| Summary: | The longissimus dorsi {LD} is the largest back muscle in the horse and has the potential to contribute substantially to the biomechanics of the back. This study aimed to investigate the structure and function of the LD in the horse. Computed tomography scans of eight cadaveric backs were used to quantify the crosssectional area of the LD and its moment arms relative to the spinal cord. These muscles were then dissected in situ and the 3D trajectories of muscle fascicles within the LD were digitized using a magnetic stylus and expressed as direction cosines. Bilateral muscle activity was measured in vivo with surface EMG at the T14, T16, T18 and L2 regions with simultaneous measures of back flexion using a fibre-optic goniometer placed {8 horses}, or 3D motion capture with passive markers placed above the spine {4 horses}. These measures were recorded for walk, trot and canter on a treadmill, and also at a 5 and 10 m radius lunge in an outdoor school. The muscle fascicle strains were modelled for different regions of the LD using the in vitro structural parameters and the in vivo measures of back flexion. Moment arms and fascicle orientations varied along the LD, and the orientations further varied in the mediolateral and dorsoventral directions. Asymmetry in muscle activity occurred when the horses moved around bends, and differences in the timing of activity occurred at different regions of the LD. Increases in muscle activity occurred when moving up inclines. The muscle fascicle strains were close to isometric which suggests that the LD muscle acts by developing high forces for force transmission or providing stiffness to the back, rather than contributing substantially to mechanical work. Regional differences in both LD structure and activity patterns suggest that its functional role may vary along its length.
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