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Sometimes, animals try out combinations of search image and movement during play and find a use for it later. ”
William Calvin Emergence of human intelligence
This article follows on from Part 5 of “The Meaning of Life” series, in which the author talks about more about training for straightness.
The horse below, Charpege, is at the canter. As the hind legs alight, gravity (G) is pulling her body down to earth and inertia (I) is pushing her body forward. At this instant, both hind legs impact, resisting the combination of gravity and inertia forces.
At impact and during about 45% of the stance, the alighting hind legs resist the combination of gravity and inertia forces (black and blue arrow). James Rooney uses the expression “acceleration of gravity.”
This is the beginning of the “braking” or deceleration phase. Braking is the scientific appellation. I prefer “decelerating” as it is truly what the hind legs are doing; they resist and absorb accelerations of gravity.
The decelerating phase lasts from impact to the “Peak vertical,” which is the moment illustrated in the picture where the hind legs are acting vertically onto the ground. (At the trot, only one hind leg is on the ground.) After the “peak vertical,” the hind legs start their propulsive activity. The propulsive force generated by the hind legs is therefore a force in the direction of the motion.
When the horse is not bouncing, as shown in this picture, the propulsive force generated by the hind legs is mostly horizontal.
During the decelerating phase, the hind leg structure resists accelerations of gravity, storing elastic strain energy in the tendons, ligaments, aponeurosis and muscles. This energy is reused for the following propulsive phase and forward swing of the hind leg.
You are now familiar with the concept of elastic strain energy stored in the long tendons during the decelerating phase and reused during the swing.
You know that the same phenomenon of storage and recoil of elastic strain energy occurs in the aponeurosis of muscles such as the serratus ventralis thoracis of the forelegs and Tensor fascia lata of the hind legs.
During our discussion about the psoas, we have explained that the elastic strain energy stored in the aponeurosis of the tensor fascia lata was greatly responsible for the forward swing of the hind limb.
Gravity is also pulling the body down to earth and the back muscles have to resist converting part of the thrust generated by the hind legs into vertical forces. This is done by the muscular system situated above the vertebrae.
The following diagram represents only the fascicles of the longissimus dorsi muscle. The diagram was created in 1946 by L. J. Slijper. It was already known at this time that the theories describing the main back muscles as long bungee cords were inaccurate. On this diagram, Slijper illustrates only the fascicles of the longissimus muscles, which are oriented forward, oblique and downward.
Oriented the other way are the fascicles of the multifidius that also bridge about three vertebrae and are oriented backward, oblique and downward. We have explained in the course how these muscles inserted in oblique form the dorsal spines to the articular facets were capable of creating upward forces. On the following picture, you can easily visualize how the back muscles resist the attraction of gravity converting a percentage of the thrust generated by the hind legs into vertical forces, which are illustrated in red arrows on this diagram.
Once the hind legs are in the propulsive phase, it is the muscular system of the thoracolumbar column which converts the propulsive thrust generated by the hind legs into greater horizontal or vertical forces. If the horse converts the thrust generated by the hind legs into a bounce, as it is the case in this picture, the back muscles convert the propulsive force of the hind legs into more vertical forces. The same muscular work will be done if the horse converts the propulsive activities of the hind legs into balance control.
It is all done from the hind legs, forward through the thoracolumbar spine. There is no backward shift of the weight as falsely promoted in the theories of half halt.
The equestrian art is pure physics and no art can be truly achieved without sound understanding of the physics of balance. At the level of the hind legs and the thoracolumbar column, it is all about the work of the muscles, tendons, fascia and ligaments storing energy during the decelerating phase and using the stored energy during the propulsive phase.
The thrust generated by the hind legs is basically a force in the direction of the motion that is converted by the back muscles into greater vertical forces.
This article is part of the 2015 Science of Motion International Conference presentation, “The meaning of life” presented by Jean Luc, both in theory and through riding and in hand demonstrations.