Conclusions regarding the influence of exercise on the development of the equine musculoskeletal system with special reference to osteochondrosis.

This paper summarises and interrelates the findings of a large-scale multidisciplinary investigation to assess the influence of exercise on the development of the equine musculoskeletal system in general and of osteochondrosis in particular, up to age 5 months. Forty-three foals, genetically predisposed to develop OC, were divided into 3 exercise groups: box-rest, box-rest with training and free pasture exercise. At 5 months, all foals were weaned and 8 foals per group were subjected to euthanasia for postmortem examination. The remaining 19 foals were placed together and subjected to euthanasia at age 11 months. Foals were clinically and radiographically monitored during life, muscle and tendon biopsies were taken and gait analysed kinematically. After euthanasia, all major musculoskeletal tissue components (bone, articular cartilage, tendon and muscle) were analysed extensively using a wide variety of techniques. Radiographic monitoring of the stifle and hock joints and postmortem analysis of all diarthrodial joints led to the conclusion that osteochondrosis is a dynamic and very common process in which lesions cannot only develop, but may regress spontaneously during the 'windows of susceptibility' of the various joints, making the clinically diagnosed forms of osteochondrosis into the tip of an iceberg. Closure of the 'window of susceptibility' may be determined by the metabolic status of the chondrocyte which was shown to be inferior in older lesions. Exercise had no influence on the number of lesions, but was related to the distribution of lesions within the joint. There was some evidence that growth rate may be one of the most important intrinsic factors that determine the occurrence of OC. Lack of exercise (box-rest) generally delayed the development of the tissues that make up the equine musculoskeletal system. This was evident in bone mineral density (BMD) at various sites, chemical composition of tendon and of articular cartilage, and in the development of gait. In most cases, this delay was compensated for when box confinement was lifted after 5 months. However, there were indications that this was not true for some collagen characteristics of articular cartilage where the withholding of exercise at early age may therefore have a lifelong effect. The training protocol used (rather high-intensity exercise superimposed on a basic box-rest regimen) appeared to have long lasting negative effects, affecting chondrocyte viability long after the training protocol had ended. A same tendency was seen in bone (decrease in BMD) and tendons (decreases in proteoglycan and hyaluronic acid content). It is concluded that, during the first months postpartum, the equine musculoskeletal system passes through a very dynamic period of growth-related development and intense alteration. In this period, the system is vulnerable to adverse influences that may result in developmental orthopaedic disease. However, regenerative capacity is still high, also in those tissues that are notorious for their lack of repair capacity in the mature individual, such as articular cartilage and tendon. Exercise seems to be an important factor in the determination of the final make-up (and hence biomechanical strength) of these tissues and, therefore, is a potentially powerful tool for the enhancement of injury resistance.