Abstract
A total of 67 healthy horses (34 Andalusian and 33 Arabian) ranging in age between 10 and 520 days were separated into three age groups (foals, weanlings and yearlings). Muscle biopsies were taken from the right gluteus medius muscle of each horse. Samples were stained for m-ATPase after acid preincubation (pH 4.4) and NADH-TR. Fibers were classified in types I, IIA and IIB (m-ATPase), and IIB non oxidative and IIB oxidative (NADH-TR). The proportion of type I fibers did not vary greatly between age groups, whereas that of type IIA increases and that of type IIB decreased significantly. Arabian yearlings showed a higher ratio of IIA/IIB than Andalusians, and it was near unity. The percentage of type IIB oxidative fibers decreased while that of IIA increased, which suggested a transformation between IIB oxidative and IIA fibers. Type IIB non oxidative fibers did not vary between age groups. These results suggest that the main effect of growth on muscle fiber composition occurs in the fast-twitch fibers (IIA and IIB), and over contractile properties more than in metabolic ones.
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