Muscle adaptation of Thoroughbred racehorses trained on a flat or sloped track

Objective-To determine histochemical and biochemical properties of muscle d uring adaptation to training on a flat or sloped track. Animals-22 Thoroughbreds. Procedure-Samples were obtained from the middle gluteus muscle before and a fter training programs were conducted, using a needle-biopsy technique. Tra ining programs consisted of horses running 1,600 m on a flat or sloped trac k for 16 weeks. Amplitude of middle gluteus muscle activity per burst was c alculated. Muscle fiber composition and area were examined on serial cross sections processed by standard histochemical staining procedures (ATPase st ain after prior incubation with an acid or base, followed by succinate dehy drogenase [SDH] stain). Furthermore, SDH and phosphofructokinase activities were determined biochemically, and composition of myosin heavy chain (MHC) isoforms was analyzed electrophoretically. Results-Training resulted in substantial adaptations. Relative contribution of muscle fiber with high SDH activity (type-l and -IIa fibers) to total c ross-sectional area, SDH activity, and composition of MHC-IIa isoforms were increased. Gel electrophoresis revealed a large amount of MHC-IIx isoform and a small amount of MHC-IIb isoform in the muscle. Although root mean squ are of muscle activity for training on a sloped track was 7.6% higher than the value obtained while training on a flat track, muscle histochemical and biochemical properties did not differ significantly between groups trainin g on fiat and sloped tracks. Conclusions-Training adaptations for contractile and metabolic properties o f the middle gluteus muscle were evident for the 2 types of training. Howev er, training adaptations did not differ significantly between groups traine d on flat or sloped tracks.