Plasma lactate response to submaximal and maximal exercise tests with training, and its relationship to performance and muscle characteristics in standardbred trotters.

Seven young Standardbred trotters, born, raised and trained at the same camp, performed submaximal and maximal work tests on a track. The submaximal test consisted of four 1,000 m runs at the trot, with increasing speed at each run. Each horse performed this test when 24, 26, 29 and 40 months old. The maximal test consisted of trotting 1,600 m and was performed at 24 and 29 months of age. Blood samples for plasma lactate analyses were obtained after each run, during the submaximal test and after the maximal test. Muscle biopsy samples were obtained at 24 and 29 months of age. The results showed marked individual differences in lactate response to submaximal effort. Early training only appeared to influence plasma lactate response in some horses. After 40 months of age the lactate response to the final submaximal test was significantly lower (7.2 mmol/l) than at 29 months (14.5 mmol/l). A negative correlation was found between lactate concentration after all runs in the submaximal test, and the horses speed over 1,600 m. No correlations were found between lactate concentration after the maximal test and the horses' speed. The activity of citrate synthase, a marker for muscle oxidative capacity, increased from 24 to 29 months of age. The proportion of Type IIB fibres decreased, whereas that of Type I fibres increased. It is concluded that lactate response to a submaximal work test on a track can be an important tool for evaluating performance capacity of an individual horse.