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

Abstract

BackgroundAlthough 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.ResultsMm. 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).ConclusionsThe 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.

Core Training and Rehabilitation in Horses

“…A high percentage of type I muscle fibers are present in m psoas minor, indicating a postural support function, whereas m iliopsoas has predominantly type IIX fibers that can actively flex the lumbosacral and hip joints. 8 Both of these actions protract the hind limbs and contribute to the horse's ability to work in collection. Not surprisingly, the sublumbar muscles can become tight in horses that perform a lot of collected work; these horses benefit from passive stretching of the hip joint in extension.…”

“…At each vertebral level, m multifidi have 5 fascicles arranged in layers. The short, overlapping, pennated fibers are separated and connected by fibrous tissue 13 and the deepest layers have the highest percentage of type I fibers 8 which are the shortest fibers spanning a single joint. These fibers oppose rotational or shear forces generated by the transverse and oblique abdominal muscles in people 9,13 and seem to act similarly in horses.…”

Whole body vibration affects the cross‐sectional area and symmetry of the m. multifidus of the thoracolumbar spine in the horse

“…In horses, the m. multifidus is comprised of approximately equal muscle fibre type (MFT–I) (slow twitch) and MFT‐II (fast twitch) fibres indicating a dual postural and locomotor role (Hyytiäinen et al . ). The functional anatomy of the m. multifidus is comparable to that in man comprising a series of overlapping fascicles running caudo‐laterally, spanning one to five intervertebral joints.…”

Clinical progress in the diagnosis of thoracolumbar problems in horses

“…This is reflected in the work done by Hyytiäinen et al . () in determining the muscle fibre types in the deep epaxial muscles of the horse. The longissimus dorsi muscles have a high proportion of fast twitch fibres, suggesting a predominantly locomotory role.…”

“…With an increase in the activity of the muscles in the horse's trunk, there was a restriction in back movement and a stabilisation of the spine. This is reflected in the work done by Hyyti€ ainen et al (2014) in determining the muscle fibre types in the deep epaxial muscles of the horse. The longissimus dorsi muscles have a high proportion of fast twitch fibres, suggesting a predominantly locomotory role.…”