Overview
The epaxial muscles form three parallel columns, from lateral to medial:
- Iliocostalis (m. iliocostalis): from the ilium and lumbar vertebrae to the ribs.
- Longissimus (m. longissimus): the largest, from the ilium to the skull, between the transverse and spinous processes. Its lumbar part is covered by a thick aponeurosis on which the cranial extension of the gluteus medius rests.
- Transversospinalis (m. transversospinalis): short bundles next to the spinous processes (spinalis, semispinalis, multifidus, rotatores).
The model shows the three columns as a single mass, from the sacrum to the axis. The horse has a well developed nuchal ligament, with a funicular and a laminar part, which passively carries the weight of the head and neck.
The equine back, with 18 thoracic and usually 6 lumbar vertebrae, is long and fairly stiff. The epaxials stabilise it during locomotion and bear the saddle and the rider's weight, which rest on the longissimus on either side of the spinous processes.
Origin and insertion
Ilium, sacrum, vertebral processes and ribs; each bundle spans one or several segments.
Action
Extend and stabilise the vertebral column. Acting on one side only, they bend it laterally. They also raise the head and neck.
Innervation
Dorsal branches of the spinal nerves.
Clinical relevance
- Back pain is a common complaint in sport horses. Tense epaxial muscles that are painful on palpation can be primary or secondary to other causes: spinous processes that touch or overlap in the saddle region (kissing spines), sacroiliac injury or limb lameness.
- A poorly fitting saddle concentrates pressure on the longissimus and causes pain and, over time, muscle atrophy beside the withers and along the back.
- In exertional rhabdomyolysis ("tying-up") the loin and croup muscles become hard and painful; blood creatine kinase rises.