Muscle characteristics and plasma lactate and ammonia response after racing in Standardbred trotters: relation to performance

Blood samples from the jugular vein and muscle biopsies (gluteus medius) in 25 Standardbred trotters were obtained 5-10 min after racing. The biopsies were analysed for fibre type composition and enzymatic profile and blood s amples for plasma lactate and ammonia concentrations. Muscle characteristics, plasma lactate and ammonia concentrations after rac ing were compared with each horse's individual performance index (IPI). The IPI is calculated annually from the individual horse's racing performance (% placing 1, 2 or 3, total annual earnings, average earning per start, and best racing record), respectively, converted to and expressed as a percent age deviation from the average record of the same sex and age group. The IPI values were 100-116. Plasma lactate concentrations were 15.0-42.7 m mol/l (mean 31.3 mmol/l) and ammonia concentrations 65-210 mu mol/l (mean 1 41 mu mol/l) after racing. Fibre type composition varied among horses (range 9-27% for Type 1, 32-54% for Type IIA, and 27-46% for Type IIB). Fibre type composition, enzyme acti vities, plasma lactate and ammonia responses to racing were not correlated to IPI. Ten of the horses also performed a submaximal test on the track, co nsisting of 5 incremental 1000 m heats at approximate speeds of 9.1, 9.5, 1 0.0, 10.5, and 11.1 m/s. Immediately after each heat a blood sample was dra wn from the jugular vein for plasma lactate analysis. Plasma lactate respon se to exercise differed between horses, but no correlation was seen with IP I. Muscle characteristics, plasma lactate and ammonia concentrations after rac ing and lactate response to a submaximal track test did not correlate with current race performance expressed as IPI in a group of horses with average or better performance capacity at the time of testing. Analysis of lactate and ammonia in blood after racing is not a valuable tool to predict an ind ividual performance index.