Marc H. Ratzlaff

Interrelationships between moisture content of the track, dynamic properties of the track and the locomotor forces exerted by galloping horses

Summary Three methods were used to examine the relationships between the dynamic properties of a track and the locomotor forces exerted by galloping horses. The impact resistance and the percentage of energy returned by the track were determined using a trailer mounted track testing device. Vertical forces were measured from instrumented horseshoes nailed to all four hooves and the velocities of each of the six horses galloping in a track straight-away were determined from slow-motion films. The moisture content of the track was altered by the addition of water. These data were analyzed to determine the relationships between changes in moisture content on the energy returned by the track, impact resistance of the track and locomotor forces exerted by the horses. There was a strong linear relationship between impact resistance and the percentage of energy returned by the track. Changes in moisture content of the track cushion resulted in similar changes in both the percentage of energy returned and the impact resistance of the track. Energy return and impact resistance decreased at 8% moisture and progressively increased from 8.5 to 14% moisture. The horses were divided into two groups based upon their speed during each trial (Group I: 14.5 to 15.4 m/sec and Group II: 15.5 to 16.5 m/sec). Changes in the moisture content of the track cushion altered the forces exerted by the horses. Forces were lowest at 8% moisture content for Group 1 and at 12% moisture for Group II. Changes in the percentage of energy returned and the impact resistance of the track also affected the forces exerted by the horses. For the horses in Group I forces increased as energy return and impact resistance increased. Conversely, horses in Group II exhibited a decrease in force as energy return and impact resistance increased. These results suggest that the dynamic properties of the track may be suitable for horses traveling at relatively narrow ranges of velocity and that when horses work at speeds outside of this range, pronounced changes in locomotor forces will occur.

Measurement of vertical forces and temporal components of the strides of horses using instrumented shoes

Summary Instrumented shoes were used to measure the vertical forces exerted by horses moving at a variety of gaits. Two types of shoes were used; one contained a single transducer positioned over the center of the frog and the second contained transducers located at the toe and both sides of the heel. Horses were shod with these instrumented shoes and walked and trotted over a force plate. Forces were simultaneously recorded from the transducers in the shoes and from the force plate. Comparisons were made between the amount and duration of the forces exerted on the transducers and the vertical and horizontal forces recorded from the force plate. Forces recorded from the single transducer shoes showed strong correlations with the forces recorded from the force plate for horses moving at the walk; however, at the trot only moderate correlations occurred between these forces. At both the walk and trot, forces recorded from each side of the heel and the total forces occurring on all three transducers from the front hooves of horses shod with three transducer shoes showed strong correlations to the vertical forces recorded from the force plate. Vertical forces were also recorded from the instrumented shoes as horses walked, trotted and galloped on a track straightaway. Forces recorded from normal horses shod with single transducer shoes on all four feet were greater on the forelimbs than the rear limbs at the walk and trot. At the gallop, forces were highest on the lead front followed by the nonlead front, lead rear and nonlead rearlimb, respectively. Forces recorded from a three transducer shoe on the right front hoof of a horse walking, trotting and galloping in a right lead were highest on the medial side of the heel and occurred during the middle of the support period. Peak forces on the toe occurred at or near the time of heel lift. The results of these studies indicate that these instrumented shoes have advantages over the methods previously used to measure locomotor forces. These instrumented shoes can be used to simultaneously record the temporal components and the amount and distribution of vertical forces exerted during consecutive strides of horses moving at a variety of gaits.

Relationships between locomotor forces, hoof position and joint motion during the support phase of the stride of galloping horses

Three methods were used simultaneously to determine the relationships between the vertical forces exerted on the hooves and the positions of the limbs and hooves at the times of peak vertical forces from 2 horses galloping on a track straightaway. Vertical forces were recorded from an instrumented shoe, fetlock joint motion was measured with an electrogoniometer and the angles of the carpus, fetlock and hoof were determined from slow-motion films. At hoof contact, the mean angles of the carpus and fetlock were 181-182 degrees and 199-206 degrees, respectively. Peak vertical forces on the heel occurred at or near maximum extension of the carpal and fetlock joints. Peak forces on the toe occurred during flexion of the fetlock joint and at mean hoof angles of 28-31 degrees from the horizontal. The mean angles of the hoof from the horizontal at the time of heel contact were 6-7 degrees. Hoof lift occurred at mean carpal angles of 173-174 degrees and mean fetlock angles of 199-200 degrees.

The interrelationships of stride lengths and stride times to velocities of galloping horses

Summary Side view slow motion movies of galloping horses were taken and analyzed to determine horse velocity, limb velocity, stride length, stride time, and the swing and support timesof the stride. Multivariate statistical methods were used to evaluate the interrelationships of theseparameters. A linear relationship between horse velocity and limb velocity was observed; however, two groups could be distinguished within the trials. In one group, in which the horses were galloping normally, a one-to-one relationship of limb velocity to horse velocity was noted. In the other group, in which the majority of the horses were either lame or restrained by the rider, limb velocities were higher than horse velocities. In general, as horse velocity increased, stride length increased and stride time decreased due to decreases in both the swing and support times of the stride. Increases in stride length contributed more to increases in horse velocity than changes in either swing or support times. Changes in stride length had the greatest effect upon the velocities of those horses that were either lame or restrained by the rider, while the effects of changes in swing and support times contributed relatively more to changes in velocities of the horses which were galloping normally. This study suggests that in lame horses, limb velocity is proportionally greater than horse velocity due to decreases in stride length and stride time.

Quantitative evaluation of equine carpal lamenesses

Summary Electrogoniometry was used to evaluate carpal joint motion of seven horses with diagnosed carpal lamenesses. Data from at least six consecutive strides of each horse were collected and compared with data from six clinically normal horses. The evaluations included the shapes and patterns of the goniograms, the ranges and amplitudes of movement, the angular velocities, and the swing and support periods of the stride. The amplitudes of movement of the affected joint were less than normal, primarily due to decreased flexion. In addition, irregularities of movement occurred during the extension phase as the affected limbs advanced. Analysis of the swing and support periods of the stride and changes in the angular velocities during flexion and extension indicated that carpal lamenesses may affect either or both phases of the stride. Therefore, the lamenesses may be classified as a swinging leg, supporting leg or mixed lameness if only one limb is involved. Other parameters, such as stride length and arcs of flight of the limbs, must be determined in addition to the electrogoniometric data to definitively classify bilateral carpal lamenesses. The results of this study indicate that electrogoniometry can provide a more quantitative assessment of the severity of carpal lamenesses than is possible by clinical examination alone. Using electrogoniometry in conjunction with the clinical examination it may be possible to determine whether a lameness is due to pain, mechanical interference or both.

Stride rates of horses trotting and cantering on a treadmill

Summary Fifteen Thoroughbred racehorses were exercised at the walk, trot and canter three times a week for seven weeks on an inclined treadmill. Stride rates of each horse were measured at the trot and canter during each exercise period. At both gaits, stride rates were consistent between trials for each horse and a general linear relationship between stride frequencies at the trot and canter occurred. Stride rates outside of the 95% confidence level occurred in only 17 of the 522 trials and, of these, gait abnormalities could be identified in only five of these trials. Three horses exhibited signs of dorsal displacement of the soft palate during some of the trials. No effects of this condition on stride frequencies were observed. The consistency of the stride rates at the trot and canter suggests that individual horses have a preferred stride frequency at a given velocity and gait. Horses that have high ratios between stride frequencies at the trot and canter would tend to have longer stride lengths at the canter at a given speed. Since maximum velocity is directly related to the length of stride, horses with longer strides should have higher peak speeds than those horses with shorter strides. Many other factors must be considered in any predictions of performance potential.

Some biomechanical considerations of navicular disease

Summary A number of theories on the pathogenesis of navicular disease have been proposed, however none of these completely explains all of the observed pathologic changes. We propose that these changes can be explained by alterations in the biomechanics of the digit caused by abnormal flexion of the pastern joint. Abnormal flexion should result in increased tension on the navicular suspensory apparatus which would compromise the blood supply to the navicular bone by mechanical compression of the vessels. Abnormal flexion should also increase the ligamentous stress on the navicular bone at the attachments of the collateral sesamoidean and distal sesamoidean impar ligaments accounting for osteophytosis at their attachments. Additionally, the increased stress on the navicular suspensory apparatus should cause a slight proximodorsal displacement of the navicular bone which would alter the normal forces on the articular surfaces of the coffin joint and increase friction between the navicular bone and the tendon of insertion of the deep digital flexor muscle. This would result in degeneration of the fibrocartilage of the navicular bone, villous hypertrophy of the bursal synovium and malacia within the deep digital flexor tendon.

The effects of a compressible plastic shoe - The Seattle shoe®- on the kinematics of the strides of galloping thoroughbred horses

Summary This study was undertaken to determine if the kinematic parameters of galloping horses were altered when these horses were shod with Seattle Shoes®. Analyses of films of six Thoroughbred horses galloping through a track straightaway, with and without Seattle Shoes®, were used to determine velocity, stride length, stride time, swing time and support time ofthelimbs, the percentages of the stride time spent in the swing and support phases, and the arcs of flight of the carpi and fetlocks of the forelimbs and the coronets of all four limbs. Descriptive statistics were calculated for each of these variables. Comparisons were made between those measured from ® horses shod with and without the Seattle Shoe,usingboththe paired-t test and analysis of covariance, which eliminated the influence of differing velocities. No significant differences occurred in the stride lengths or timing parameters when these horses were shod with Seattle Shoes®. The Seattle Shoe® had its most significant effect on the arcs of flight of the limbs. The compression of the Seattle Shoe® appears to store a portion of the potential energy normally stored in the suspensory appa- ratus and results in a decreased extension of the fetlock. This energy is then released, causing significant increases in the heights of the forelimb during retraction and protraction. Although kinematic adjustments are required by horses to maintain the level of performance, the Seattle Shoe® appears to have no detrimental effects on the performance of galloping horses and it may decrease strain on the suspensory apparatus.

Rupture of the distal sesamoidean ligaments of a horse: Vertical forces and temporal components of the strides before, during and after injury

Summary The temporal components and vertical forces of the strides before, during and following rupture of the distal sesamoidean ligaments of the right forelimb were recorded from instrumented shoes. The right front hoof was shod with an instrumented shoe containing transducers located at the toe and both sides of the heel. The left front hoof was shod with a shoe containing a single transducer positioned over the center of the frog. The Thoroughbred mare had galloped through a track straightaway and entered the turn when the injury occurred. The horse had changed from a right to left lead five strides before the breakdown. In the three strides prior to the injury, no abnormalities, such as stumbling, were observed. During the stride in which the breakdown occurred, the right front hoof landed first on the lateral side of the heel rather than on the medial side as in the preceding strides. The forces on both sides of the heel initially increased in a normal pattern and at the time of peak force, rupture of the distal sesamoidean ligaments apparently occurred. Following the injury, marked changes were observed in the force-time curves which included a pronounced impact force on the toe and very low peak forces on all transducers.