Summary Muscle biopsies were taken from the middle gluteal muscle of 68 stallions (52 Andalusian [AN] and 16 Arab [AR]) ranging from six to 12 years of age. Seventeen AN horses and eight AR horses were untrained, while the remainder underwent active endurance training for 6 months. Fifteen AN horses were moderately endurance‐trained while the other 20 AN horses and eight AR horses were strongly endurance‐trained. Percentages of type I and type II fibres were similar in all groups (P>0.05). The group of horses with the hardest training had a higher percentage of type HA fibres (AN P<0. 01; AR P<0.02) and a lower percentage of type IIB fibres (P<0.05 for AN and AR) than the untrained horses. All trained horses had a higher percentage of type IIB oxidative fibres and a lower percentage of type IIB non‐oxidative fibres than the untrained horses (P<0.01 for both). These results suggest that the proportions of type I and type II fibres are highly stable within a given breed, but the stimulus of training facilitates changes both in contractile properties and, particularly, in the oxidative capacity of type II subgroups.

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