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

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Reasons for performing the study:

There is little information about the muscle fibre recruitment pattern during sloped and flat track running in Thoroughbred horses.


To examine the glycogen depletion pattern of each muscle fibre type during running on a flat and sloped treadmill.


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% V̇O2max. Needle biopsy samples were obtained from the middle gluteal muscle immediately after the running at 90% V̇O2max for 4 min and 60% V̇O2max for 12 min on 10% and 0% inclines treadmill. Four muscle fibre types (Types I, IIA, IIA/IIX, andIIX) 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.


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% V̇O2max (8.4-9.4 m/sec on 10%, 13.9-14.1 m/sec on 0%), and 12 min running at 60% V̇O2max (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.


The recruitment pattern of muscle fibre type is mainly determined by exercise intensity (%V̇O2max) 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.

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