In this blog we take a look at how the anatomy and biomechanics of creating 'bend' with horses. If you would like to learn more check out the webinar: Bending Biomechanics.
Lateral bend refers to the curvature of the horse’s spine around the rider’s inside leg on circles and corners. Theoretically, this should be moderate and regular within the spine from poll to tail following the curve of the ground track. In reality this does not happen as throughout the spine the sections vary in flexibility.
The Neck
The neck, from the poll to the withers, is the most flexible part of the spine with the majority of the flexion occurring at the cervicothoracic junction at the base of the neck. Excessive bend in the neck can cause the hindquarters to swing out or the shoulder to fall out.
The Back
The thoracic spine, one of the most rigid sections of the spinal column is further hampered by the saddle.
Spinal bend in this area is only achieved by the accumulation of tiny movements between each vertebra along the length of the spine and is dependent on the amount of movement within each intevertebral joint. This varies from horse to horse but is generally in the range of 1-3 degrees; the most being around the 9th- 14th thoracic vertebrae. There is a very small amount of lateral flexion in the lumbar spine and none in the sacrum.
Lateral flexion of the thoraco-lumbar spine can be stimulated by a reflex positioned beneath the rider’s leg. When stimulated on one side, this reflex causes slight rotation of the ribs and flexion of the spine away from the pressure. When stimulated on both sides simultaneously, this reflex causes the horse to lift his back and ribcage.
If you would like to learn more check out the webinar: Bending Biomechanics
Fact File – Muscles creating bend in thoraco-lumbar region:
The Longissimus Dorsi is part of the erector spinae group of muscles. This long, strong muscle runs along the top of the thoracic and lumbar spine to the pelvis attaching to each vertebrae. It supports and extends (hollows) the spine and contributes to lateral flexion.
The Internal and External Abdominal Oblique Muscles originate at the tuber coxae and insert onto linea alba, the last few ribs and the costal cartilages. They support the abdomen and assist in defecation and expiration. When working together they create flexion and lateral flexion of the thoraco-lumbar spine.
The ribs attach to the thoracic vertebrae via small synovial facet joints. These allow the chest to expand for breathing. Movement here is limited to a few centimetres. A slight shift of position in one rib affects next in line which results in a cumulative change along the whole thoracic spine. Movement within the synovial joints also allows the ribcage to rotate to one side. This can be seen clearly when the horse turns on a small circle. As he steps through with the inside hind, the neck is flexed to the inside, the ribs on the inside compress and the outside ribs open.
The muscles contributing to bend on either side of the spine are continually suppled and strengthened by alternately actively contracting then passively stretching their antagonists. One set of muscles contract to flex the joints and to bend the neck and ribcage. The opposite set of muscles then relax and sometimes stretch to allow the bend or flexion. Tension in the muscles can affect the ability to bend. For example, if the horse has shorter muscles on the right, he may find it harder to bend to the left and visa versa.
The Action of the Thoracic Sling
Unlike us, the horse’s forelimb is not attached to the rest of the skeleton by bone. The horse does not have a collar bone. This has great impact on the use of the forelimb and thorax during movement. The thorax is suspended between forelimbs by the thoracic sling. This group of muscles and connective tissue including the cervical serrated muscle, ventral serrated muscle and the pectorals, allow the thorax to rotate as well as move in the dorsal/ventral and caudal/cranial directions within the confines of the bony pillars of the forelimbs. This greater freedom of movement allows the shoulders work independently as one can slide more than the other. When the inside leg is placed on the ground, the outside shoulder rises, comes further forward and round This allows the horse to bend around a fixed point although the actual body may not be bending at all. Due to the rigidity of the spine, if the horse were to have a collar bone, this would greatly impede his ability to turn and bend. He almost certainly would not be able to perform a 10 metre circle!
Fact File - The Thoracic Sling Muscles attach onto the forelimb and support the thorax. They include the Serrated Ventralis, the Ascending Pectoral and the Subclavian muscles. These are postural muscles. The thoracic sling muscles that attach the forelimb to the thorax include the Trapzius, Rhomboid, Latissimus Dorsi and the Superficial Pectoral muscles.
Abduction and Adduction
By adducting or abducting, that is taking the limb across or away from the body, both the hind and the fore legs contribute to bend. The smaller the circle, the more pronounced the effect. A stiff horse that finds it difficult to bend his spine, may swing his quarters out to the side or be reluctant to adduct his inside hind leg.
Summary
Dressage requires the horse to be supple and places demands on the spine in terms of coordination, engagement and balance and bend. Understanding how horses bend is a complex subject and is dependent on the anatomical and conformational limitations of individual horses.
If you are interested in learning more about how the horse bends you will be interested in How Your Horse Moves, Posture and Performance and our Applied Anatomy and Biomechanics Course.
If you would like to learn more check out the webinar: Bending Biomechanics