Is the Equine Bow and String Theory Still Relevant

The bow and string theory was first proposed by Barthez in 1798, but was mostly ignored until 1946 when Slijper published the theory. That was a long time ago, and since then our understanding of the body has vastly improved – mostly owing to further research published on the biomechanics of the spine in motion (both in vitro and in vivo).
We have research that shows us how specific muscle groups respond to specific exercises over a period of time, and research on the muscle activation patterns of certain muscles during various gaits.
Our understanding of the muscular system now includes recognition of functional fascial chains, rather than a focus solely on the action of single muscles in isolation. 

So the question that naturally arises is, ‘Is the bow and string still the best analogy we have for understanding the relationship between the spine, its motion and the muscular chains that influence it?’ 

The Bow and String Described

Let’s start off with understanding what the bow and string theory actually states:

“The horse has a very flat-shaped bow which is made up of the vertebral column, its epaxial muscles and ligaments. The whole structure is kept rigid and under tension from the string formed by the sternum, abdominal muscles, linea alba and the muscles of the limbs.” (Slijper, 1946).

The original bow and string theory included a consideration of the muscles of the limbs, while our interpretation and understanding of the theory usually exclude them.  This is evident in two diagrams commonly used when discussing the Bow and String Theory:

Images similar to these are commonly used to illustrate the bow and string theory. They clearly omit a reference to the forces produced by the limbs.

In Functional kinematics of the equine back, Renee van Weeren (2006) discusses the various theories preceding this one. The author points out that the bow and string theory was the first attempt to consider the equine trunk as a whole. Slijper (1946) considered the entire trunk to be part of one system under intrinsic tension. He recognised that changes in one area would affect other areas. Thus the theory recognises the dynamic nature of the horse’s structure and allows us to view the body more holistically.  

Issues Often Raised in Opposition to Bow and String Theory

1. The theory was proposed in 1946, and our knowledge has progressed since then.
2. Considering the thoracolumbar spine as a unit that only flexes and extends as a whole is not accurate, since this depiction does not capture the spine’s full range of motion.
3. A change in orientation of the Dorsal Spinous Processes in the thoracic spine indicates a change in the action of the muscles attaching to the DSPs.
4. The vertebrae and their supporting musculature provide a channel for transmission of horizontal force, or forward motion, and vertical force, which resists gravity.
5. The abdominal and back muscles need to occur in balance with one another to maintain the straightness of the thoracolumbar spine.
6. From an anatomical perspective, the back muscles are large while the abdominal muscles are thin, and have a limited ability to generate power.
7. The function of the back muscles is not to facilitate movement of the back, but rather to provide support and stability.
8. The thoracolumbar spine has a limited available range of motion.
9. Lateral bending of the TxLx spine is always coupled with rotation of those vertebrae.
10. Muscle contraction and action have far more to do with the transmission of force than with the movement of a bone or joint.
11. How could the abdominal and pectoral muscles make fine adjustments to the position of individual vertebral levels?

Before we continue, it may be helpful to reflect on each of these points, considering whether or not each statement is true, and whether or not it is a valid objection to the bow and string theory.

What Movement is Available in the Spine?

We know that there are three primary movements that can occur throughout the length of the spine: flexion–extension, lateral bending and axial rotation. The cervical spine contains by far the greatest available movement of the entire spine. The cervical spine has a great deal of available flexion and extension, as well as lateral bending, and a limited range of axial rotation that is comparable to the amount available in the thoracolumbar spine.

Image: Renee Van Weeren, 2006

In a walk, Faber et al (2000) found that the range of motion for flexion–extension is fairly constant caudal to T10. Lateral bending is most evident in the cranial thoracic vertebrae and in the pelvis, and less evident between T17 and L5.

Axial rotation increases gradually from T6 to the tuber coxae.

In a trot,  Faber et al (2001) showed that the range of motion in flexion–extension, axial rotation and lateral bending was lower than in any other gait. The trot had the most ‘stable’ spine. 

Heim et al (2015) showed that trotting under saddle significantly reduced the ROM of the spine; a sitting trot even more so.

Another interesting finding from this study was that during a trot the poll showed the lowest amount of dorsoventral motion when measured with an IMU in comparison to the wither, lumbar, SI and sacrum. This indicates that the head and neck or poll acts as a point of stabilisation for the rest of the vertebral column during movement. It also shows that there is a gradual decrease in ROM from the whither caudally along the spine.

In the canter, Faber et al (2001) showed that there was very little variability between the included horses in terms of flexion and extension; there was slightly more variability in terms of axial rotation, with the largest variability occurring during lateral bending.

In addition, Johnson and Moore-Colyer (2009) showed that the lumbo-sacral flexion–extension increased as the speed of the canter increased.

While the information gathered during these studies is great, it actually has limited clinical value.  The studies conducted by Faber used Steinmann pins inserted into the dorsal spinous processes. This is an invasive technique that must have some impact on the motion of those vertebrae. In addition, the study was performed on five warmblood horses; a sample population of this size cannot be considered representative of the population of horses we treat and work with on a daily basis.

Are We Aiming for Stability or Mobility of the Spine?

“Deeper muscles create stability, more superficial muscles create movement. There has to be stability before movement. From research, we know that the stabilising factor of the deep muscles is lost when there is injury to the spine, at the level of that spinal lesion. Muscle properties will also change or be lost as a result of this change. The superficial muscles now have to perform two functions – creation of stability and mobility. These muscles will then palpate as taut, hard, braced, etc.” Nicole Rombach, Movement Retraining in Rehabilitation for Equine Spinal Dysfunction.

Can we truly have one without the other?

When there is a lack of healthy stability of the spine coming from the spinal stabilisers, the body responds through compensatory mechanisms to stabilise the area. This can be in the form of the fusion of spinal levels, or of the larger muscles taking on more of a stabilising or bracing role.

If there is a lack of mobility in the spine, or if certain spinal levels are fused and there is bracing through the back, the result is a lack of healthy mobility of the soft tissues and resulting pain and dysfunction.

We must have both a healthy range of motion that is free from pain, and healthy paraspinal stabilising muscles (i.e. a healthy multifidus) to ensure that the spine is stabilised.

If we think of the muscles responsible for stabilisation of the spine at the level of the spine itself, we see that there are muscles along the entire dorsal line of the spine. These are primarily the multifidus supported by a range of additional muscles. On the ventral aspect of the spine, there are muscles in the cervical, cranial thoracic, lumbar and sacral areas. There are no ventral paraspinal muscles under the mid-thoracic region.

This makes the role of the abdominals and thoracic sling essential in the stabilisation of this area for functional support in movement. The psoas, one of these ventral muscles, also connects to the diaphragm and may be providing additional components of support and stability to the ventral aspect of the thoracic spine. This observation would appear to support our bow and string theory.

How Does the Head Influence the Back and Spine?

According to Zsoldos and Licka (2015), during both movement and stance, the head and neck are major craniocaudal and lateral balancing mechanisms employing input from the visual, vestibular and proprioceptive systems.

With recent interest in head and neck positions, a plethora of research has been published on the subject.

The considerable effects of neck movement and posture on the whole trunk and even the limbs are transmitted via bony, ligamentous and muscular structures.

1. We can influence the head and neck position through training – this makes it an important consideration in the training of the whole horse.
2. The neck position affects the cervical spinal cord and the nerve roots.
3. In locomotion, the neck stores elastic energy within ligaments, joint capsules and fascia.
4. To adequately stabilise, coordinated muscle activity is necessary. This is learned and developed and requires constant muscle activity to prevent excessive wear and tear on the vertebral joints, and repetitive trauma to the spinal nerves and/or the spinal cord.

Points reported through various studies:

• The head and neck work like a cantilevered beam, creating forces that act on and between the vertebral joints. At any given joint, the forces exerted by cranial loads on the caudal segments increase, depending on the mass and length of the cranial segment. These forces are partly counteracted by the muscular and ligamentous structures that create compression and counteract the leverage created by the cranial segment.
• Head and neck length and position affect the movement of the whole horses back and limbs.

Gomez Alvarez et al (2010) evaluated the effect of head and neck position on thoracolumbar kinematics in the unridden and ridden horse. They concluded that head and neck position has a significant influence on the flexion and extension of the back.

Positions that elevated the head and neck tended to create an extension in the thoracic region and flexion in the lumbar region. Lower neck positions resulted in the opposite – flexion in the thoracic region and extension in the lumbar region. High head positions generally resulted in restricted ROM of the spine, especially the lumbar spine, while low neck positions increased ROM (which raises the question of which is preferable – mobility or stability?). An excessively high head and neck significantly disrupted normal gait parameters – much more than an over-flexed position.

Constanza Gomez Alvarez: The Relationship Between the Limbs and Vertebral Column in Health and Pain

Constanza Gomez Alvarez: The Relationship Between the Limbs and Vertebral Column in Health and Pain. Different head and neck positions evaluated in the ridden and unridden horse, and their effect on spinal kinematics and gait parameters. 

How do Fascial Lines Affect the Movement of the Back?

The fascial network and its many connections provide us with an entirely different perspective on the body. It helps us to visualise and see connections throughout the body, from the tongue to the hind hoof and every system in between.

“The whole connective tissue matrix is one single net. All the divisions we find in the books and the anatomy trains themselves are all made with a knife from this single, indivisible whole net. As we look at the parts, we must remember that the whole is real. Any division is conceptual.” Tom Myers, on anatomy trains.

What does this mean in our current conversation? The division between the spine and the abdominal muscles is a concept, not a reality. The deep and superficial ventral and dorsal fascial chains in the horse are connected to one another – we can only separate them with a knife in our hands (and in our minds).

With this visual, we can start to see that the spine and epaxial muscles are connected to and dependent on the abdominal muscles and the connected viscera. They function as a whole.

While we have identified these chains, we do not yet have research on their function and interaction.

Superficial Ventral Line, Superficial Dorsal Line, and the connection between the two. Illustrations: Anna Lloyd

Muscle Size, Fibre Type and Function

Fibre direction also gives us insight into the function of a muscle. Prime movers have long parallel fibres, capable of rapid movement over a large range of motion. Joint stabilisers, especially muscles like the multifidus, have short pennate fibres that are strong but short; therefore, they have a limited range of motion. Included within them are bands of fibrous tissue or fascia.

Where do the abdominal muscles fall within this categorisation? We know that they are relatively thin muscles that span a large area. They contain a huge amount of connective tissue or fascia, and they have a limited capacity to shorten in contraction. The abdominal muscles lie on top of one another in layers, with their fibres running in different directions.

The argument is commonly made that the abdominal ‘string’ lacks the necessary size to affect the back. We can clearly see how small the muscle mass is when we compare it to the sizes of the dorsal and ventral muscles of the body at one spinal level. However, I propose two thoughts:

1. If we were to consider the entire mass of the abdominals as they wrap around the trunk together with the fascial connections into the abdominal cavities, would there still be such a large difference in the size or mass between these structures?
2. We are comparing muscles that have entirely different functions, and therefore different forms. We cannot consider the abdominal muscles through the same lens as the epaxial muscles, as they have a form that supports their function.

My Conclusions

None of these observations lead me to regard the bow and string theory as false or even outdated. I do absolutely feel that the bow and string theory is not complete, and does not provide us with all the information we need to consider – but almost nothing ever does. We must consider the patient as a whole, first and foremost. We must consider how different structures affect one another. And we must support our patients to the best of our ability. I still feel that the bow and string theory (or analogy!) provides me with a valuable image to share with clients, and helps me to clarify my intention when I am working with a horse in the real world.

So – the upshot is that I do not disregard the theory at all, but intend to add information or layers of knowledge to my understanding of this very useful image.

A discussion with Celeste Lazaris on this topic was summarised by an attendee as follows:

*As is often the case, consider the whole body.

*There are many more muscles involved, particularly in the limbs.

*Tensegrity is a concept that may help to contextualise the bow and string theory.

*Increasing stability through increased range of motion or mobility in some areas can improve stability in other areas.

*Muscle mass is an important consideration but not the be-all and end-all because other tissue is involved. For example, a low abdominal muscle mass does not mean the ventral chain is unimportant to the dorsal chain. In fact, the ventral aspect of the horse is very important to the dorsal aspect, since the entire ventral fascia, rather than the muscle tissue alone, is involved in motion.

*Head and neck involvement. Long and low, when done correctly, is an eccentric contraction and can be very useful for strengthening the back. However, the exercise is not a static, one-size-fits-all solution for every horse.

*In a room of experts (i.e., actual researchers) you will get multiple opinions.

The more you know, the more you know you don’t know. 

You can read more thoughts and opinions here:

https://www.scienceofmotion.com/documents/time_to_get_out_of_the_museum.html

https://www.scienceofmotion.com/equine_back_research.html

References

1. Clayton, H. M. & Townsend, H. G. (1989). Kinematics of the cervical spine of the adult horse. Equine Veterinary Journal, 21(3), 189-192. https://doi.org/10.1111/j.2042-3306.1989.tb02139.x
2. Clayton, H.M., Kaiser, L.J., Lavagnino, M. & Stubbs, N.C. (2010). Dynamic mobilisations in cervical flexion: Effects on intervertebral angulations. Equine Veterinary Journal, 42: 688-694. https://doi.org/10.1111/j.2042-3306.2010.00196.x
3. Faber, M., Johnston, C., Schamhardt, H. C., Van Weeren, P.R., Roepstorff, L. & Barneveld, A. (2001). Three-dimensional kinematics of the equine spine during canter. Equine Veterinary Journal, 33(S33), 145-149. https://doi.org/10.1111/j.2042-3306.2001.tb05378.x
4. Gómez Álvarez, C.B., Rhodin, M., Bobbert, M.F., Meyer, H., Weishaupt, M.A., Johnston, C. & Van Weeren, P.R. (2006). The effect of head and neck position on the thoracolumbar kinematics in the unridden horse. Equine Veterinary Journal, 38(S36), 445-451. https://doi.org/10.1111/j.2042-3306.2006.tb05585.x
5. Gómez Álvarez, C.B., Rhodin, M., Bobbert, M.F., Meyer, H., Weishaupt, M.A., Johnston, C. & Van Weeren, P.R. (2006). The effect of head and neck position on the thoracolumbar kinematics in the unridden horse. Equine Veterinary Journal, 38(S36), 445-451. https://doi.org/10.1111/j.2042-3306.2006.tb05585.x
6. Heim, C., Pfau, T., Gerber, V., Schweizer, C., Doherr, M., Schüpbach-Regula, G. & Witte, S. (2016). Determination of vertebral range of motion using inertial measurement units in 27 Franches-Montagnes stallions and comparison between conditions and with a mixed population. Equine Veterinary Journal, 48(4), 509-516. https://doi.org/10.1111/evj.12455
7. https://veteriankey.com/kinematics-of-the-equine-back-and-pelvis/
8. Hyytiäinen H.K., Mykkänen A.K., Hielm-Björkman A.K., Stubbs N.C. & McGowan C.M. (2014). Muscle fibre type distribution of the thoracolumbar and hindlimb regions of horses: relating fibre type and functional role. Acta Vet Scand, 56(1):8. doi: 10.1186/1751-0147-56-8. PMID: 24468115; PMCID: PMC3922740.
9. Johnson, J. L. & Moore-Colyer, M. (2009). The relationship between range of motion of lumbosacral flexion-extension and canter velocity of horses on a treadmill. Equine Veterinary Journal, 41(3), 301-303. https://doi.org/10.2746/042516409X397271
10. Prinsen, H., Springer, N.C., Schreuder, M. & Muller, M. (2009). The effect of rising and sitting trot on back movements and head-neck position of the horse. Equine Veterinary Journal, 41(5), 423-427. https://doi.org/10.2746/042516409X371387
11. Rhodin, M., Gómez Álvarez, C.B., Byström, A., Johnston, C., Van Weeren, P. R., Roepstorff, L. & Weishaupt, M.A. (2009). The effect of different head and neck positions on the caudal back and hindlimb kinematics in the elite dressage horse at trot. Equine Veterinary Journal, 41(3), 274-279. https://doi.org/10.2746/042516409X394436
12. Weishaupt, M.A., Wiestner, T., Waldern, N., Roepstorff, L., Weeren, R.V., Meyer, H. & Johnston, C. (2006). Effect of head and neck position on vertical ground reaction forces and interlimb coordination in the dressage horse ridden at walk and trot on a treadmill. Equine Veterinary Journal, 38(S36), 387-392. https://doi.org/10.1111/j.2042-3306.2006.tb05574.x
13. Zsoldos, R.R. & Licka, T.F. (2015). The equine neck and its function during movement and locomotion. Zoology, 118(5), 364-376. https://doi.org/10.1016/j.zool.2015.03.005

Resources

1. Equine Effect of Head and Neck Position on Movement of the Back
2. Movement Retraining in Rehabilitation for Equine Spinal Dysfunction
3. Structure and Function of the Equine Locomotor System
4. Objective Gait Analysis in the Horse
5. The Relationship Between the Limbs and the Vertebral Column in Health and Pain