long period of
time. This implies a need for economy of movement to reduce the energy
expenditure. During locomotion, swinging the limbs back and forth uses
a considerable amount of energy. Mechanisms for reducing energy expenditure
include having the weight of the limbs concentrated in the proximal limb
and folding the limbs during protraction, both of which reduce inertia.
The addition of shoes and other equipment to the distal limbs increases
the dynamic load and decreases the ease of rotation. The further down the
limb the weight is added, the greater its effect. Therefore, heavy shoes
and wraps add to the energy expended in every stride, a factor that is
particularly significant in horses competing over long distances that require
thousands of strides to complete the race.
Since it is a subjectively judged sport, esthetics are of primary importance in dressage, which is in contrast to many other sports. Recently, the gait qualities that are evaluated subjectively in warmblood horses have been correlated with kinematic variables measured during gait analysis (Back et al., 1994; Holmstrom et al., 1994). In The Netherlands, potential sport horses are judged according to the subjective qualities of length, strength and suppleness of movement. A score in the range of 0-40 is awarded for each of the three qualities, with a score of 20 representing the average for the population. A score lower than 20 is awarded for better than average performance, a score higher than 20 is given when the performance is judged to be worse than average. A study was performed in which the scores awarded by an experienced judge for length, strength and suppleness as the horses trotted over ground were compared with kinematic variables measured as the horses trotted on a treadmill (Back et al., 1994). The variables that had the highest correlations with the judged score were stride duration, rotation of the scapula, and maximal dorsiflexion of the fetlock joint in the fore limb stance phase. Stride duration was highly correlated with the judged quality of length, which is not surprising since a long stride duration implies a longer stride length at the same speed. For warmblood dressage horses, average stride durations are 55 strides/min for the walk, 80 strides/min for the trot and 100 strides/min for the canter. The importance of limb length as a determinant of stride length was discussed above. In the fore limb, suppleness in the shoulder region is particularly important since a small increase in range of motion of the proximal limb translates into a large increase in range of motion of the distal limb. An increase of 7o in scapular range of motion lengthens the stride by about 20 cm. There was also a significant correlation between fetlock joint motion and the judged quality of suppleness; horses with high gait scores showed greater dorsiflexion of the fetlock joint in the fore limb. Another study of gait quality (Holmstrom et al., 1994) compared the movement patterns between Swedish Warmblood horses classified as good movers with those classified as poor movers. Stride duration again emerged as an important determinant of gait quality, together with a short stance percentage, i.e. stance duration expressed as a percentage of stride duration. A short stance percentage may indicate a superior ability to generate impulsion by producing a higher force over a shorter period of time. Another variable that differed with gait quality was diagonal dissociation, which measures the separation of the diagonal limbs at ground contact. In good movers, the hind limb made contact in advance of the diagonal fore limb at the trot, and the magnitude of the diagonal dissociation was correlated with gait quality. It was suggested that a longer dissociation indicated better balance and an ability to carry more weight on the hind quarters. The swing phase retraction of the fore limb (i.e. the time between the maximal forward position of the limb during the swing phase and ground contact) was longer in good movers than in poor movers. Furthermore, at the position of full protraction, the fore limb had a characteristic position in the good movers with the elbow joint flexed, the forearm elevated, and the carpus slightly flexed. This is in contrast to the extended carpus and the upward flip of the toe that sometimes occur in poor movers. A sloping conformation of the shoulder facilitates forward and upward movement of the fore limb at the end of the swing phase, and is, therefore, a favorable type of conformation (Holmstrom et al., 1993). Since the height of the flight arc of the fore hoof does not change much from trot to passage, the natural fore limb movement at the trot may predict expressiveness in piaffe and passage. Good movers generally show more overall retraction of the fore limb and more protraction of the hind limb but, contrary to the opinion of many trainers, hind limb protraction did not increase as training progressed (Holmstrom et al., 1995). In other words, the horses did not learn to step further underneath themselves during training. Therefore, it is very important to select a horse that steps well under itself naturally. Conformation is another subjectively evaluated area in which there is little solid evidence to support or refute a preference for certain morphological types in relation to performance or soundness. Elite dressage horses and show jumpers were found to have a more sloping scapula, a flatter pelvis and more open (extended) hock joint angles than a population of riding school horses of the same breed (Holmstrom et al., 1993), which suggests that these are favorable traits for performance horses. In addition the dressage horses had a shorter neck and radius, a larger elbow joint angle and a more upright femur. The show jumpers had more sloping pasterns in the fore limbs and smaller hip joint angles. Assessment of conformational ‘defects’ indicated incidences of about 80% for toed out hind limbs, 50% for toed in fore limbs and 60% for bench knees (cannon bones set too far laterally relative to the central axis of the proximal limb). Since the frequency of these traits did not differ between riding school horses, elite show jumpers and elite dressage horses, it was concluded that they did not have an adverse effect on performance (Holmstrom et al., 1993). It would be advantageous if movement patterns were stable enough during
growth and development to allow superior movement to be detected at a young
age. In fact, horses seem to have an inherent intra-limb coordination pattern
that is maintained from foal to adulthood (Back et al., 1995). Stride duration
and stance duration increase with age and size, but swing duration, protraction/retraction
angles, and relative timing of the joint angular events are consistent
from foal age to adult. This has given rise to the concept of the gait
fingerprint - a characteristic kinematic profile that doesn’t change significantly
with growth and aging.
When choosing a work surface for performance horses at least two aspects of the interaction of the horse's limb with the ground must be considered. These are the impact resistance and the shear resistance of the surface. Impact resistance describes the ability of the footing to absorb impact energy. Surfaces with a high impact resistance (e.g. concrete) absorb little energy on impact and are associated with a high amplitude impact shock. Surfaces with a lower impact resistance (e.g. wood chips) absorb more energy by deformation, so there is less concussion on the limbs. Shear resistance describes the ease with which the footing is displaced by a shearing force, as occurs when the limb pushes off at the end of the stance phase. For sport horses, the ideal surface has an intermediate shear resistance - low enough to allow the toe to penetrate as the hoof breaks over, thereby reducing tension in the DAL and reducing pressure on the navicular region, but not so low as to slide away from the hoof as it pushes off at the end of the stance phase. Hard surfaces have a high shear resistance, which does not allow the toe to penetrate. Surfaces with a very low shear resistance allow the toe to penetrate deeply but tend to give way as the horse pushes off. It is useful to compare the physical characteristics of different surfaces in relation to their effect on the horse's stride. Hard surfaces (concrete, asphalt, hard soil) have a high impact resistance and a high shear resistance. Consequently, the limbs are subjected to considerable concussion, and the toe is unable to penetrate the surface, which produces high loads in the navicular region in the terminal stance phase. Therefore, hard surfaces are particularly damaging to horses with navicular problems. Sand has a somewhat lower impact resistance than hard soil, combined with a low shear resistance which allows the toe to penetrate deeply. Both the SL and the DAL are subjected to lower strain on sand than pavement. However, deep sand tends to give way resulting in a loss of traction. Since horses must use a greater muscular effort to overcome the tendency of the sand to give way, the working heart rate will be up to 50% higher on deep sand. This is why sand is so tiring for the horse to work on. When sand has a high moisture content, the particles adhere to each other due to surface tension, so wet sand is more stable and less tiring to work on than dry sand. There are some commercial products that incorporate fibers or shredded materials to stabilize the sand particles. On turf, the rooting system of the herbage has a stabilizing effect on the soil particles. As a result, turf has an intermediate shear resistance which is ideal because it allows the toe to penetrate the surface as the hoof rotates, but it does not give way as the horse pushes off. The impact resistance of turf depends on several factors, notably the moisture content of the soil. As the soil dries out the impact resistance increases. Although a high moisture content lowers the impact resistance, too much moisture allows slipping. Well-maintained turf provides excellent footing, but it is difficult to keep the turf in this condition. Deterioration in surface characteristics under conditions of drought or excess rainfall are a problem for turf arenas and tracks. There can be no doubt as to the importance of good footing, and it is
useful to have an appreciation of the advantages and disadvantages of different
surface types in relation to performance and soundness. The ideal footing
for a specific arena or track varies with the sport, local climate, natural
ground type and gradient, and location (indoors or outdoors). It is easier
to choose a suitable surface for a single sport in an indoor arena than
it is to cater to the needs of several different sports in an outdoor arena,
where the unpredictable effects of the weather play a role. The capital
investment and the practicalities of maintaining the surface on a day-to-day
basis are also important and, as a result, the end product is often a compromise
between the ideal and the practical/affordable.
Back, W., Barneveld, A., Bruin, G., Schamhardt, H. C., & Hartman, W. (1994). Kinematic detection of superior gait quality in young trotting warmbloods. Vet Quart, 16, Suppl 2, S91-96. Back, W., Schamhardt, H. C., Hartman, W., Bruin, G., & Barneveld, A. (1995). Predictive value of foal kinematics for the locomotor performance of adult horses. Res Vet Sci, 59(1), 64-69. Drevemo, S., Fredricson, I., Dalin, G., & Bjorne, K. (1980). Equine locomotion. 2. The analysis of coordination between limbs of trotting Standardbreds. Equine Vet J, 12(2), 66-70. Holmstrom, M., Fredricson, I., & Drevemo, S. (1994). Biokinematic differences between riding horses judged as good and poor at the trot. Equine Vet J, Suppl 17, 51-56. Holmstrom, M., Fredricson, I., & Drevemo, S. (1995). Biokinematic effects of collection on the trotting gaits in the elite dressage horse. Equine Vet J, 27(4), 281-287. Holmstrom, M., & Philipsson, J. (1993). Relationships between conformation, performance and health in 4-year-old Swedish warmblood riding horses. Livest. Prod Sci, 33(3-4), 293-312. Seder, J. A., & Vickery, C. E. (1993). Double and triple fully airborne phases in the gaits of racing speed Thoroughbreds. Abstracts, Second International Workshop on Animal Locomotion, 59-60. Wilson, B. D., Neal, R. J., Howard, A., & Groenendyk, S. (1988).
The gait of pacers 2: factors influencing pacing speed. Equine Vet J, 20(5),
347-351.
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