The iliopsoas of the horse

This time we are going to discuss a muscle about which a lot is written recently, the iliopsoas.

If we compare the anatomy of the horse with the anatomy of the human being – it’s quite similar, although it is not exactly the same, especially the biomechanics. The way we deal with the anatomy, the function of muscles and body, determines our ideas. If we look at the body at the “old school” or according to the modern insight – it’s a different way of thinking.

Isolated movement of one muscle in the body does not exist. Every movement of the body is a result of synchronized action of muscles belonging to a group of muscles (agonists) or to a “chain” of muscles working in synergy.

Simple bending of the arm is not caused by a simple contraction of the biceps, as you will find in the anatomy books. Research has proven that 40% of the muscle contraction is transmitted into another muscle – surrounding the biceps or in distance. In fact, lifting an arm produces activity in the gluteal muscle on the other side of the body!

To get a good insight of the function of muscles, whole body dissections are inevitable.
At these dissections you can clearly see the structure of muscles. The function of a muscle is determined by its location, construction of the muscle and tissue structures within it.

Muscles with a more stabilizing function are located closely to the joint. They have usually more attachments to the bones and the surrounding of the joint and may have more connective tissue/ fascia within it. Large, long muscles with less connective tissue/fascia are mostly “movers”.
The stabilizing muscles contain mostly slow twitch fibres that can withstand sustained work.  Opposite of it phasic muscles contain mostly fast twitch fibres producing fast movement.

If we look at the position and structure of the iliopsoas (it is red without any visible sign of connective tissue or short insertions to individual vertebrae) we can eliminate that iliopsoas is more phasic muscle (responsible for a movement) and not a major stabilizer of the spine.
The iliopsoas is lying against the lumbar spine and runs toward the hip (femur), where it is attached. It is covered with strong iliac fascia, which ensures that movements can be made. The fascia of the iliopsoas “holds” the muscle against the lumbar spine allowing a little displacement of the muscle. The effect of contraction of the lumbar part of the iliopsoas is a dynamic stabilisation of the lumbar spine. The part from the lumbar-sacral-junction to the hip allows movement of the pelvic and hip joint.

Because all muscles move in chains of movement, a muscle cannot act as a standalone/isolated muscle. A relatively small muscle, like the iliopsoas, cannot have a big impact on the movements of the hindquarters. The movements of the hindquarters are three-dimensional movements, like all the movements in the rest of the body.

As we said earlier, to have a good movement pattern, there should be no limitations in the body.

If there are no limitations, muscles cannot become overloaded or wrongly strained as that could result in an injury.