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Horsewalkers - should they be round or oval?

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Horsewalkers (electro-mechanical devices that allow multiple horses to be exercised simultaneously in a controlled fashion) are used extensively in the management and training of horses. They permit controlled exercise of horses at walk and trot. They are less labour intensive than most other forms of controlled exercise, such as walking in-hand, lunging, riding, swimming or running horses on treadmills. The exception might be ride and lead, but this is not a widely used technique, except perhaps in polo.

Horsewalkers may be used for a variety of reasons including warming-up or cooling down prior to or following ridden exercise, as a way to relieve boredom in stabled horses, for controlled exercise as part of a rehabilitation programme and to supplement ridden exercise. Horsewalkers are often also used where ridden exercise is not desirable or possible, such as in preparation of young animals for sale or in animals that may have injury to the back and therefore cannot be ridden. The majority of horses can be trained to accept being exercised on a horsewalker within a short period of time.

Any form of exercise carries a risk of injury and whilst there does not appear to be any objective information on the safety of this form of exercise, it would generally be considered that the horsewalker is a very safe form of exercise. Until recently, horsewalkers have been exclusively of a round design in which the horse is constantly turning on a circular track. The radius (tightness) of the turn is determined by the diameter of the walker - the larger the walker, the more gradual the turn. At present commercial round horsewalkers vary from around 10 to 30 metres in diameter (i.e. 5-15 metres in radius). The conventional design is of a centre post from which radiate arms that support the moving dividers that separate the horses but also encourage them to walk as the centre post rotates, in turn moving the dividers.

Other designs do not incorporate dividers but horses are hitched to arms radiating from the centre post. Whilst the majority of walkers can operate in either a clockwise or anti-clockwise direction, on the walker the horse is still turning constantly. Exercising at walk or trot on a circle for prolonged periods of time must be considered to a large extent unnatural for a horse. Horses at pasture, whether grazing or exercising, move in all directions and never in one continuous direction. The same is true of ridden exercise. No rider would work his or her horse continuously for 30 minutes on a circle, even when working in a confined area. For example, a Dressage test incorporates many changes in rein and exercise in straight lines as well as on turns. Lunging is another mode of controlled, unridden exercise that is commonly used by horse owners or trainers.

Lunging may be used in place of ridden exercise or to train riders or as a warm-up for the horse prior to it being mounted and ridden. Lunging may also be used in situations where a horse requires to be exercised but where fitting a rider and saddle is not desirable, for example, in the case of a sore back. However, prolonged lunging is not advisable and in addition, as with circular walkers, changing the rein frequently is common practice. Continual turning may be deleterious to the musculoskeletal system (muscles, bones, tendons, ligaments and joints). For example, it is widely recognised that signs of lameness are exacerbated in horses exercised on a circle. This is commonly used by veterinary surgeons in lameness investigations. It is also suspected that sharp turns may contribute to injury of distal limb structures (i.e. those structures furthest from the body such as the foot).

This implies that turning exercise changes the weight distribution through the limbs. The surface on which a horse is lunged may also determine whether lameness is apparent or not; a horse may not exhibit lameness when lunged on a soft surface but may do so when lunged on the same size circle on a firmer or uneven surface. Most research into how horses move has been concentrated in horses walking and trotting in straight lines, or on treadmills, and there are only a limited number of studies relating to horses turning on a circle. Only one kinematic (movement) study has evaluated the effects of turning a corner on the distal joint motions. Horses turning in a sharp (1.5m diameter) left circle showed a shorter stride length, but stance duration (the amount of time the foot is on the ground) was longer. This work also showed that the lower leg and foot rotate as the weight of the horse moves over the limb. Research from Australia showed that the outside edge of the cannon bone is not loaded significantly during exercise in a straight line on a flat surface. The same group of researchers also showed in a separate study that surface strains on the cannon bone vary between inside and outside forelimbs during turning.

On the inner surface of the cannon bone, compression of the bone is greatest in the outside limb, and stretching of the bone is greatest on the inside limb. On the outer surface of the cannon bone, both compressive and tensile peaks are largest on the inside limb, which also showed the largest recorded strains in compression. On the dorsal (front) surface of the bone (where bucked shins occur in young horses), compressive strains were largest on the outside limb, and were greater on larger circles. They concluded that turning exercise is required to maintain normal bone, in that low-speed exercise in a straight line only loads the outer edge of the cannon bone. In 2006 workers from the USA studied the effect of trotting in a circle on the centre of mass of the horse. The centre of mass is a point within or on the body at which the mass of the body is considered to act.

The centre of mass may vary according to gait, speed and direction of travel. The location of the centre of mass affects the distribution and size of the loads on the limbs. These researchers showed that in horses trotting on the lunge on a 6m diameter circle at a speed of ~2 metres/second, all horses leaned inwards at an angle of ~15°. The speeds attained by these horses at trot on a circle are lower than those typically seen for horses on a straight line. As the speed was slower, the implication is that stance proportion was increased (i.e. the weight bearing phase of the stride was longer on a circle than would be expected in a straight line). Furthermore, the researchers pointed out that “horses may behave differently when turning clockwise versus counter-clockwise due to asymmetries in strength, suppleness and neural programming…”. Thus, whilst it is often assumed that an equal amount of exercise on each rein on a circular horsewalker should be applied, this may not be the case for many horses and may actually be counter-productive. The potential negative impact of circular exercise has also been highlighted with respect to the muscular system: “Especially in the initial stages of a return to work avoid lunging, horse walkers, or work in tight circles, as well as hill work”; a quote from veterinary surgeon and muscle specialist Dr Pat Harris from the Equine Studies Group at the WALTHAM Centre for Pet Nutrition, UK. Exercising on a circle also requires more effort than exercising in a straight line (Harris, Marlin, Davidson, Rodgerson, Gregory and Harrison (2007) Equine and Comparative Exercise Physiology, in press).

For example, being lunged on a 10 metre diameter circle was around 25% more work than being ridden on a large oval track in an indoor school. In addition, being lunged on a 5m circle was around 12% more work than being lunged on a 14 metre diameter circle. Even accounting for the weight of the rider, lunging is harder work than ridden exercise, which is most likely due to the continual effort required by the horse to balance itself on a continual turn. Oval walkers are a new concept. The premise of using oval walkers is that continual exercise on a small circle is unnatural for horses and could even lead to injury and that a walker incorporating both straight line and turning exercise would represent a more appropriate form of controlled exercise.

As so little information exists on turning in horses, a study was designed by us [Dr David Marlin (Physiologist) and Paul Farrington (Veterinary surgeon)] to investigate turning stress in horses in more detail. The work was undertaken in collaboration with Dr Bob Colborne (a specialist in Biomechanics) at Bristol University, UK. A

SUMMARY OF THE RECENT RESEARCH ON TURNING

The purpose of this study was to record the forces acting on the lower limb as horses walked in a straight line, on a 14 metre diameter circle, and on a 10 metre diameter circle to provide insight into the horizontal forces transmitted up the limb during locomotion in a straight line and whilst turning. Three fit, sound Thoroughbred horses, ages 3, 5 and 12 years of age were used in the study. Horses were walked across a force-plate (a metal plate placed on the ground that measures the force with which the horses’ foot is placed on the ground) both in a straight line and on a 10 and 14 metre diameter turn. For the turns the horse was always walking on a left-turn. The results showed that the coffin joint had the greatest degree of abduction (movement of the limb away from the body), adduction (movement of the limb towards the body) and axial rotation (twisting movement) and that these movements were greatest at the time of impact and break-over. The first point of contact with the ground has a significant influence on the line of stress through the foot and up the limb, as does the position of the body at the same moment.

On a turn the horse abducts the inside forelimb away from the body towards the line of the circle with rotation of the foot in the direction of the turn. The stride length is dictated by the tightness of the turn, as is the stance time (when the foot is on the ground). As the horse then moves forward the horse’s body moves towards the inside limb increasing the loading on the limb. The results showed that on average the forelimbs tended to behave asymmetrically (i.e. the two front legs did not behave the same) on a circle so that the forces and movements differ to produce different torque effects (twisting forces). The hind limbs tended to behave more symmetrically except when the size of the circle was reduced from 14 to 10 metres in diameter.

IMPORTANCE OF HORSEWALKER SURFACES

The walking surface will likely have an effect on the stresses experienced by a limb. If the surface allows reasonably free twisting of the hoof when weight bearing, the stresses between the hoof and ground will be small. However, any ground surface that holds the hoof and impedes this horizontal rotation will probably impart higher loads to the joints of the lower limb. Large turning forces should be avoided when the limb is vertically loaded (i.e. when the weight of the horse’s body is over the limb and the limb is on the ground). It is also important that the walking surface is level to avoid tilting of the hoof during weight-bearing. A walking track that is worn in the middle and that causes rotation of the joints in the foot is likely to cause larger and uneven forces to the lower limb joints and associated tendons and ligaments.

IMPLICATIONS FOR OVAL VERSUS ROUND HORSEWALKERS

Our recent research and a review of other scientific studies show that turning is not equivalent to exercise in a straight line. Turning exercise is harder than exercise in a straight line and loads the bones in a different way. Furthermore, on small turns the inner and outer limbs may not behave in the same way as on larger circles. This may have implications for horses with pre-existing musculoskeletal injuries. The potential advantages of an oval walker is that it combines straight line and turning exercise that more closely mimics the exercise that a horse will do when being ridden or when free at pasture. The results of our small study have shown that the hind limb patterns were quite different on the tighter radius turns, indicating a different strategy for turning, and supporting the notion that both straight line and turning exercise should be recommended for overall loading patterns that are healthy for maintaining bone that can withstand loading forces in a variety of directions. The results also make clear that small diameter round walkers (~10 metre diameter or less) are less desirable than round walkers of 14 metre diameter or greater. Small diameter round walkers increase the loading and asymmetry and increase the work compared with larger diameter walkers. In conclusion, there appear to be significant advantages to using a walker of an oval design as opposed to a round design, as exercise on an oval loads the limbs with a combination of straight and turning movements, as would be experienced during riding or in free movement.