The study of biomechanics has been advanced through two avenues of research: analyzing films of moving horses and generating models. Making a biomechanical model is almost exactly the reverse of studying film. With film, you observe what is; in making a model, you construct a simplified version of reality to better understand it.
Model making, often done today via computer simulation, is enormously useful because it allows us to predict the effects of variables such as different rider weights, curving versus straight lines of travel, greater or lesser energy levels, and changes in the horse's conformation. As useful as models are, however, I think there is no way to create an accurate one unless the model maker rides well enough to have experienced, through feel, the horse's elastic spine oscillating and bending beneath her. In other words, a thorough grounding in reality tends to eliminate egregious errors.
In the history of equitation, there have been both good models and bad ones. A good model does two things:
1. It makes accurate predictions: Anatomical structures in the live horse resemble those of the model; film study and experience under saddle prove that the structures function the way the model says they do.
2. The model, carried as a mental image, causes the rider to do things that help the horse bear weight on his back, do his work better and maintain soundness over the long term.
A bad model is just the opposite. It causes a rider to do things that reduce a horse's athletic abilities and hurt him physically. Two really bad models of equine function have been used for centuries by riding instructors. Insofar as they are false and destructive, I would be quite happy to see them eliminated from all schools of equitation.
Two Bad Models
The first bad biomechanical model compares the horse's body to a rocking horse. According to this way of thinking, a horse's forehand can be lightened by pulling his head up and back. What is wrong with this model is that real horses are not made of wood. They are not rigid, and pulling back on the reins does not rock a horse back. Instead, it merely deforms the vertebral chain and interferes with its functioning.
Film study shows that riders operating with the mental picture of a rocking horse get results that nobody really wants: elk-necked posture in which the horse raises his poll above the bit, drops the base of the neck and rigidifies his loins. When the loins are unable to coil, the horse cannot properly use his hindquarters.
The second bad biomechanical model involves the misapplication of the physics concept of the center of gravity. When a rider visualizes the center of gravity as a heavy ball inside the horse that can be rolled to the rear, she leans back and pulls up. In reality, there is nothing inside a horse that can move to help him "weight" the hindquarters. Rather than try to shift something inside the horse, riders should focus on helping him change the posture of his back and thus the actions of his limbs.
Two Good Models
Here are two good models of the structure and function of the equine body:
The first compares the horse's back to a suspension bridge. This is a classic model that has been used to study function in a wide array of mammals, but it is particularly apt for horses. The stiffly springy quality of the horse's back and many details of his anatomy are comparable.
The second useful biomechanical model compares the horse's vertebral chain to a compound, cantilevering suspension bridge. The back from withers to croup constitutes one suspension bridge unit, while the neck constitutes a smaller one. The structural point of connection of the two bridges is the withers. Muscles located below the vertebral chain--the floor of the bridge--contract, coiling the loins and tensioning the bridge cables. Ultimately, with sufficient coiling, the whole forequarter is pulled off the ground.
The most important conclusions we may draw from studying these two models are that:
1. Stretching of the passive-regulator "bridge cables" (the dorsal ligament system) holds up the horse's back.
2. Effort made by three key muscles lying below the vertebral chain (longus colli, the iliopsoas complex and the rectus abdominis) stretches the "bridge cables."
3. Muscles lying above the vertebral chain--primarily the longissimus dorsi and rhomboideus--have more leverage than muscles lying below it. Thus, in order not to block the effort of the muscles lying below the chain, the longissimus dorsi and rhomboideus must contract minimally--no more forcefully at any time than absolutely necessary to act as passive regulators.
There are, as I have said, both good models and bad. The ring of muscles is itself a model, one that has helped both riders and horses. Yet a model is a metaphor, and all metaphors can be stretched to the breaking point.
To read more about biomechanics, see Dr. Deb Bennett's article "The Anatomy of Collection" in the September 2005 issue of EQUUS magazine.