Musculoskeletal diseases in racehorses are common and can lead to catastrophic injuries requiring euthanasia of the horse.
Consequently, results of intensive studies concerning the pathogenesis of these injuries have revealed that many of these
problems are due to chronic fatigue damage in the tissues from repetitive stress of racing and training.1 A horse's predisposition to tissue damage may be due to high mechanical loads imposed on a particular tissue, relatively
poor material properties of a specific tissue or both. Abnormal mechanical loading on a particular tissue can be due to a
number of factors, including neurologic dysfunction and remote pain leading to overload of an opposing limb. In addition,
we know clinically that conformation can greatly influence the forces on a specific joint, tendon or ligament, often leading
to clinically detectable diseases. Recent evidence is also starting to show that certain joints may be predisposed to high
mechanical loads due to subtle geometric differences within the joint tissues.2 The material properties of a tissue, whether it is bone, articular cartilage, ligament, tendon or muscle, are dictated by
its collagenous and noncollagenous protein characteristics. For bone, the material properties are also dictated by the quantity
and quality of mineral.1 Therefore, aberrant characteristics in any of these matrix components can lead to reduction in strength of the tissues.
Matrix components within tissues can be influenced by things such as genetics, nutrition and physical loading history. As
an example, in people, there is considerable evidence that genetics is a strong factor in dictating the presence of osteoporosis.3,4 Nutrition is also a factor, which is suggested in horses as well.3 Recently, exercise has been shown to strongly influence tissue material properties. In humans, it has been shown that regardless
of genetic and nutritional influences, people with a long history of moderate levels of exercise have a protective effect
for osteoporosis.3,5,6 To further those investigations, it appears that when exercise was imposed during the greatest growth period, bone strength
was maximized later in life, providing a protective factor from osteoporosis.7-11 In addition to these clinical results, experimental studies in several different species show, in general, that there is
a threshold of exercise beyond which tissues are strengthened, but as importantly, a threshold above that which can be damaging.1,12 The problem with these findings is that usually only one tissue, such as bone is studied, and there are no conclusions as
to the effects of a particular exercise level on all tissues.
Exercise studies in horses have shown that beyond a certain level, tissue damage can occur to certain tissues.1,12 In addition, limited exercise or nonweight-bearing events, such as casting, can also cause tissue damage.13-15 Therefore, it appears that the results for horses are similar to other species. However, in order to use physical loading
to positively affect tissue material properties guidelines are needed to show whether loading during growth will be protective
of all tissues.
The goal of the current study was to determine the effects of exercise at an early age on musculoskeletal tissues in the horse.
Our hypothesis was that early imposed exercise would strengthen all tissues, thus preventing tissue damage later in life.
Materials and Methods
Thirty-three Thoroughbred foals were divided into two groups that were subjected to different exercise regimens. In phase
1 from birth until 18 months of age, the conditioned group was raised on pasture as well as subjected to a conditioning program
(1020 meters) of increasing exercise level from approximately 10 days of age. The control group exercised spontaneously at
pasture. At 18 months of age, 6 random foals from each group were euthanized for post mortem analysis. The remaining 21 foals
entered Phase 2 during which they were trained for 2-year-old racing.
Horses were observed daily for general health, and clinical examination was carried out by a veterinarian at approximately
four days of age and monthly thereafter in Phase 1. This examination consisted of a general physical and lameness examination.
At the end of Phase 1, horses underwent clinical examinations, together with full radiographic, scintigraphic and ultrasononographic
examinations at the Massey University Equine Hospital. The behavior and plasma cortisol levels of the foals between average
ages of three and five months was quantified.17,18 The following detailed evaluations have been completed on the tissues acquired at 18 months.
Effects of early exercise on articular cartilage viability: In order to determine the effects of early exercise on articular
cartilage and subchondral bone in specific sites of the metacarpophalangeal and metatarsophalangeal joints of young Thoroughbred
horses, articular cartilage samples from four sites of the distal third metacarpal/metatarsal bones were stained with calcein-AM
and propidium iodide, and confocal laser scanning microscopy was used to count live and dead cells. Proteoglycan scoring and
modified Mankin scoring were also determined. The subchondral epiphyseal bone mineral density at the sites was measured using
Effects of early exercise on mechanical properties of articular cartilage: The objective of the study was to determine (1)
the site-associated response of articular cartilage of the equine distal metacarpal condyle to training at a young age as
assessed by changes in indentation stiffness and alterations in cartilage structure and composition, and (2) relationships
between indentation stiffness and indices of cartilage structure and composition. Four osteochondral samples were harvested
per metacarpal condyle from dorsal-medial, dorsal-lateral, palmar-medial, and palmar-lateral aspects. Cartilage was analyzed
for India ink staining (quantified as reflectance score), short-term indentation stiffness (sphere-ended, 0.4 mm diameter),
thickness, and biochemical composition.20
Effects of early exercise on subchondral mineralization pattern in the third metacarpal condyles: Metacarpophalangeal joints
were scanned using a conventional computed tomographic scanner and the files were exported to a custom-designed program for
3-dimensional analysis of the joint. In the computer program, the third metacarpal condyles were disarticulated and analyzed.
The bones were further cut into slices at 20, 30 and 40 degrees palmar from the mid-frontal plane. This allowed analysis of
bone density and density pattern in the area most susceptible to injury.
Effects of early exercise on osteochondral tissues: Articular cartilage was harvested for analysis of glycosaminoglycan content
and synthesis. In addition, synovial membrane, articular cartilage and osteochondral samples were collected for histologic
analyses using published techniques. All data were analyzed to determine the effects of exercise and site within the joint
on dependent variables.