FIBER COMPOSITION AND CAPILLARY SUPPLY OF THE GLUTEUS-MEDIUS MUSCLE IN RESTING CHILEAN CRIOLLO HORSES

A study to analyze fibre composition and capillary supply of the Glute us medius muscle of the Chilean criollo horse under resting conditions was carried out. Six adult (5-10 year old), clinically healthy Chilea n criollo horses were sampled through a muscle biopsy technique during the period previous to the rodeo competition. Muscular biopsies were obtained from the left Gluteus medius at 6 cm of depth below the fasci a. Muscle samples were serially sectioned and stained for myofibrilar ATP-ase and amylase-PAS, in order to identify types I, IIA and IIB fib res and to evaluate the fibre type distribution, the cross sectional a rea and capillary density. The results showed that the Chilean criollo horse has a greater proportion of fast twitch type II fibres than typ e I, whereas type IIB fibres (42.5%) showed a slightly greater proport ion than type IIA (39.0%), which suggests that this breed of horses is best adapted to muscular work of speed and resistance. The greatest f requency of type IIB fibres and the low capillary density (253 +/- 26 capillaries/mm(2)) seem to demonstrate that these characteristics are associated to a loss of fitness due to a prolongued resting period to which the horses were subjected.