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Background  Although the majority of equine muscles have a mixed fibre type distribution indicative of diverse functional roles, the predominance of a fibre type can indicate the primary function of a muscle. The deep epaxial musculature has an important role in core spinal stability in humans, reflected as a predominantly muscle fibre type (MFT) I or postural fibre type. The fibre type of the deep epaxial musculature has not been determined in horses. The objective of the study was to determine the MFT distribution in selected muscles of thoracolumbar and hindlimb region of horses. This included deep epaxial and hypaxial muscles that were hypothesised to have a postural stabilising role. A second objective was to examine differences in MFT distribution between horses bred for endurance (Arabian) and sprinting (Quarter horse). Muscle biopsy samples were obtained from selected thoracolumbar and hind limb muscles of 5 Quarter horses, 4 Arabians, and 2 Thoroughbreds. The myosin heavy chain distribution was determined by gel electrophoresis. Mann–Whitney rank test was used to compare the proportional MFT and differences between breeds.    Results   Mm. sacrocaudalis dorsalis medialis  and  diaphragm  had the highest proportion of MFT-I. The remaining deep epaxial muscles and the hypaxial muscle  m. psoas minor  had approximately equal MFT I and II proportions.  Mm. psoas major, iliocostalis, longissimus dorsi  and the hind limb muscles contained mostly MFT-IIX. The fibre type distribution was similar between Arabians and Quarter horses, although Quarter horses had more MFT-IIX fibres in  psoas major  ( P  = 0.02) while Arabians had more MFT-I fibres in  m. longissimus dorsi  ( P  = 0.03).    Conclusions  The fibre type distribution of the deep epaxial muscles,  mm psoas minor  and  diaphragm  varied from approximately equal MFT-I and II proportions to predominantly MFT-I suggesting a postural stabilising role possibly important in core spinal stability. In contrast the fibre type proportions of  mm psoas major, iliocostalis, longissimus dorsi  and the hind limb muscles were mainly MFT-II suggesting a locomotory role. Knowledge of fibre type distribution in such a clinically important area can direct diagnosis, prevention and treatment of muscular or neuromotor dysfunction.

Muscle fibre type distribution of the thoracolumbar and hindlimb regions of horses: relating fibre type and functional role