RESUMO
UNLABELLED: REASONS OF PERFORMING STUDY: To obtain broader insight into the muscle of foals with different genetic background, muscle fibre composition, its post natal development and citrate synthase (CS) activity of the gluteus medius were investigated. HYPOTHESIS: Because muscle properties are influenced genetically and related directly to performace ability, muscle characteristics should be affected by selection and correspond with the requirement of ths sports selected for. METHODS: The foals were divided into Group A, (n = 16), considered an average of the population, and Group B (n = 36), selected for showjumping. Biopsies were taken deeply from the gluteus medius muscle. Fibre type identification was performed immunohistochemically at ages 6 and 12 months. Citrate synthase activity was measured at age 6 months. RESULTS: age 6 months statistically significant differences were found between Groups A and B in the proportion of type I, Had and IId fibres. Oxidative fibres (I and IIad) were more abundant in Group B, while fast-glycolytic fibres (IId) were more abundant in Group A. Except for IIa, the fibre type composition at age 12 months had become equal in the 2 groups. The difference in the sum of oxidative fibres (I + IIad + IIad) between Groups A and B at age 6 months was similar to the difference in CS activity at that age, although the latter was not significant. CONCLUSIONS: At age 6 months the 2 groups had different proportions of fibre types I and IId, but these differences had disappeared at age 12 months. POTENTIAL RELEVANCE: The muscular response to training at an early age can be interpreted with knowledge of muscle characteristics and its post natal development in foals selected for showjumping.