Muscle biopsy index for discriminating between endurance horses with different performance records.

Biopsies were taken from three different depths of the gluteus medius muscle of 36 endurance horses aged between four and 17 years. Twenty of the horses were considered excellent performers according to their three fastest records in endurance events over the previous two or three years. The other 16 horses were moderate performers, with a mean racing speed < 3.5 ms-1 (in 120 to 180 km endurance rides), < 4.0 ms-1 (in 80 to 120 km endurance rides) or < 3.75 ms-1 (in 40 to 60 km endurance rides). The biopsy specimens were analysed for fibre type distribution (type I, IIA and IIB), fibre area and relative fibre area. The activities of the enzymes citrate synthase, 3-hydroxy-acyl-CoA-dehydrogenase and lactate dehydrogenase were also determined. The data were subjected to discriminant analysis and principal component analysis. It was possible to discriminate correctly between the horses according to their performance records by means of the discriminant analyses when the histochemical and morphometric data from all three muscle biopsies were used, but not when data from only a single muscle biopsy were used. Principal component analysis confirmed that the most efficient muscle parameters describing the athletic potential of endurance horses were the percentages and relative areas of the fibre types. Horses with a better endurance performance had higher percentages and relative areas of types I and IIA fibres and lower percentages and relative areas of type IIB fibres than moderate performers.