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Skeletal System Development

Go Slow and Get to the Finish Quicker
By Bruce A. Weary DC

There are many issues to consider when preparing a horse for endurance ride participation, whether it is to finish at a leisurely pace in an LD ride, or to compete head to head with the front runners in a 100-miler. Strategies for the horse’s diet and hydration, as well as cardiovascular and muscular systems, must certainly be considered, but this article will address another very important system that must be conditioned and managed carefully—the skeletal system.

The bones and joints serve several functions, some more apparent than others. First, the skeleton provides a framework for structural integrity and the attachment of the various muscles of locomotion. Joints act as a series of levers that transfer muscular power into movement.  Cartilage surfaces in joints must function nearly friction free so that all movement can occur in as fluid and coordinated a manner as possible.

Both bones and joints must withstand the loading, strain and concussive effects of athletic performance so that the horse’s structural strength and soundness are not compromised. In addition, bones act as a dynamic storehouse of important minerals, such as calcium, phosphorus and magnesium, that move in and out of the bone matrix depending on the horse’s health and physical demands placed on him.

Lastly, bone marrow produces both white and red blood cells, and also participates in the lymphatic system. Exercise can help keep bone marrow from becoming too fatty, and possibly enhance the marrow’s blood cell producing capabilities.

Since bone and joint injuries can account for up to 70% of downtime in the athletic horse, some understanding of the effects of exercise on the skeletal system is helpful in developing safe conditioning strategies for the endurance horse.

Bone strength development

The proper development of bone strength in the horse begins in utero, during the last three months of gestation.  During this time, the cartilaginous skeleton mineralizes into bone as the unborn foal doubles in size. During the first year of life, the foal’s skeleton matures from 15% mineralized at birth to about 75% mineralized at one year.

This process is stimulated by the foal’s activity and increasing body weight.

The key to creating healthy, sustained skeletal integrity depends largely on understanding the relationship between exercise and rest as they affect bone and joint development within safe margins. Cyclic exercise and rest periods cause the micro-structure of bones and cartilage to remodel, especially in the first year of life.

It has also been largely established that the amount and type of exercise has a direct influence on joint cartilage and bone density in both young and mature horses as they develop. One study by Raub, et. al., showed that weanlings that were exercised by trotting up to 2.5 miles a day reflected a 25% increase in bone density as compared to those that were stalled at night and simply turned out during the day. Conversely, such things as high energy feeds, relative inactivity, rapid weight gain and inadequate intake of calcium and essential minerals can combine to adversely affect proper bone ineralization and maturation of the young horse.

Training and bone health

Another potential challenge lies in managing a horse’s skeletal development and joint soundness within the stress of everyday training. The bones must continually adapt and remodel themselves to maintain strength in response to exercise. The stress of continuous and overdone highloading exercise can result in bones actually becoming less dense if there is less than adequate time for proper remodeling between workouts.

During such overtraining, the high stress loading on the bones can also cause micro-fractures or other adverse microscopic changes within the cortical bone. One such example is pedal osteitis, which can be induced from excessive exercise/concussion and can lead to demineralization of the coffin bone.

Once horses reach maturity at 4 years of age, the bone remodeling process is less active. Some investigations have shown that, in both developing and mature horses, the stimulation of increased bone remodeling/density is relatively flat until the horse is exercised at the gallop.

Whether it is a Thoroughbred surging around a track or an endurance horse doing speed work, the increased loading has been shown to stimulate the formation of increased cortical bone and thus, structural strength, when done within reasonable limits.

Studies of shin soreness in Thoroughbred racehorses have shown that after a hard gallop over 200 to 300 meters (1 to 1½ furlongs), the cannon bone in a young horse becomes reactive and attempts to strengthen itself by depositing calcium within its front cortical wall for the next 10 to 12 days (Nunamaker et. al., 1990; Davies, 2001).

When too many “breeze-up” or “all-out” gallops are given successively at two- to three-day intervals in an accelerated “get fit” or “too fast—too early” training program, the bone itself cannot respond rapidly enough and can become inflamed, as occurs in shin soreness (Davies, 2001).

Studies have shown that in the young horse, an injury to a bone can bring about this response within two to five days, but in an older horse this may take up to 10 to 12 days, depending on the type and extent of the exercise overloading (Firth, 2003b). Joint cartilage is even slower to adapt to the effects of exercise, and must be considered in conditioning regimens as well.

Heed the conditioning timeline

A general rule of thumb that has stood the test of time in endurance riding is that it takes two years or more to fully develop all body systems to a level of durability that will withstand the rigors of long distance work. Certainly, bone and cartilage are the slowest to develop, cardiac and skeletal muscle the fastest, with tendons and ligaments optimally strengthened somewhere in between.

While the softer tissues can often exhibit more detectable signs of trouble from overconditioning, such as heat, swelling, inflammation, lameness, etc., the “harder” tissues can often be silent for long periods of time while tissue compromise and ultimate injury may be brewing.

There have been many cases in our sport where riders have been misled by a horse’s apparent muscular and cardiovascular fitness and willingness to work. If such horses are allowed to exert themselves prematurely, without time for proper bone and joint development, the unfortunate result can be an injured horse and a rider scratching his head in confusion. For this reason, moderation is the key in developing the endurance horse for a lifetime of skeletal soundness and, hopefully, thousands of miles of trails to be enjoyed by horse and rider alike.

Final recommendations

Some final recommendations from experts in equine physiology regarding the healthy skeletal development of the athletic horse include:

1. Provide adequate physical activity throughout the horse’s life, especially during its first year.

2. Allow for proper rest between workouts, based on the horse’s age and development, especially after high intensity/speed work. If in doubt, more rest is less risky.

3. Provide competent shoeing/trimming for proper hoof health, angles and biomechanics.

4. Avoid speed work in tight circles, such as arenas. It may cause abnormal angular strain on bones and joints.

5. Provide adequate intake of macro and trace minerals, especially during the horse’s growth phase of life.

6. Avoid heavy carried weight in the early conditioning phase of any horse, especially during speed work.

7. Recognize that the horse that is resting for more than a few weeks will undergo slow skeletal demineralization, and that return to work should be somewhat gradual to allow time for all body systems to reestablish proper integrity and function before returning to competition.

8. Check the horse for soundness after each workout by palpating the legs for heat, swelling or tenderness, as well as lunging in both directions to detect any abnormality in gait.

9. “Go slow and get there quicker.”

10. Have fun out there!

References

Raub, R.H., S.G. Jackson and J.P. Baker. 1989. The effect of exercise on bone growth and development in weanling horses. J. Anim. Sci. 67:2508.

Nunamaker, D.M., C.M. Butterweck and M.T. Provost. 1990. Fatigue fractures in thoroughbred racehorses. Relationship with age, peak bone strain and training. J. Orthopedic Res. 8:604-605.

Davies, H.M.S. 2001. The relationship between surface strain and measurements of bone quality, quantity and shape. Proc. 4th Int. Workshop. Anim. Locomotion (1WAL2000) Vienna 22-28 May, 2001. Equine Vet. J. 33:16–20.

Firth, E.C. 2003b. Methods of assessing bone growth and development in young horses. In: Proc. 2003 Equine Nutrition Conference. Growth and Development of the Equine Skeleton. Kentucky Equine Research, Sydney, Australia pp. 175-190.

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