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| LOCOMOTION, LAMENESS AND PERFORMANCE
LOCOMOTION Evaluation of a horse’s movement by a trainer or veterinarian is a subjective process and the information gained is qualitative in nature. Gait analysis applies measurement techniques to quantify the characteristics of locomotion. Kinematic analysis describes the movements of the limbs and body in terms of timing, distance and angular variables. Kinetic analysis describes the internal and external forces, such as the force between the hoof and the ground (ground reaction force - GRF) and the torques around the joints. Gait Analysis During each stride, every limb has a stance phase when it is in contact with the ground and a swing phase when it moves forward in preparation for the next stance phase. When trainers assess quality of movement, they tend to focus on the swing phase, since this is when the expressiveness or extravagance of the horse’s movement are apparent. In the etiology of lameness, however, the swing phase is relatively unimportant because the associated forces are small. It is during the stance phase that large forces are applied to the musculoskeletal system, so evaluation of the stance phase is usually more informative in evaluating performance-limiting factors or lameness. Loading of the limbs during the stance phase occurs in two stages. Immediately after the hoof contacts the ground it is rapidly decelerated giving rise to a shock wave that travels proximally through the bones and joints during the impact phase. During the remainder of the stance phase, the limb gradually accepts the body weight then pushes off against the ground. Since the behavior of the musculoskeletal tissues depends on the rate of deformation, the tissues react differently during impact than during the later part of the stance phase. In general, the hard tissues (bones and joints) are more vulnerable to injury during the impact phase, whereas the soft tissues are more likely to be injured during the loading phase. An understanding of the characteristics of the impact and loading phases leads to a better appreciation of the development of lameness in athletic horses and to the assessment of strategies for prevention and treatment. Impact Phase Repetitive limb loading during locomotion generates intermittent waves of deceleration that are attenuated by the body’s natural shock absorbers as they travel proximally in the limbs. The impact shock wave is characterized by having a high amplitude with a rapid vibration frequency. Energy enters the locomotor system when the foot strikes the ground, and some of this energy is dissipated through attenuation by the body tissues. Consequently, the amplitude of the impact wave decreases as it travels proximally in the limb. Ineffective attenuation of impact shock causes microtrauma to bone and articular cartilage and plays a primary causative role in acute and chronic skeletal and articular injuries. The hoof acts as the initial shock absorber for the skeletal system by absorbing impact shock in the laminae and digital cushion. Our studies of cadaver limbs (Lanovaz et al., 1998) indicate that the soft tissues inside the hoof, such as the laminae and digital cushion, are primarily responsible for attenuating the frequency of the impact vibrations, whereas the bones and joints are more important for attenuating the amplitude. As the shock wave travels up the limb, it is further dissipated by subchondral bone, articular cartilage and other periarticular tissues. Subchondral bone is a fairly efficient shock absorber, but excessive impacts lead to sclerosis and microfractures. Articular cartilage is an even more effective shock absorber than an equivalent amount of bone, but because it is present in such a thin layer in the joints it makes a relatively small contribution to overall shock attenuation. Factors that exacerbate the damaging effects of impact shock include faster speed, a harder work surface and perhaps certain types of shoes. In racehorses that accumulate a large mileage at high speed, impact shock is likely to be an important etiological factor in breakdown injuries, e.g. long bone fractures. Catastrophic fractures of the long bones are often preceded by fatigue fractures (Stover et al., 1992), which have been identified as a consequence of impact loading (Radin et al., 1973). In short bones and sesamoid bones, sclerosis often occurs prior to fracture in well-defined patterns that are related to the stress lines (Pool, 1992). Sclerosis of subchondral bone is also a consequence of impact loading (Radin et al., 1973). Sport horses train at slower speeds than racehorses, leading to lower impact shock and, consequently, the effects may accumulate for many years before becoming clinically apparent. Articular cartilage is fatigue prone, and there may be a threshold of impulse intensity above which cartilage damage is progressive and irreparable. Changes in cartilage in response to repetitive impact loading include metabolic and biochemical alterations that are consistent with cartilage degeneration and the development of osteoarthritis, which further reduces the ability of the joint to attenuate impact shock, resulting in damage to the more proximal and distal joints. The role of exercise in initiating and perpetuating damage to articular cartilage in degenerative joint disease is well-established. For example, sheep housed on concrete floors are more prone to develop osteoarthritis than those housed on dirt due to the effects of repeated impulsive loading over a prolonged period. In sport horses, osteoarthritis is the primary reason for premature retirement. The damage is initiated long before it becomes clinically apparent and an awareness of the predisposing factors can be applied to ameliorate impact shock and so prolong the horse’s career. In this case a prime consideration is the nature of the work surface used for daily training – a cushy, resilient type of footing is less damaging than a harder surface. Even at relatively slow speeds of around 4 m/s the hoof experiences impact accelerations in the order of 80 to 100 g (Benoit et al., 1993). The amplitude and vibration frequency of the shock wave vary with gait, speed, fatigue, ground surface and shoeing. Both the amplitude and vibration frequency increase with speed. The onset of muscular fatigue is associated with a significant increase in amplitude of impact shock (Pratt et al., 1976), which may play a role in impact-related injuries. Steel shoes increase the amplitude of impact shock measured on the hoof wall, whereas certain shoes and pads reduce both the amplitude and frequency of the impact accelerations (Benoit et al., 1993). The amplitude also increases with the density of the surface material, but is reduced when the surface has a higher content of water or organic material (Barrey et al., 1991). Loading Phase The carpus rapidly snaps into the close-packed position after initial ground contact, and this allows the fore limb to act as a propulsive strut throughout most of the stance phase. The fetlock joint and the palmar soft tissues behave like an elastic spring to conserve energy. In the early part of the stance phase the fetlock joint extends as it sinks toward the ground, reaching maximal extension at midstance, which corresponds with the time when the cannon bone is vertical. As the fetlock joint extends, the palmar soft tissues are stretched. After midstance the fetlock rises allowing the elastic structures to recoil, thereby releasing elastic energy that was stored during the stretching process. This energy helps to flex the distal joints during the swing phase. The limbs accept the body weight in the early part of stance then push off against the ground in the later part of stance. Limb loading can be evaluated using a force plate, which measures the magnitude, direction and point of application of the GRF vector. This is resolved into three perpendicular force components: vertical, longitudinal (from head to tail) and transverse (side-to-side). When a horse moves in a straight line, the transverse force is small and relatively unimportant compared with the vertical and longitudinal forces. The vertical force is responsible for overcoming the effects of gravity and for raising the body mass. At the trot, the vertical force peaks at midstance, when the cannon bone is vertical and the fetlock is at its lowest position. Maximal fetlock extension is directly correlated with peak vertical force. The longitudinal force, which is concerned with braking (deceleration) and propulsion (acceleration) of the horse in a forward direction, shows negative and positive components during each stance phase. In early stance, the longitudinal force retards the forward movement, later in the stance phase it provides propulsion. The direction of the longitudinal force changes around the time of midstance. Strains in the tendinous structures of the distal limb have been measured directly using strain gages. They can also be calculated from a knowledge of the GRFs and limb kinematics. The extensor branches of the suspensory ligament orient the hoof for contact and prevent buckling forward of the interphalangeal joints during early stance. As the limb accepts weight, the superficial digital flexor (SDF) muscle generates tension to stiffen the limb. Consequently, peak tension in the SDFT occurs during the loading phase. The suspensory ligament (SL) together with the sesamoid bones and the distal sesamaoidean ligaments acts as a passive system to support the fetlock. Since it has no muscular component, SL strain depends entirely on the angle of the fetlock joint. As the fetlock extends, SL strain increases and reaches its maximal value at midstance. The deep digital flexor tendon (DDFT) reaches peak strain later in the stance phase around the time when the propulsive force is maximal. The DDF is thought to be involved in providing forward propulsion. Since it has no muscular component, strain in the distal check ligament of the DDFT (DCL) depends on the angles of the fetlock, pastern and coffin joints. Maximal strain corresponds with the start of breakover. Due to its dependence on coffin joint angulation, the DCL is particularly sensitive to changes in hoof angle and toe length, and to different surface types particularly with regard to the ability of the hoof to rotate during the stance phase. The DCL is maximally strained at the start of breakover. The entirely passive structures (SL, DCL) are loaded and strained more than the DDFT and SDFT which have an active muscular component that adjust tension in the tendons. Maximal strains in the SDFT and DDFT are higher when the horse carries weight, moves on a hard surface or travels at higher speeds. During the swing phase the flight arc of the hoof reaches its highest point soon after lift off with a second, smaller elevation coinciding with an upward flip of the toe at the time of maximal protraction. This gives a slightly biphasic flight arc. Effects of Footing 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. When choosing a work surface for performance horses two important properties are the impact resistance and the shear resistance of the surface material. 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, such 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 DCL 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 terminal stance. These effects are used to advantage in a lameness examination. Sand is the most commonly used footing in the midwest due to its availability and cost effectiveness. However, sand varies widely in its physical properties and some types of sand are much better than others. The relevant considerations are size, shape and hardness. 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 DCL 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 leading to early onset of fatigue. 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 (think of running on the beach). Some commercial products incorporate fibers or shredded materials to stabilize the sand particles. This mimics the effect of the rooting system of turf, which has a stabilizing effect on the surrounding soil particles. 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 turf in this condition. Deterioration in surface characteristics under conditions of drought or excess rainfall are a problem for turf arenas and tracks. Effects of Farriery Trimming and shoeing have a marked effect on performance and soundness of the equine athlete. Ideally, trimming optimizes the interaction between the hoof and ground during locomotion. The objectives of trimming include aligning the dorsal hoof wall with the pastern axis, and ensuring that the bearing surface of the hoof lies beneath the weight-bearing axis of the limb. This is accomplished by adjusting the absolute and relative lengths of the heels, the quarters and the toe. When the hoof is balanced in this manner, it usually contacts the ground flat-footed or slightly heel first. Radiographic studies have shown that P1 is always a little more upright (vertical) than P2 and P3 (Bushe et al., 1988), with the three phalanges being most closely aligned when the hoof is trimmed with the dorsal hoof wall parallel to the pastern axis. This configuration is favoured because it optimises the forces on the supporting soft tissue structures. However, it should not be assumed from this statement that the pastern angulation is constant and unchanging. In fact, there is an inverse correlation between the hoof and pastern angles; an increase in hoof angle (more upright) is associated with a reduction in the pastern angle (more sloping) within an individual horse and vice versa. Older texts generally recommend an angle of 45-50o in the fore hooves and 50-55o in the hind hooves, but recent studies indicate that in most horses a steeper hoof angle is needed to align the hoof-pastern axis. For example, one study of Thoroughbred-type horses showed that the hoof-pastern axis was aligned at a mean hoof angle of 54o (range 48-55o) in the fore limbs, and 55o (range 49-60o) in the hind limbs (Clayton, 1988). Another consideration is the location of the bearing surface of the hoof relative to the weight-bearing axis through the cannon bone. The bulbs of the heel should lie vertically below the central axis of the cannon bone in a sagittal plane. In some horses, although the hoof-pastern axis is aligned, the whole hoof capsule is located too far forward with the bulbs of the heels ahead of the central axis of the cannon bone. The resulting caudal concentration of the weight bends the hoof tubules at the heels, reducing their ability to withstand compression, and resulting in under-run heels. Concentration of weight on the heels has been associated with the development of chronically bruised heels, navicular disease, degenerative joint disease of the fetlock joint, chip fractures of the fetlock and carpus, and strains of the DDFT. Mediolateral balance evaluates the hoof in a frontal plane. The objectives are to optimize weight-bearing on the medial and lateral sides of the hoof, facilitate breakover at the natural position (toe, medial side, lateral side) and straighten the flight arc of the limb when viewed from in front or behind. The first objective is to facilitate breakover in the natural position, which will often produce a straighter flight arc. The result is evaluated and, if necessary, wedges are added to the medial or lateral side in an attempt to achieve a flat-footed contact. At gaits faster than a walk it is difficult to evaluate hoof motion with the unaided eye. In this situation slow motion replay of a video recording is invaluable. Video recordings made from in front and/or behind the horse as it walks and trots on a straight line are replayed at normal speed, in slow motion and in single frame advance mode to evaluate the flight pattern of the limb and hoof contact with the ground. During corrective shoeing, video recordings can be used to evaluate the effects of each stage in the process. Acute Hoof Angle Some trainers favor an acute hoof angle because they believe that the long toe-low heel conformation enhances performance by increasing stride length. When the trot stride was compared for a normal hoof angle versus an acute hoof angle, there were no significant changes in stride length or suspension, and the flight arc of the hoof was almost identical with the two angulation (Clayton, 1990a). However, the acute hoof angle was associated with an increased frequency of toe-first contacts, which was thought to be a consequence of the proprioceptive reflexes ensuring a fairly flat placement of P3 regardless of the shape of the hoof capsule. Toe-first contacts are associated with a tendency to trip or stumble. The duration of breakover was prolonged with the acute hoof angulation and the orientation of the limb segments at the start of breakover suggested an increased tension in the DCL and navicular ligaments. However, the effects of an acute hoof angle on breakover may be mitigated on a softer surface that allows penetration of the toe during the terminal part of the stance phase, since flexion of the coffin joint reduces tension in the DCL and navicular ligaments. The study failed to reveal any enhancement of performance due to an acute hoof angle, and this type of conformation or trimming may predispose to navicular disease due to the greater tension in the navicular ligaments and the DCL/DDFT which then exerts pressure on the navicular bone. Other pathological conditions that have been associated with an acute hoof angle include osteoarthritis of the fetlock and interdigital joints, chip fractures of the fetlock and carpal joints, sesamoid fractures and sesamoiditis. Horses trimmed with normal angles in their fore hooves and acute angles in their hind hooves showed a prolongation of breakover and delayed lift off in the hind hooves. However, the normal limb coordination pattern was restored by the time of ground contact (Clayton 1990b). Therefore, delaying breakover in the hind hooves is unlikely to have a beneficial effect in horses that interfere. A more effective solution to interference problems may be to hasten breakover and lift off in the fore hooves. Heel wedges cause only slight changes in strain of the SDFT, DDFT, and SL during walking (Riemersma et al., 1996a), though a larger increase in SDFT strain has been recorded at the trot (Stephens et al., 1989). With heel wedges the onset of breakover is delayed due to a more gradual increase of tension in the DDFT. With a toe wedge, strain in the DCL increased, whereas strain in the SDFT and SL decreased. Since the DCL has no muscular component that can actively change its length, strain in this structure is totally dependent on the limb configuration especially the angle of the coffin joint. The DCL is normally maximally strained at the start of breakover, which is when heel wedges have their greatest effect on the GRF. This emphasizes the importance of the DCL in influencing limb forces and movements in the final part of the stance phase. Raising the heels seems appropriate in DCL injury, though this may slightly increase SDFT loading. During recuperation from DCL injury, however, the limitations on exercise make it unlikely that the safety margin of the SDFT will be exceeded even with heel wedges in place. Hoof Length In a study designed to investigate the effects of overall hoof length on the flight arc, pads were applied to increase hoof length by 5 cm (Balch et al., 1994). Compared with a normal hoof length, the long hooves were associated with a prolongation of stride duration, swing duration, and breakover, but did not affect overall stride length or stance duration. The flight arc peaked earlier and higher with the longer hooves, but the normal movement pattern was re-established in the later part of the swing phase. Although stride length did not change, the prolongation of the swing phase may be esthetically pleasing. Shoe Weight Weighted shoes are used to give horses more action. Doubling the
weight of the shoe did not affect stride length, stride duration,
or breakover, but it did increase the maximal heights of the hoof,
fetlock and carpus during the swing phase. The peak height of the
flight arc tended to occur later in the swing phase (Balch et al.,
1996). Also, the hoof and pastern segments had a more acute angle
at ground contact, probably as a result of increased momentum of
the distal limb during the swing phase. Heavier shoes require greater
energy expenditure in the elbow flexors to overcome inertia at
the start of the swing phase and in the elbow extensors to overcome
the limb’s momentum at the end of the swing phase.
Lamenesses have traditionally been classified as supporting limb, swinging limb or mixed. Gait analysis shows that almost all lamenesses show deficits in both the stance and swing phases though one or other is likely to predominate. Changes in the stride timing vary with the limb and site of lameness. An absence of a suspension phase following the lame diagonal stance phase is a fairly consistent alteration. In a predominantly supporting limb lameness, the horse carries less weight on the lame limb with a corresponding reduction in the peak vertical force on that limb. This is reflected by reductions in coffin joint flexion and fetlock joint extension at midstance. There are compensatory increases in the vertical force and joint angles in the contralateral (sound) limb. The maximal angles of the fetlock and coffin joints are one of the most consistent and sensitive indicators of lameness. In contrast to the distal joints which show reduced flexion in the lame limb, the proximal joints, notably the shoulder in the fore limb and the tarsus in the hind limb, may show increased flexion in the lame limb. This is a load-damping effect that reduces the peak forces in the lame limb (Buchner et al., 1996a). The range of protraction and retraction of the limb as a whole may change when the horse is lame. With a fore limb lameness retraction of both fore limbs tends to be reduced, whereas with a hind limb lameness protraction of both hind limb tends to be reduced. This is because the fore limb is closer to the horse’s center of gravity (and carries more weight) when it is retracted, whereas the hind limb is closer to the center of gravity (and carries more weight) when it is protracted. With regard to protraction and retraction, it is interesting to note that the hind limbs are retracted further during walking than during trotting. This explains why the signs of fibrotic myopathy, which affects the hind limb in the fully retracted position, are more obvious in the walk than in the trot. The head, withers and tuber sacrale normally show a sinusoidal pattern in their vertical displacement during trotting. Changes in the head movement pattern are the best indicators of a fore limb lameness, with the head being raised during the lame fore limb stance phase and sinking during the sound fore limb stance phase. It has traditionally been assumed that the mechanism of action was by shifting the center of mass, but more recent studies have shown that the asymmetrical head movements act through the inertial interaction between the trunk and the head/neck segment. The resultant reduction in neck joint sagittal torque reduces the maximal vertical force on the trunk (Vorstenbosch et al., 1997). During both fore and hind limb lamenesses at the trot, the trunk has a lower vertical velocity at impact of the lame limb, and during the lame limb stance phase the trunk is kept higher above the ground than during the stance phase of the compensating limb (Buchner et al., 1996b). In effect, the sound diagonal pushes off into a suspension and the trunk is highest at the start of the lame diagonal stance phase. It sinks gradually during the lame diagonal stance phase, and continues to sink as weight is transferred from the lame to the sound diagonal without the intervention of a suspension. After midstance on the sound diagonal the trunk is again raised into a suspension in preparation for the next stance phase of the lame diagonal. The best indicator of hind limb lameness is the movements of the hips and croup. When a trotting horse is viewed from behind, the oscillations of the tuber coxae show a small elevation during stance and a larger elevation during swing in sound horses. During hind limb lameness, movement of the tuber coxae is increased on the side of the lame limb (Buchner et al., 1996a). The asymmetry is perceived most easily just before ground contact of the lame limb when the tuber coxae on that side is elevated rapidly. These movements can also be seen by watching the croup movements in the lateral view; the croup is highest just before the lame limb makes contact with the ground. Center of Pressure Analysis Neurological diseases can pose a diagnostic challenge. We are currently investigating the value of center of pressure analysis as a possible diagnostic aid for detecting balance deficits in horses with neurological diseases. The center of pressure (COP) is defined as the centroid of force distribution on a force plate. Even during quiet standing, there are small adjustments of tension in the muscles that are reflected by movements of the COP. These movements are often referred to as postural sway. The amount of postural sway (or the stability of the COP) is an indicator of balance. Somatosensory, vestibular and visual pathways provide feedback for the maintenance of balance. The balance mechanism is such that the COP is allowed to drift a short distance before being corrected. It is these drifts and corrections that are measured in postural sway analysis. In people postural sway analysis is used to detect balance deficits, for example in patients with Parkinson’s disease and in astronauts returning from space flight. Many neurological diseases of horses are associated with disturbances of balance, such as cervical vertebral malformation-malocclusion, EPM and EDM. We are investigating the possibility of using postural sway analysis as an aid for detecting neurological diseases in horses. The force plate in the McPhail Center measures 2’ by 4’ and is the largest model available for equine studies. Even so, it is not big enough for large horses to stand with all four hooves on the plate simultaneously. In small horses postural sway analysis is performed with all four hooves on the force plate, but in larger horses separate analyses are performed for the fore and hind limb pairs. The position of the COP is measured at intervals of one hundredth
of a second for a period of 10 seconds and the recordings are repeated
four times to allow the calculation of mean values. The sequential
positions of the COP are plotted graphically as a stabilogram in
which the longitudinal (craniocaudal) motion is plotted against
the medialateral motion. The following variables are measured to
make comparisons between horses: The COP velocity appears to be the most sensitive indicator of differences between individuals. We are currently building a database of normal horses and horses with known neurological diseases with the objective of determining whether characteristic changes can be detected that might have some diagnostic significance. In addition, we are investigating the effect of detomidine on the horse’s balance.
Traditionally, selection of horses has been based on subjective
evaluation of factors that include 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: On the contrary, gait patterns that are not pleasing to the eye, that make athletic performance difficult, or that predispose to locomotor unsoundness are avoided. Gait analysis has been used to identify patterns of movement that are associated with success in different sports, allowing a more objective selection of equine athletes. However, equestrian sports cover a wide range of disciplines, and different sports require specific athletic talents, so there are marked differences in the selection criteria for different occupations. The following paragraphs will describe the locomotor characteristics of two very different types of sports, racing and dressage, to illustrate the diversity of talents required of the equine athlete. Racing Horse 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. Speed = Stride Length x Stride Rate 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. Human runners sometimes use this type of over-striding, with its consequent decelerating effect, to slow down after the end of a race. 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 summation of the forces exerted over a period of time). It has been suggested that 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 make the greatest contribution to changes in stride rate, so race horses must develop sufficient muscular strength to generate higher forces over a shorter period of time to maintain the impulse. 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. Sprint Racing 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 ipsilateral 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 leaving the ground 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 in a horizontal direction. 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 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 in which 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. The gallop normally has a single suspension in each stride; this is a gathered suspension that occurs between lift off of the leading fore limb and contact of the trailing hind limb. At extreme speeds there may also be a short suspension between lift off of the leading hind limb and contact of the trailing fore limb (as in galloping dogs), and there may even be a very short suspension between stance phases of the trailing and leading fore limbs. The frequency and duration of these multiple suspensions increases with speed. Endurance Racing 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 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. Dressage 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 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 extension angle 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. Limb length is an important determinant of stride length. 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 extension 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 phase which 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 (also known as diagonal advanced placement – DAP), 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 and Philpsson, 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. Contrary to the opinion of many trainers, hind limb protraction was not shown to 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. 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.
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.
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