"The pathologist explained that in order to create horizontal forces, (forward movement,) and vertical forces, (resistance to gravity and balance control), two muscles are needed, one acting horizontally and one acting vertically, or, a single muscle acting in an oblique manner. Such insertion allows the same muscle to create both horizontal and vertical forces. This is exactly how the fascicles of the main back muscles are oriented and function.
This was the beginning of a long series of research aiming toward a functioning of the horse’s back muscles based on the subtle management of forces instead of increasing the movements of the vertebrae. This was 1969 and we were, at that time, already far away from the infantile idea that a single action such as lowering the neck could flex the whole thoracolumbar spine and also that gaits and performances can be improved by increasing the range of motion of the horse’s thoracolumbar column.
Rooney also questioned the veracity of the bow and string concept. As a pathologist, Rooney observed firsthand the discrepancy between the large mass and power of the back muscles and small mass and limited power of the abdominal muscles....
...In 1980, Leo Jeffcott measured the range of possible movement of the horse’s vertebral column. Many studies after Jeffcott found differences in the location of vertebral column movements but they all found a limited range of motion. Basically, the back muscles do not increase the vertebral column range of movement but, at the contrary, resist forces induced on the horse’s vertebral column in order to maintain the vertebral column movements within the limits of its possible range of motion
...One of the most common deceptions is the belief that the lowering of the neck flexes the lumbar vertebrae and increases their range of motion. The optical illusion was explained in 1986 by Jean Marie Denoix. The lowering of the neck reduces the mobility of the lumbar vertebrae.
This is true for every horse. Stiffening of the lumbar vertebrae hampers proper dorso-ventral rotation of the pelvis and therefore sound kinematics of the hind legs
. In order to compensate for the stiffening of the lumbar vertebrae, the horse increases the work of the iliopsoas muscles, which swings the hind limbs forward. Since the iliopsoas is placed under the lumbosacral junction, increased work of the iliopsoas muscle does induce greater rotation of the lumbosacral junction. This lumbosacral rotation does give the optical illusion that the whole lumbar region moves. In fact, the lumbar vertebrae do not flex. Instead, the horse compensates for the rigidity of the lumbar spine, that was created by the lowering of the neck, with greater intensity in the lumbosacral junction that is situated behind the lumbar vertebrae. The theories of relaxation, stretching and greater mobility of the vertebral column are naďve interpretations of a mechanism which in fact, is working exactly the opposite way..." Equine Back Research