Muscle fibre variation in the gluteus medius of the horse.

The gluteus medius of two killed Thoroughbred horses were sampled along the muscle and across the muscle at four different depths. The distribution of fibre types in these two horses was assessed by staining cross sections of the muscle sample for ATPase. A non-uniform distribution of fibre types was found within the gluteus medius in both horses and there was a significant increase in percentage of slow twitch (ST) fibres from the surface to the deeper regions of the muscle. The rate of increase, however, depended on the individual site along the muscle. Averages ranges from a low of 2.4 per cent ST fibres to a high of 64.5 per cent. This study shows that small biopsy samples of fibres are not necessarily representative of the whole muscle and that there is a need in subsequent studies to define precisely biopsy location.