Regionalisation of the muscle fascicle architecture in the equine longissimus dorsi muscle.

The research investigates whether there are regional variations in the muscle architecture of the longissimus dorsi, the largest muscle in the horse’s back, using computed tomography scans and 3D orientation measurements.

Study Objective and Methodology

• The study aimed to identify if there are regional variations in the muscle architecture of the longissimus dorsi, a critical muscle in a horse’s locomotion and performance.
• The researchers utilized computed tomography scans acquired from eight horse cadavers to determine the muscle’s cross-sectional area and volume.
• They also quantified the 3D orientations of the muscle fascicles (bundles of muscle fibers) in situ or in their natural place by their direction cosines using a magnetic digitization system. This process involves analyzing the muscle from various points to determine variation in structure and anatomy.

Findings

• The average volume of the muscle was found to be 2.34 liters per side with a standard error of 0.18 liters.
• The mean length of the vertebral column from T4 to L6 dorsal spinous processes was measured at 56.57 cm with a standard error of 2.0 cm.
• Significant differences were identified in the cross-sectional area and the moment arm lengths between muscle segments. The moment arm length is a measure of the distance between the line of action of the force (in this case, represented by the muscle fibres) and the centre of rotation (in this case, the joints).
• In the lumbar region of the back, the mediolateral moment arms (those pointing side to side) were two to three times greater than the dorsoventral moment arms (those pointing from back to underside).
• There were significant variations in the direction cosines (orientation of the muscle fibers) between different segments of the muscle and between different regions within each separate segment.

Conclusions

• These findings suggest that the muscle fascicle architecture of the longissimus dorsi differs along its length and between different regions within each segment.
• The variations in muscle anatomy could predispose different sections of the longissimus dorsi to different functions or roles during horse locomotion.
• The researchers have established the need for further studies incorporating 3D anatomical measurements with in vivo (live) measurements of function such as back motion and muscle activity to get a more comprehensive understanding of the muscle’s functionality.