RACING PERFORMANCE AND LONGITUDINAL CHANGES IN MUSCLE CHARACTERISTICSIN STANDARD-BRED TROTTERS

Biopsy specimens from the gluteal muscle were taken from 8 Standardbre d trotters at 7-8 months of age and on 3 or 4 occasions during the fol lowing 4 years. All horses were trained from the age of 18 months. Sev en of them started to race at the age of 3 years and one at 4 years. M uscle characteristics changed significantly with age and training. Typ e IIA fibers increased from a mean percentage of 29 to 53 and type IIB fibers decreased correspondingly from 55% to 30% (mean). Citrate synt hase activity increased from 31 to 55% mmol/kg/min and lactate dehydro genase activity decreased from 1,063 to 1,004 mmol/kg/min. Oxidative c apacity increased in type IIB fibers, since approximately 50% were low -oxidative in the foals, though most fibers were oxidative in the musc le of the adult racehorses. Studbook performance records such as earni ngs, track record, proportion of races placed 1-3, earnings per start and number of starts, were obtained during the period when the horses were 3-5 years old. An individual selection index and Best Linear Unbi ased Prediction (BLUP) were calculated as measures of the horses' meri ts for racing performance. BLUP values differed between these horses f rom 92 to 112 and individual indexes from 98 to 111. These racehorses had a high type IIA/IIB fiber ratio and a high oxidative capacity in m uscle. No correlations were found between muscle characteristics and r acing performance. The 2 horses with the lowest individual selection i ndex value had more low-oxidative type IIB fibers than the other horse s with higher individual selection index values.