Equine locomotory muscles : postnatal development and the influence of exercise

The Dutch warmblood horse is widely used in different types of sport. The individual capacity to perform depends on factors as character and the quality of the cardiopulmonary and musculoskeletal system. These factors are partly genetically determined; in the postnatal phase of growth and maturation they develop via a natural pattern, influenced by environmental factors. This way, the capacity to perform can be affected by training and exercise. Many studies about the influence on functional muscle properties like power and resistance against fatigue have been executed with humans and (young) adult horses. These studies have proven the positive effects of training and exercise many times. The question remains if further benefit can be achieved when young foals are submitted to training already in their first year of life. It can be expected that in the early postnatal period, when muscles adapt to the new environment, the sensitivity of muscles for training is high. If improvement occurs, the capacity for later athletic performance might be affected as better possibilities are created for further adaptation during the conventional training period later in life. In this thesis, a group of 38 Dutch warmblood horses from the EXOC (EXamination OsteoChondrosis) project (1996) is described. It was an unique opportunity to examine such a large group of horses. The group was divided in 3 groups in the first week of life. The first group was the Boxrest group. The animals from this group were permanently kept in the stable with their mares. The second group was the Boxtraining group. The animals from this group were also kept in the stable but underwent forced movement (gallop sprints) every day. The third group was the Pasture group. These animals moved freely in the pasture. At 22 weeks of age, half the foals were euthanised for other purposes of the EXOC project. The other half was kept in a paddock, as one group, with the possibility of some free movement. At several ages between 0 and 48 weeks, biopsies were taken from two important locomotory muscles (gluteus medius and semitendinosus). In these biopsies indicators for muscle power and contraction speed and resistance against fatigue (expressed in blood perfusion parameters and oxidative enzyme activities) were analysed. Measurements were done with the aid of modern techniques such as the usage of monoclonal antibodies and computer imaging systems. The studies in this thesis are the first to describe the occurrence of 2 uncommon muscle contractile proteins in locomotory muscles of newborn horses. These proteins are normally present in heart muscle and in the recovering skeletal muscles after traumata. Furthermore, it appeared that functional muscle properties underwent major changes in the first year of life. Dutch warmblood horses are born as sprinters and develop towards more endurance-like athletes. The resistance against fatigue is high at birth, decreases until 22 weeks of age and then becomes stable. It appeared when foals are restricted in movement (Boxrest group), the blood perfusion and the capacity of muscles to produce energy efficiently remains underdeveloped. Movement (forced or freely) is necessary for normal growth and maturation of the locomotory muscles. Further benefit from forced movement (Boxtraining group) above free movement in the pasture (Pasture group) was not demonstrated. For this reason, it is recommended to leave young foals in the pasture as much as possible.