The psoas muscle is one of the most important muscles in a horse’s body. It is located deep within the abdomen and runs along the back of the horse from the 12th thoracic vertebrae to the 5th lumbar vertebrae. It then travels down to attach beneath the lesser trochanter of the femur.
This muscle is used to move the pelvis and create large gymnastic movements such as a canter or piaffe. During these movements it is really important that the correct amount of flexion and extension muscle tension is maintained throughout. If the flexor and extensor muscles are not properly balanced it can cause a horse to lose control of the movement and become unbalanced.
To maintain balance during a movement it is essential that the psoas and other muscles are able to contract both eccentrically and concentrically. This helps to prevent the muscles from getting too fatigued and allows them to keep moving and allow a horse to continue with its intended movements. However, as with all muscle groups, problems can occur if the horse is asked to work in too advanced an outline for too long. This can result in the horse trying to compensate by using its other muscles in a different way. This can lead to a loss of the ability of these muscles to contract isometrically.
In a horse, the psoas muscle is attached to a number of structures such as the lower ribs, intercostal cartilage and the lumbar spine. It also has branches that attach to the head, tail and hind quarters. It is an important muscle because it is responsible for the movement of the pelvis and creates power in large gymnastic movements such as a canter.
A 3D model of the psoas can be found here from Horse Anatomy Software. This software allows the user to explore the internal systems of a horse layer by layer, and can even remove certain layers to reveal the structure underneath. The software allows the user to view a range of different anatomical structures such as the psoas, and it can be rotated in any direction and at any angle.
The tuber coxae (or “hook bones”) of the pelvis are a series of bony projections on the ilium and ischium. Between these is the tubeer ischii or sacrosciatic ligament. This is the site of the lesser sciatic foramen, where the caudal gluteal nerve passes through.
The tibia articulates with the talus via the medial malleolus and lateral malleolus, which are the two prominent points on the distal end of the tibia. The talus has two trochleas, a proximal trochlea and a distal trochlea, as well as a calcanean tuberosity and sustentaculum tali (Figs 5B-6 through 5B-8). The calcaneus is the largest of the weight bearing distal tarsal bones and consists of a central shaft and lateral shaft with a rounded distal surface. The calcaneus is reinforced by the fibula and a fusion of the lateral and medial malleolus to form the sutural ridge.