Muscles of the Dressage Horse

By Hilary Clayton, DVM

Coordination and the quality of a horse's movement result in the horse's "gaits."

Dr. Clayton recommends the book The Rider Forms the Horse by Udo Bürger and Otto Zietzschmann, which has been newly translated into English.

The first part of this lecture will focus on the functions of the various muscles.

• The hind limb generates propulsive power and carries weight.
• The back transmits force from the hind legs forward (and if it is TOO relaxed, it does not allow the hind legs to push forward--it therefore requires appropriate tone).
• The forelimbs serve as struts to support the trunk, preventing the horse from falling on its forehand.
• The neck is a cantilevered beam, affecting weight distrivbution, balance and the mechanics of the back via position and carriage.

Muscle contracst ACTIVELY to generate tension and stretches passively when relaxed. Muscle cannot actively lengthen. It functions to move limbs in the swing and stance phases, stabilize joints in weight bearing and to generate force for locomotion while the hoof is on the ground.

Skeletal muscles are under voluntary control, but many movements involve a chain of events that are NOT under CONSCIOUS control. As a result, injury may lead to a habitual change in activation patterns that are for the better or worse (often for the worse), leaving the trainer with the task of teaching the recovered horse to return to a normal pattern of movement. This field of medicine is rapidly expanding.

Muscles are divided into AGONIST (those that SHARE the same function) and ANTAGONIST (those that work in opposition). There is a great deal of redundancy amongst agonist muscles--an example would be the biceps and brachialis, which both act as elbow flexors. Antagonist muscles work together with agonist muscles to produce smooth and elegant movement.

Tendons attach muscles to bone (although some muscles attach directly to bone). Muscle fibers run lengthwise and ratchet over each other in order to contract. The farther over each other they "ratchet", the greater the degree of shortening, pulling their attachment points closer together so that the joint angle becomes more acute (the joint is flexed). This is known as a "concentric contraction."

If a larger force is stretching the muscle than is generating tension, it is known as an "eccentric contraction." (An example would be when an antagonist muscles regulates the action of an agonist muscle, or relaxes against gravity--when you are holding a bicep curl against gravity and gradually release it, your bicep is performing an eccentric contraction, because the stretching force is greater than the tensile/shortening force.)

If the muscle length is constant and tension and force are equal, then the contraction is "isometric."

The horse uses the FLEXOR muscles in the swing phase and the EXTENSOR muscles in the stance phase.

The nuchal ligament and muscles on the top of the neck CONTROL the effect of gracity on the neck. The vertebrae do not follow the crest of the neck, but are close to the underside at the base of the neck. The horse's neck is S shaped: ventrally concave at the poll and dorsally concave at the base.

NECK MUSCLES:

1. Splenius: Originates at the withers/shoulder and nuchal ligament and inserts at C3-C5 and poll. It ELEVATES the head and EXTENDS AND BENDS neck. It runs at the top of the neck.
2. Semispinalis: Originates C3 - T7 vertebrae and inserts at the poll, and is made of tendinous bands. It also supports and elevates the head and neck. It runs down into the shoulder.
3. Nuchal ligament: Originates at the supraspinous ligament on the thoracic spines and inserts at the poll and C2 - C6. It is VERY ELASTIC. Its strongest insertion is at C2. It serves to help the muscles to support the weight of the neck. When the horse is relaxed and trotting, the nuchal ligament does about 1/3 of the work.

We don't want these muscles (above) actively to pull, because they would put the horses head up in the air (as in an evasion). Instead we want them simply to support the neck as it "nods" a bit up and down in the strides of the gait. These first 3 muscles are CONTROLLING muscles in eccentric contraction.

1. Serratus Ventralis: Raises the base of the neck and elevates the lower curvature. Because of flexion at the poll, it exaggerates the upper curvature. THIS IS ONE OF THE GOOD GUYS!

The head and neck is 10% of the horse's body weight, so a 10 cm change in placement changes the placement of the horse's center of gravity (back to front) by 1 cm.

Movement returns at the LS joint because the thoracic ligament fades out, allowing undulation (it blocked movement at prior joints). Tension from the nuchal ligament pulls on the thoracic ligament, raising the back just behind the withers. This, however, causes the back to hollow in the lumbar region which has implication sfor rollkür (you'll see a hollow in front of the croup). When the neck is raised, the nuchal ligament is saggy.

Young horses need their necks to help them lift their backs. The rider's weight should be forward at this stage to allow them to develop their back muscles.

BACK MUSCLES

1. Epaxial Muscles = above vertebrae. This includes the longissimus (big), spinalis (central) and multifidus (smaller deep central). The longissimus contracts rhythmically in an eccentric contraction to stabilize flexion of the back.
2. Sublumbar muscles- =below vertebrae. These flex the LS joint, resulting in pelvic undulation. Also flex and lift lumbar region and pull femur and hind leg forward--all very important functions for a dressage horse.
3. Abdominal muscles =encase viscerae (abdominal organs)
   • Viscera weight almost 600 pounds in a 1000 pound horse, so that when the horse moves, the guts slosh around. A major function of the abdominal muscles is to hold the viscera in place--and to hold them more tightly!
   • Assist in flexing LS joint, particular the rectus abdominus
   • Increase intra-abdominal pressure, which helps to stabilize the back.

As the horse's body moves, the back goes up and down. In the rising phase (suspension), the back flexes and in the falling phase (diagonal stance phase), the back extends. The longissimus muscles are active as the back flexes to limit flexion and the abdominal muscles contract as the back extends. The motion of the back decreases as the speed increases.

The Epaxial and Hip muscles raise the forehand in highly collected movements, acting in conjunction with the gluteals.

The Hind Leg protracts (swings forward) and retracts (swings backward) around the hip joint. Bending of the haunches causes increased flexion in weight bearing and increased torque on the joints. It requires greater strength in the muscles to stabilize the flexed joints.

The STIFLE is stabilized by the quadriceps. The hamstrings pull the entire limb backward, causing forward propulsion. The hamstrings are made up of the biceps femoris, semitendinosis and the semimebranosus.

The forelimb also provides expression.

The SLING MUSCLES have a STRUT FUNCTION. They consist of the SERRATUS VENTRALIS which attaches inside the shoulder blade and fans to the ribs, the RHOMBOID, which also attaches inside the shoulder blade but runs opposite to the scapula, and the PECTORALIS, which runs from the sternum and rib cage to the humerus. When the sling muscles are contracted, the chest is elevated and the withers are higher. This is the source of additional "growth" and change of shape in the withers after starting a young horse.

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