Example of a Transitional Stride from Trot to Canter
Transitions from walk or trot to canter begin in 'phase two' of traditional hoofbeat analysis, where the arbitrary designation for stride initiation is landing of the outside hind leg. BUT traditional analysis of the gait assumes the horse is already cantering, a condition that is not true for transitions from halt, walk or trot. The transitional stride begins with the ARBITRARILY CHOSEN left hind toe down (Frame 3 below). Note that the spacing of the diagonal pair is much closer than the trot diagonal in Frame 2. In addition, the transitional diagonal is accompanied by the repositioning swing legs in different phasing from the phasing of the trot. This transition reflects the problems a horse solves for dressage transitions: velocity changes plus phase changes. In the still frames you can see that Rio works on a given rein and engages the muscles of his top and bottom lines (another way of saying he has a flat tummy in the transition).
In a fully trained dressage mount this is a diagonal pair opposite the normal diagonal of the canter lead chosen (Frame 4) and it establishes the FIRST STEP OF THE CANTER (Frame 5), WHICH WOULD BE THE LANDING OF THE OUTSIDE HIND LEG.
The first full stride of canter, with its characteristic timing of steps, is the one after the transition. When I explained this to one of my instructors (after showing him the animation on my portable computer at a lesson), he stared at me for a moment, then said, "So that is why one of my instructors always said 'the horse rolls over into the canter'!" The instructor had seen and described the qualities of a dressage transition from trot to canter.
Here I ride Rio Sereno (Rhinelander Canadian Hunter cross) from a walk into a trot stride before left canter on the right rein. This is a dressage transition, although it could be performed in still higher collection. It is still acceptably level, prompt and in stride.
The transitions for dressage take place on diagonal pairs of limbs at some part of the stance phase. Transitional strides have their distinctive limb positions as their timing encompasses the leg swaps that transit from one gait pattern to the next. Dressage transitions are a small set from a spectrum of limb positions that a quadruped like a horse can make in changing gaits.
For comparison, here is Rio a year prior to the movie above with a green dressage transition. Note that in Frame 1 below, the inside foreleg is not as vertically (mid stance) placed during the transition as it is in the still Frame 3 from the movie. His outside hind (RH) is not placed as far forward under his body in this forward, fundamental dressage transition. Even at this beginning stage, the basics for a specific transition have been established for his gymnastic development.