It is generally accepted that muscle enzyme activity (CK and AST) levels in plasma may increase in Standardbreds and Thoroughbreds after exercise and racing, indicating muscle damage. However, in one recent study on healthy Standardbred trotters fed a forage only diet muscle enzyme activity levels were very low after race-like exercise tests. Interestingly, lowered muscle enzyme responses after exercise has also been observed in horses with recurrent exertional rhabdomyolysis as well as in healthy athletic horses when feed low starch diets So, could it be that it is the diet and not the exercise that is causing muscle damage and enzyme leakage? The aim of the project is to evaluate the effect of different energy substrate intakes on plasma muscle enzyme activity post exercise. The hypothesis is that the plasma muscle enzyme activity levels considered normal for athletic horses are highly dependent on starch rich diets and to a lesser degree dependent on exercise.