The Equine Iliopsoas Muscle

Following my attendance to an insightful and thought-provoking lecture and demonstration by Tom Beech (The Osteopathic Vet), Ricky Gache (Farrier) and Mandy Miller (Olympus Saddlery Ltd) at Lomond Classical Equestrian Centre… this weeks muscle focus will orientate around the Iliopsoas muscle group of the horse. This blog post will also touch on “The Psoas Theory” which is a train of thought that injury to the Iliopsoas muscle group is far more common than we think, and injury can be responsible for the primary cause of secondary pain and poor performance seen in modern day equines.

Anatomy

The Iliopsoas muscle group is comprised of the Psoas Major and the Iliacus muscle.

Psoas Major

Origin:

• Transverse processes of the lumbar vertebrae
• Ventral surface of the proximal ends (underside of the top) of the last two ribs.

Insertion:

• Lesser trochanter of the femur. It is here where it fuses with the Iliacus.

Innervation:

• Local intercostal; lumbar nerve (ventral branch); lumbar plexus.

Iliacus

Origin:

• Sacroiliac surface of the ilium
• Wing of sacrum
• Sacroiliac ligament
• Psoas minor tendon

Insertion:

• Lesser trochanter of the femur. It is here where it fuses with the Psoas Major.

Innervation (same as that of Psoas Major):

• Local intercostal; lumbar nerve (ventral branch); lumbar plexus.

General Function

Both the Iliacus and Psoas Major that make up the Iliopsoas have the same muscular function.

• Hindlimb protraction
• Outward rotation of the hindlimb
• Hip (coxofemoral) joint flexion
• Stabiliser of vertebral column when the hindlimb is in a fixed position.

The Iliopsoas muscle group connects the trunk to the hindquarters, enabling hindlimb muscle engagement whilst maintaining correct posture. As is easily determined by noting the function of the Iliopsoas, correct and healthy function is required for efficient hindlimb protraction.

Problems

The Iliopsoas is particularly vulnerable to strain due to its anatomical positioning and function. Therefore, the likelihood of a force being applied beyond the muscles capabilities is increased.

Pre-disposing Factors to Injury

• Inadequate warm-up
• Existence of a previous injury to the muscle or associated structures
• Fatigued muscles.
• Loss of balance – ie. poor surfaces
• • This particularly places a lot of stress on the muscle insertion at the lesser trochanter of the femur.
• Muscular tension/hypertonicity
• Head and neck in a deep and round position
• Serious fall
• Becoming cast

Whilst the injury may have occurred in the Iliopsoas muscle group, a direct negative effect on the function of the Psoas Minor muscle may occur.

Injury can occur unilaterally or bilaterally.

Clinical Signs

In a case where there is an injury to a muscle, it would be expected to see lameness… locomotive signs of discomfort and pain. However, the Iliopsoas muscle group can be injured without causing apparent lameness. Instead, more subtle signs may be evident.

• Loss of muscle tone to Iliopsoas muscle group
• • This leads to the horse adjusting his movement = compensatory locomotive patterns.
• “Hip drop” on the side of injury as hindlimb flexion is limited.
• Unlevel points of tuber coxae when assessed statically.
• • The Iliopsoas muscle group influences the position of the pelvis.
• Over-compensation of the uninjured hindlimb or diagonal pair.
• • Demonstrated by the uninjured hindlimb taking a wider and larger step, increased tarsal flexion, increased MCP joint “sinking” indicating increased weight-bearing
• Tail carried to the uninjured side.
• In ridden work, the rider may feel:
• • an uncomfortable twisting motion instead of impulsion
• • resistance to specific movements that encourage hindlimb muscle engagement ie. collection.
• • inability to track up or “step under”
• • … this can cause the horse to stiffen the back, and increase the weight bearing load onto the forelimbs.

A DIAGRAM DEMONSTRATING THE HYPAXIAL AND ILIOPSOAS ACTIVATION REQUIRED FOR COLLECTION (Sustainable Dressage)

Acute clinical signs can then develop into chronic problems.

• Increased muscle tension/hypertonicity of uninjured side and surrounding hip (coxofemoral) joint.
• • Eventually muscular tension will be mirrored in the shoulder and neck area. Palpable in the brachiocephalic, pectorals and deltoid.
• • If left long enough, this muscular tension can progress to the point of causing lameness.
• • Muscular tension can then have a knock on effect on the poll and jaw area.
• • This can be attempted to be solved superficially by the use of training aids such as side reins to improve the head and neck position. This can often lead to worsening of current problems, or developing of new ones.
• Atrophy of middle gluteal and hamstrings muscle group.
• • Atrophy to the superficial gluteal and proximal bicep femoris on the side of injury may occur due to lack of elasticity of the Iliopsoas prevents full hinblimb retraction.
• • • This may present by the horse being uncomfortable for the farrier.
• Engagement of the hindquarters becomes increasingly difficult due to muscular atrophy and tension caused by distorted neuromuscular feedback. Consequently, core (hypaxial) muscle engagement is difficult reducing the horses ability to work naturally in self carriage.
• Under saddle, the horse may tilt their head slightly and over-flex on one rein… whilst feeling stiff and unbalanced on the other.

The Link between Iliopsoas Pain and Back Sensitivity

Sensitivity located over the lumbar portion of the spine and behind the saddle can potentially be associated with Iliopsoas discomfort.

The Link between Iliopsoas Discomfort, Diet and Kidneys

In close contact with the Iliopsoas muscle group are the kidneys. As the kidneys hold a lot of toxins, the Iliopsoas muscles can act as a “biological dustbin”. Preservatives in feed will increase the amount of toxins in the body, for example, increasing the amount of waste products to be broken down by the kidneys.

FEMALE KIDNEY POSITIONING

The Link between Hormones and Iliopsoas Sensitivity

The ovaries of a horse sit just behind the last rib. When a mare is in season, she may be particularly sensitive over this area of skin. The Iliopsoas muscle is also located in this area, and may become hypertonic (harder/increased tone) or contain excess fluid (oedema) when in season.

The nerves that determine whether a horse develops ovaries or testicles when they are an embryo are the same. With this in mind, the process of gelding a horse involves the cutting of a neuromuscular bundle. This can mean that branches of nerves that come from the same area of nerves that innervate the ovaries in a mare can be damaged. Hence, a gelding can also become sensitive in the same area that a mare would during her seasons.

MALE KIDNEY POSITIONING

A Vicious Circle

When muscles do not function correctly, additional strain on associated tendons, ligaments and joints will undoubtedly occur.

Prehabilitation – the art of prevention

In order to prevent injury and consequent dysfunction of the Iliopsoas muscle, methods of prehabilitation can be implemented:

• Gymnastic exercises
• Eliminate restrictions
• • Poor fitting tack
• • Poor hoof conformation and care
• • Unbalanced rider
• • Poor warm-up

Rehabilitation

• Ensure the horse has been checked by an equine dentist or qualified dental technician to allow for correct alignment of the TMJ – this can have a direct effect on the pelvis.
• Foot balance is essential as this will have a direct impact on posture and strain on the spine.

As the Iliopsoas muscle group is a very deep muscle, it cannot be directly palpated. Therefore, massage directly on to the muscle is not a feasible option. Instead, the muscle has to be released actively by the horse themselves. An example of a method used to release this muscle is the Masterson Method, whereby the practitioner learns to engage the horse in a series of exercises that can release tension deep inside the body. Additionally, massage of the surrounding superficial muscles can have a secondary effect of Iliopsoas relaxation.

WHY TO TREAT THE ILIOPSOAS?

ILIOPSOAS MUSCLE RELEASE TECHNIQUE

ILIOPSOAS MUSCLE RELEASE TECHNIQUE AND EXPLANATION

ILIOPSOAS MUSCLE STRETCH

Varying Opinions in Rehabilitation Methods

The simplistic concept of releasing a muscle in order to promote physical and mental wellbeing for the animal is commonly accepted. However, there is a belief that by releasing muscles that hold tension we are hampering the capacity of the tendons to store adequate elastic potential energy for efficient locomotion. This belief is derived from observing and treating the animal from a locomotive perspective, rather than a muscular one. This is overviewed in the diagram below.

References:

Horse Wellness Blog – What Lies Beneath the Rider’s Seat: The Horse’s Psoas Muscle!

Abi Lewis Equine Therapy – The Psoas Theory

Lecture by Tom Beech (The Osteopathic Vet)

Budras, K., Sack, W., Rock, S. (2012) Anatomy of the Horse