Yes, it really does exist, the flexion of the spine. Contrary to what some coaches claim.
The abdominal muscles and the diaphragm muscles are involved in both the flexion of the spine and its maintenance, as well as its rotation and lateral inclination. However, these movements are fine-tuned by the muscles and ligaments of the spine. These muscles are also involved in the distribution of the rider’s weight as well as the momentum of forward movement on the most important shock-absorbing structures of the horse’s body. Namely the shoulder girdle, facilities of the Energy Storing System of the anterior limb (flexor tendon) and the ligamentous apparatus of the sacroiliac joint.
There is first of all the M. longissimus dorsi.
Then we need to look at the M. multifidii.
In order to bend the spine in a stretched posture, the M. multifidii work in concentric (shortening) work. The M longissimus dorsi and the M. iliocostalis however in eccentric (strain).
Another important stabilizer of the posterior thoracic and lumbar spine is the M. iliopsoas.
The M. latissimus dorsi also affects the stability of the spine.
These are the symptoms of hypertension in the M. latissumus dorsi. Nevertheless, the latissimus is an important muscle in straightening. So you see how in medicine: the dose makes the poison. Tonus yes, training too, but no hypertension.
From: Science in Motion
Apart from the fact that this posture makes it easier for our horses to bend, breaking impact is of fundamental importance for the training and use of our riding horse.
The ligamentum longitudinale dorsal – spanned while the spine is in flexion – serves as the main shock-breaking device of the spine. It is physiologically stretched by the flecting muscles of the horse (no overstretching possible). Of course the M. multifidii are involved in the fine tuning. In contrast to the ligamentous apparatus, which does not fatigue, the permanent tension of the M. multifidii would lead to fatigue of these muscles. The stretching of the dorsal ligamentous apparatus through back and abdominal muscles thus creates a fatigue-free carrying apparatus, while the individual muscle groups involved in flexion, depending on the movement, can tense and relax again and again.
Ligaments have receptors for overstretching injuries, which cause the surrounding musculature to relieve the injured ligament by an increased tone in the corresponding muscle groups.
The dorsal ligamentous apparatus absorbs the impact acting from above by the rider’s weight, in which it springs from an extension into relaxation.
During this movement the spinous processes approach each other.
Since the dorsal ligamentous apparatus relaxes, it cannot be damaged by this movement.
The Ligamentum longitudinale dorsale is completed by the Ligamentum longitudinale ventrale.
In contrast to the dorsal ligament, however, this is stretched by an impact from above and thus limits the approximation of the spinous processes. The amplitude of the stretching is not physiologically influenced by the muscles themselves, but by external influences that cannot be controlled by the horse. This ligament can very well be overstretched and thus be damaged. Overstretching causes a reaction of the stretch receptors and stiffening of the affected area of the spine by the surrounding muscles.
This applies in particular to the sacroiliac joint.
The sacrum is freely suspended between the pelvic belt. A massive ligamentous apparatus is responsible for its stabilization, but also for its free and high mobility. Until now it has been assumed that ligaments have only very few pain-sensitive structures. This is probably not true for some of the ligaments of the sacroiliac joint, they are extremely well nerved.
Now ligaments, which are relieved by the hypertension of individual muscles, could be protected and healed. On the contrary. The increased muscle tone reduces blood circulation in the damaged area and prevents healing. Therefore, it makes sense to release lesions shortly after the injury.
It is the point with the greatest mobility, the highest shear load and the transition between different load effects: Extremity (vertical) to spine (horizontal).
I will certainly write another chapter on lesions of the sacroiliac joint.