Mia is not in any sense a WP horse or even a reasonably trained horse. This is the only picture I have of her doing her 'jog' - a relaxed, easy, very comfortable-to-sit trot. It looks to me like her legs are moving in a trot, but her she seems to have 3 feet on the ground at that moment...and my family jokes about this being Mia 'pretending to trot':
The author of the 2010 WP study argues that movement in horses in a continuum, and that they are capable of a much greater variety of motion than the categories we put them in to. That was the problem faced by one study I read on dressage horses. It tried to define "collection", but it seemed as if horses achieved that balance shift in a variety of ways.
"The assumptions by riders and trainers of the motion pattern of collection at trot may have arisen from observations made at slower speeds such as walk, piaffe and passage, and may only in part be valid for lower degrees of collection at trot. The rider’s experiences of balance and weight shift can for example be a result of changes in timing between the fore- and hind limb placements and changes in stride frequency and/or stance durations. Holmström et al. (1994) suggested that a large positive diagonal advanced placement indicates good balance and ability to carry weight on the hind limbs and he also showed that good moving horses had higher diagonal advanced placement (Holmström et al. 1994). Highly trained dressage horses have longer stance duration in the hind limbs than in the forelimbs at collected and extended walk (Clayton, 1995), indicating that these horses move in more self carriage which implies lightness of the forehand and greater reliance on the hind limbs for propulsion (Clayton, 2001). The complex concept of collection can not be fully defined in kinematic, kinetic and temporal terms from this thesis, but it is clear that the key lies in combining several parameters characterizing the horse as a whole, rather than focusing on single values."http://pub.epsilon.slu.se/1680/1/Rhodin_2008_1.pdf
Relationship Between the Head and Neck Position, Vertebral Column and Limbs in the Horse at Walk and Trot, by Marie Rhodin"
This is a graph of how someone displayed the variation back in 1965:
It seems to me that horses defy our ability to limit them. It certainly highlights the problem judges have, because the motion needs to be judged by far more complexity than the simple rules an Internet poster like myself tries to use. It also reminds me how easy it is for me to train Mia for the wrong thing - to think "X" means she is moving right, when she is really cheating and my uneducated butt won't catch her deception!
I guess this is why a person can spend a lifetime involved with horses and still have plenty left to learn...
...particularly those of us who started at 50!