Influence of age on fibre type characteristics in the middle gluteal muscle of Andalusian foals.

34 Andalusian foals of both sexes were divided into three age-groups (A = mean age 1 month, B = 7 months, C = 14 months). Samples of the right m. gluteus medius were stained for m-ATPase and NADH-TR in order to determine fibre type composition and size as well as the relative area occupied by each type. Results revealed no statistically significant variation in the proportion of type I fibre among the three age-groups. Significant differences were recorded, however, for type II fibres; an increase in the proportion of IIA fibres was accompanied by a decrease in IIB ones, the difference being most marked between groups A and B. IIB high-oxidative fibres also decreased, while IIB low-oxidative ones showed no significant variation. All fibre types increased significantly in size; types I and IIA recorded a threefold increase, whereas type IIB showed least growth. The relative area occupied by each type increased significantly between groups A and B, but only IIA fibres recorded a significant increase in relative area between groups B and C.