Postnatal muscle fibre composition of the gluteus medius muscle of Dutch Warmblood foals; maturation and the influence of exercise

The fibre type composition of the deep gluteus muscle was studied in biopsies of Dutch Warmblood foals from birth until age 48 weeks. Half the foals were given box-rest, the other half received exercise consisting of an increasing number of gallop sprints. The muscle fibre types were determined using monoclonal antibodies discriminating against the following myosin heavy chain (MHC) isoforms: types I, IIa, IId, Cardiac-alpha and Developmental. During the first 48 weeks there was a consistent increase of fibres expressing types IIa MHC, replacing fibres expressing IId MHC. This change was reflected in the presence of a quite large population of fibres co-expressing MHC IIa and IId. The difference between the exercised (training) and nonexercised (box-rest) groups was small but suggested that the increase of type IIa fibres was larger in the training group. It appeared that after birth a significant number of fibres coexpress either Developmental and type IIa-MHC or Cardiac-alpha and type I-MHC. The Developmental isoform disappears during the first 10 weeks after birth and almost all the alpha isoform expression during the first 22 weeks. It is concluded that a fast turnover of fibre types takes place in the deep gluteus medius in the first months postpartum. Potentially, exercise could have an effect on the rate of change of these fibre types.