Major back muscle of the horse revealed in all its digitized glory

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The model of the longissimus dorsi muscle based on the fibre digitalisation of the study. The withers are protruding at right. The model was used to simulate muscle activity using software. Image: Dietrich et al. https://doi.org/10.3390/ani11030915

The very complex inner structure of the largest muscle in the equine back has been revealed in never-seen-before detail.

The longissimus dorsi muscle has a great influence on the stability of the spine and facilitates proper locomotion.

Researchers with the University of Veterinary Medicine Vienna, writing in the journal Animals, said muscle function is determined by its specific intramuscular architecture.

However, only limited three-dimensional information is available for the inner organisation of this crucial muscle.

The study examined the thoracic and lumbar longissimus muscles of five horses who had died or were euthanized at the equine hospital at the university for reasons unrelated to the study.

The muscles were dissected layer by layer from the front to the back.

Three-dimensional coordinates along individual muscle fibre bundles were digitised from the origin to the insertion and three-dimensional models were created using imaging software and computed tomography.

The muscle was divided into functional compartments and careful measurements were taken of bundle lengths, muscle volume, and cross-sectional areas.

Bundle lengths were the greatest in the thoracic region and decreased from front to back. The muscle bundles toward the rear were less than half the length of those in the thoracic parts. The bundle angles differed between different parts of the muscle.

Towards the head, the bundles were also almost parallel to the horizontal plane, while further back the angles increased up to an average of 38 degrees.

The volume of the muscle ranged from 1350 cubic centimeters to 4700 cubic centimeters, depending on body size.

The authors said that, with the help of three-dimensional reconstructions, regional differences in the muscle architecture of the longissimus dorsi muscle were able to be quantified and documented for the first time.

They said the inner structure of the longissimus in the lumbar area indicates that, in this region, it can exert large forces and plays an important role in stabilisation of the spine.

“Our results are in line with previous studies, which found that the lumbar longissimus dorsi has the potential to exert greater forces than the thoracic longissimus dorsi, due to its inner architecture.”

The researchers said their anatomical observations and three-dimensional measurements reveal a very complex inner structure of the muscle.

The regional anatomical differences identified indicate that the longissimus dorsi performs different functions along its length.

The use of digitisation tools meant that the data for the muscle bundle architecture, including their length and angles, can be shown in its entirety.

The obtained details had created a base for enhanced understanding and further biomechanical studies, they said, such as simulating the muscle function.

The university study team comprised Dietrich, Stephan Handschuh, Robert Steid, Alexandra Böhler, Gerhard Forstenpointner, Monika Egerbacher, Christian Peham and Hanna Schöpper.

Dietrich, J.; Handschuh, S.; Steidl, R.; Böhler, A.; Forstenpointner, G.; Egerbacher, M.; Peham, C.; Schöpper, H. Muscle Fibre Architecture of Thoracic and Lumbar Longissimus Dorsi Muscle in the Horse. Animals 2021, 11, 915. https://doi.org/10.3390/ani11030915

The study, published under a Creative Commons License, can be read here

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