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To test the hypothesis that exercise at very young age may influence the eventual molecular composition (and hence the biomechanical properties) of tendon tissue in the horse, 43 Dutch Warmblood foals were allotted to 3 differently exercised groups (box-rest, box-rest with training and pasture exercise). Twenty-four superficial digital flexor tendons (SDFTs) were collected at age 5 months (8 from each exercise group) and the others were obtained at 11 months after an additional period of light exercise that was equal for all remaining foals and was intended to see if any induced changes would be reversible or not. Significant changes in DNA content (cellularity), hyaluronic acid (HA) and polysulphated glycosaminoglycans (PSGAGs) were found after the 5 month period of different exercise regimens. There was a tendency towards an exercise-related effect on hydroxylysine content and number of hydroxylysylpyridinoline (HP) crosslinks. Levels of Cartilage Oligomeric Matrix Protein (COMP), measured by homologous inhibition ELISA, showed significant differences at 5 months and were highest in foals kept at pasture and lowest in foals maintained in a box but given enforced exercise. At 11 months, the biochemical parameters of the tendons from the foals of the former box-rest and pasture groups became similar, indicating the capacity of the immature tendon to recover from a retarded development. However, the ratio of PSGAGs per unit of DNA of the former training group was significantly lower than those from the other groups, suggesting that the training regimen in this study had a lasting negative effect on the tenocytes resulting in a decrease of the production of PSGAGs. Therefore, inappropriate or excessive exercise may damage developing tendon, with limited recovery after normalising the exercise level. These possibly deleterious effects of a training regimen on tendon development may be important for the management of young would-be equine athletes.

In human athletes, conditioning, training and competition are commenced before skeletal maturity. Yet in equine athletics, racing of young (age 2 years) horses remains contentious. Tendon injury persists as major causes of wastage in equine athletes. Minimising injury and associated welfare issues could involve a radical approach to the timing and implementation of conditioning and training. Tendons were examined from Thoroughbreds, Dutch Warmblood foals, working horses and also a group of wild horses to evaluate effects of age, function and exercise. Gross mechanical properties did not differ significantly with age or exercise, but showed a high variance within each group. Mechanical properties of tendon tissue showed significant differences as a function of age and location. The collagen fibril crimp angle and length showed a regional reduction in the central core with exercise and age, with a synergistic effect. Regional differences in collagen fibril diameter were seen in long-term exercised older horses, but not in short-term exercised, or younger, horses. The higher proportion of small fibrils in the central region of the long-term exercised horses did not correlate with new collagen formation and therefore appear to result from disassembly of the larger diameter fibrils. Fibril diameter distributions were influenced by exercise regimens in the growing foal. Changes in molecular composition occurred in longer-term exercise and older horses, in the centre of the tendon, with higher levels of type III collagen and changes in glycosaminoglycan (GAG) content. Cartilage Oligomeric Matrix Protein (COMP) levels also appear to be modulated by age, function and superimposition of exercise. These changes were all exacerbated with age and exercise, suggesting appropriate exercise in young horses may lead to a lower incidence of injury than in older horses. An hypothesis is advanced that immature tendon can respond to exercise while mature tendon has limited, if any, ability to do so. These findings support potentially controversial earlier conditioning and racing of younger, rather than older, equine athletes.