Recruitment pattern of muscle fibre type during flat and sloped treadmill running in thoroughbred horses.

REASONS FOR PERFORMING THE STUDY: There is little information about the muscle fibre recruitment pattern during sloped and flat track running in Thoroughbred horses. OBJECTIVES: To examine the glycogen depletion pattern of each muscle fibre type during running on a flat and sloped treadmill. METHODS: Thirteen Thoroughbred horses (3-9 years old) were used. They were initially subjected to incremental exercise tests on a treadmill at 10 and 0% inclines in each horse to determine running speed at 90 and 60% VO2max. Needle biopsy samples were obtained from the middle gluteal muscle immediately after the running at 90% VO2max for 4 min and 60% VO2max for 12 min on 10% and 0% inclines treadmill. Four muscle fibre types (Types I, IIA, IIA/IIX, and IIX) were immunohistochemically identified, and optical density of Periodic Acid Schiff staining (OD-PAS) in each fibre type and the glycogen content of the muscle sample were determined by quantitative histochemical and biochemical procedures. RESULTS: The changes in OD-PAS showed that the recruitment of all fibre types were identical after each exercise bout, i.e., 4 min running at 90% VO2max (8.4-9.4 m/sec on 10%, 13.9-14.1 m/sec on 0%), and 12 min running at 60% VO2max (5.4-6.0 m/sec on 10%, 7.9-11.2 m/sec on 0%). No significant differences were found in the recruitment patterns of each muscle fibre type between 10 and 0% inclined exercise bouts at the same exercise intensity. CONCLUSIONS: The recruitment pattern of muscle fibre type is mainly determined by exercise intensity (%VO2max) and duration, but not by running speed. POTENTIAL RELEVANCE: The results of this study indicate the possibility that up-hill running results in the same training effect as faster running on a flat track.