Spiral Line

The spiral line has been so much fun going in to and there is a lot to write about it so this section is a bit long but it's definitely worth a read. If you are not too bothered by the anatomy you can skip down to what it does :D.

The Spinal Line plays a significant role in spinal rotation, particularly during flexion. It comprises a right and left side, each with a winding section that curves beneath the abdomen and a straight section that follows the Superficial Dorsal Line (SDL). These lines intersect the body's midline three times: just ahead of the withers, under the abdomen, and over the sacral tubers.

Beginning beneath the ear, the line traces the splenius muscle, crossing the midline above the C6-C7-T1 vertebrae. It then continues along the rhomboid muscle on the inner side of the shoulder blade and finally reaches the thoracic ventral serratus muscle.

From that point, the line progresses into the external abdominal muscle, then crosses the linea alba (the white line woven into the internal abdominal muscle on the opposite side). It continues to the coxal tuber, merging with the fascia lata and extending distally past the hock, through the extensor tibialis cranialis and peroneus tertius muscles. These muscles attach to the tarsal and metatarsal bones on both sides, and the plantar surface of the tarsus, integrating with the Achilles tendon and the caudal head of the biceps femoris. From the ischial tuberosity, the line travels via the dorsal sacroiliac ligament to the sacral tuber. The third crossover occurs through the dorsal sacral ligaments, after which the line mirrors the Superficial Dorsal Line's (SDL) path towards the head, but on the opposite side from its initial origin.

So What Does it Do?

The Spiral Line (SL) helps the spine rotate, bend, and move the neck sideways. It has two parts: one that curves under the belly for twisting and bending, and one along the back that straightens and stabilizes. There's a left and a right SL, which need to be balanced for the body to stay straight. Starting at the head, the SL twists down the body, turning the belly left and the back right. This spiral movement helps with overall body stability and flexibility.

When the Spiral Line (SL) twists the belly, it bends the neck sideways and lifts it slightly. At the chest, it pulls the shoulder blade closer and upward on the opposite side. Further down, it twists the body, pulls the hip forward and down, and slightly bends the spine. At the hock, it stabilizes the joint. At the pelvis, it lifts the back of the hip. As it runs up the back, it straightens the back and neck, and untwists the body before the other side of the SL starts to twist it the other way.

When Things Go Wrong

A common pattern is to find the Spiral Line (SL) more contracted on the right side. This might be due to the horse's position in the womb, the birth process itself (as suggested by veterinarian Ian Bidstrup), or simply natural internal anatomical differences. We believe this unevenness in the SL may be the cause of what's often called 'right or left front limb dominance' or 'natural crookedness' in horses.

 The Spiral Line (SL) has a twisting section and a straightening section that work against each other. When the SL is constantly tight, the tightest part shows symptoms. If one SL is persistently contracted, the neck will bend sideways and lift, while the opposite shoulder blade and front leg will be pulled inwards and backwards towards the chest. This might seem opposite to how the Front Limb Lines work, where moving forward/inward and backward/outward usually go together. However, the SL uses different muscles for this inward movement (specifically, the thoracic serratus muscle). The body will twist and bend to the same side as the neck bend. This puts more weight on the front leg that's pulled back (opposite the neck bend), limiting its shoulder movement.

Because the tight shoulder moves less, its muscles, both on the outside and inside of the shoulder blade, don't develop as much, often making the shoulder blade appear more upright. The latissimus dorsi muscle, behind the shoulder blade, can become enlarged, bulging out when the leg is lifted. This is caused by the leg being constantly pulled back. On the other side, the shoulder is more mobile and has more muscle mass, and the shoulder blade lies at a more angled position against the body.

The line's connection to the hip on the side that's bent causes that hind leg to be placed further under the belly, closer to the middle. This results in the horse being bent and twisted to one side, with that hind leg stepping further underneath. The opposite front leg is pulled back and inwards, limiting its ability to move forward and outwards. The horse appears curved on that side. This front leg can then seem shorter than the other.

The section of the line around the hock can limit hock movement or cause imbalances, and can also affect the horse's stay apparatus, which is linked to the stifle joint.

Looking from behind, the hip bone is lower on the side where the line begins, and the outer thigh muscles are more defined on that side. On the opposite side, the hip bone is higher, and the outer thigh muscles are less developed.

Often, the horse will stand with one front leg extended forward, bearing more weight on the heel, while the opposite leg is slightly pulled back, carrying more weight on the toe. This can lead to uneven hoof growth. This stance is the horse's way of balancing the rotated spine and distributing weight evenly. Some believe this uneven stance in young foals contributes to later crookedness. Others suggest it's a compensation for an already rotated spine at birth. It's a classic 'chicken or the egg' scenario.

Pics from:

EQ Saddle Science

Equine Myofascial Kinetic Lines - For Professionals by Rikke M. Schultz, Tove Due & Vibeke S. Elbrond