The bow and string theory, or analogy, helps us to understand how the horse supports the weight of a rider on their backs, as well as how they achieve collection and self-carriage. An understanding of the mechanics of this process is essential if we are to develop good rehabilitative exercise programmes. Weak or painful backs are often the result (or cause) of less than optimal interactions between the back and the abdominal muscles and thoracic sling, leading to changes in the biomechanics of the head and neck, and the position of the limbs in protraction and retraction.
Let’s look at the bow and string theory to start, and then discuss the effect of head and neck positions on back mobility and strength.
Consider the longbow
A longbow consists of a bow that can flex and extend, and a string that acts on the bow to flex or extend it. When the string is relaxed and at it longest length, the bow is extended, to a degree. The string has a limited length, and the bow will extend only to the degree that is allowed by the string. When the string is pulled, however, or shortened, the bow will respond by flexing – curving noticeably.
In the horse, the thoracolumbar spine and pelvis are the bow, and the abdominal muscles or the ventral line form the string. Let’s look at each of these individually.
Horses have seven cervical, eighteen thoracic, six (usually) lumbar and five caudal vertebrae. Each vertebral segment has different ranges of motion in different planes, and all work together to form a functioning spine. I love these images from Horse Movement: Structure, Function and Rehabilitation by Gail Williams, adapted from research conducted by Clayton et al (2012).
These images allow us to see just how much (or little) motion our bow actually has.
The string is made up of the abdominal muscles and the ventral muscle chain. These include the Rectus Abdominus, External Oblique Abdominal, Internal Oblique Abdominal, and Transverse Abdominal, and the Sublumbar Iliopsoas and Psoas Minor.
The tightening and shortening of this string, or the ventral muscle chain, allows for the flexion of the thoracolumbar spine. Perhaps it is worth considering the thoracic sling as a part of this system, as the thoracic sling can aid in lifting the chest between the forelegs, helping with overall flexion of the completed bow. In the same way, the stepping under of the hindleg and rotation of the pelvis will add to the overall amount of flexion available through the body of the horse, leading to collection.
The head and neck
The head and neck of the horse act as a pendulum or counterbalance to the rest of the body. The position of the head and neck can be used to influence the dorsal and ventral muscle chains, the movement of the back, and the stride and footfall patterns. The position, movement or tendencies of the head and neck can also reveal a great deal about the body and the movement of the horse as we assess and evaluate them.
We know that the position of the head and neck in treatment stretches or dynamic baited exercises can be used to build up the multifidus muscle (Clayton et al, 2011), activate the abdominal muscles and improve mobility and stability of the spine.
Taking a good lateral flexion with a low head position into a walk is one of the ways we can take a version of these baited stretches into movement, allowing us to influence the dorsal muscle chain by lengthening and stretching the outside dorsal muscle chain and activating the ventral chain on the inside of the horse.
From a recent Research Refresh on the Equine Platform (The position of head and neck position on the kinematics of the back), we also saw that placing the head and neck in certain positions will impact differently on the kinematics of the spine, depending on whether the gait is a walk or trot. If the head is placed in a high position with the aid of side reins – above the level of the whither – at a walk, the flexion-extension mobility of the lumbar spine will decrease, while the stride length will shorten. With the same head position at a trot, the thoracic spinal flexion and extension will decrease.
This shows that it is important to build the horse up gradually to a raised head position. The high head carriage should not come from a direct lifting of the head but rather from the activation and stepping under of the hindlimbs, the activation of the ‘string’ and a corresponding flexion of the ‘bow’ or spine.
Clayton (2011). Dynamic mobilisation exercises increase cross sectional area of musculus multifidus
Clayton et al (2012). Evaluation of intersegmental vertebral motion during performance of dynamic mobilization exercises in cervical lateral bending in horses
Rhodin et al. (2005). Research Refresh. The influence of head and neck position on kinematics of the back in riding horses at the walk and trot
Williams, G. Horse Movement: Structure, Function and Rehabilitation.