The loin is the topline region of the horse situated between the last rib and the croup, spanning the lumbar vertebrae and the associated musculature above them. In the standard points of the horse, the loin occupies the junction between the back and the hindquarters and is one of the most biomechanically critical areas of the horse’s body: it is the region through which propulsive forces generated by the hindlimbs are transmitted forward into the back and ultimately to the rider. A short, strong, well-muscled loin is associated with athletic capacity, whereas a long or weak loin — sometimes called a “slack loin” — is considered a conformational deficiency in working and performance horses.
The loin overlies the lumbar transverse processes, and the muscles spanning this region — primarily the longissimus dorsi and the iliopsoas group — are responsible for spinal extension and flexion, including the engagement of the hindquarters under the body that defines collection in dressage and the rounding required for jumping. The loin is adjacent to the withers anteriorly through the back and connects posteriorly to the croup and the attachment of the hindquarters at the hip. Asymmetry or muscle wastage visible across the loin can indicate unilateral hind leg unsoundness or sacroiliac dysfunction.
Horses in work should be assessed across the loin during regular handling to detect pain on palpation, asymmetric muscle development, or reduced flexibility. A farrier’s observation of an uneven weight-bearing pattern in the hindlimbs often correlates with loin or sacroiliac discomfort. Maintaining a correct body condition score is important for loin health, as excessive weight increases compressive loads on the lumbar spine, while insufficient muscle mass reduces spinal support. Loin soreness is commonly reported in horses returning to work after a period of rest or in those ridden with poor saddle fit that concentrates pressure directly over the lumbar region.
Further Reading: The Wikipedia entry on the loin region covers the anatomical boundaries and muscular composition across species. The primary muscle spanning this region is described in the Wikipedia article on the longissimus muscle, which is the dominant force-transmitting structure between the hindquarters and the thoracic spine.