Physiology Links about Equine Movement

So basically, the horse’s body in movement is a set of opposing forces—one set from the hind limb which propels the horse forward, and one set from the fore limbs which prevent that force from pushing the horse down. The conundrum is that the harder the hind limbs push and the longer they stay on the ground (so, increased engagement), the greater the tendency is for this force to rotate the horse onto the forehand. When the hind limbs trail behind the horse, the forcepushes the horse onto the forehand. The role of the forelimbs becomes to maintain an uphill balance and counteract the tendency to fall forward.

As horses do not have a clavicle, there is no bony connection between their forelimbs and trunk. Instead, the limbs are attached and supported by a network of muscles, tendons and ligaments. In addition, the horse’s scapula is highly mobile and can rotate and move up, down, forward and backwards across the rib cage.

The thoracic (sling) muscles suspend the ribcage between the forelimbs. These muscles attach behind the scapula as well as to the cervical vertebrae and to the ribcage. The Serratus ventralis thoracis muscle is the most important sling muscle; its contraction raises the ribcage. When the sling muscles are engaged, the horse’s withers lift. When they are relaxed, the withers are low and the horse rolls onto his forehand. Therefore, to develop uphill balance, a rider must work to develop the ability of these muscles to engage and lift.

Image taken from: http://www.riaflex.co.uk/articles/post/ ... d-training (www.riaflex.co.uk)
It is the equal activity of the left and right sling muscles which holds the ribcage centrally between the forelimbs. However, horses must also learn to use these muscles unilaterally to raise and stabilize the rib cage when one of the front limbs is lifted. In most horses, the sling muscles are weaker or less active on one side.

Riders will experience the effects of this asymmetry under saddle in particular when turning. Horses tend to prefer to collapse their weight onto the inside shoulder and to push off of that limb, rather than taking the weight onto the outside forelimb, particularly on their weaker/less developed side. Therefore, they actually have to learn to use the outer forelimb to support and lift the inner forelimb when turning. This is opposite of their natural tendency when moving without a rider.

When the sling muscles are engaged, the horse’s withers lift. When they are relaxed, the withers are low and the horse rolls onto his forehand. Therefore, to develop uphill balance, a rider must work to develop the ability of these muscles to engage and lift.

As was mentioned earlier, the horse’s scapula is highly mobile due to a lack of a clavicle. When it slides upwards, the withers are down; when the scapula slides downwards, the withers are raised, and when it slides backwards, the shoulder is tucked in. To see the full range of the scapula was truly impressive, and for me it drove home the importance of ensuring that the horse’s saddle is not impeding this movement.

Throughout the stance phase of the stride, the horse’s forelimb is retracted. The trunk is pulled forward over the grounded hoof. In the swing phase of the stride, the forelimb is protracted, then retracted; the retraction during the swing phase helps to reduce the forces of concussion on the limb.

As the degree of collection increases, the legs move through a smaller range of motion and become more vertical at contact and lift off, which facilitates the elevation of the horse’s forehand. The trapezius, pectoral muscles and the bones of the shoulder and forelimb work together to help turn the horse, as well as to execute lateral movements like leg yield or half pass.

http://www.banhdc.org/images/ch-exp-equineanatomy.pdf

http://www.vipsvet.net/articles/biomechanics.pdf

http://holistichorse.com/equine-therapy ... the-horse/