You often read and heard people talking about lateral flexion & bend and longitudinal
flexion & bend. The terms are sometimes used interchangeably, but you’ll see what that isn’t so in a moment.
1. Longitudinal - is concerned with details that affect the topline of the horse in the straight line.
2. Lateral - is through the left or right side of the horse. Longitudinal - is concerned with
details that affect the topline of the horse in the straight line.
3. Bend - is through all of the horse from ears to tail.
4. Flexion - is just at the joint below the poll.
Longitudinal flexion is therefore when the poll area softens such that the nose in a straight line comes closer to the vertical line. It is purely talking about the area of the head joining on the neck
in the straight line.
Lateral flexion is where the horse looks slightly to the left or right in the poll area with the neck and the rest of the body straight.
People will often talk about the poll as if it is a joint, even calling it the poll joint, but this is incorrect. The actual poll is the top portion of the skull and as such is immovable. The joint that we refer to is where the first cervical vertebra attaches to the skull which is slightly lower than the poll region
The up down movement of the head, known as longitudinal flexion, occurs at the atlanto-occipital joint; the point where the Occipital bone and Atlas meet. The side to side movement of the head, known as Lateral flexion
( or position), occurs in the second cervical (neck) joint -the atlanto-axial joint.
Phew, having to say
that during every lesson makes me understand why someone decided to call it flexing the poll ;-)
Lateral bend is where the horse is bent through the whole body from ears to tail, as on a circle.
Longitudinal bend is where the horse is lowered through the hindquarters from the correct engaging of the hindquarters. The three joints of the hind legs are flexed equally which results in a higher,
raised neck and poll flexion.
NB. - There can be flexion without bend but not bend without flexion.
The horse is made calm, forwards and straight and the poll area softens as the
horse steps into the hand - longitudinal flexion. The rider next works towards lateral flexion, then lateral bend and proceeds on to lateral work (sideways movements) which in turn gymnasticises and
strengthens the joints of the hind legs. The horse can then be collected resulting in longitudinal bend.
As for “Flexed straight” this comes mainly from the (Austro-Hungarian) tradition, although
not all are united in riding a horse flexed at all times. It comes, primarily, from those close to the SRS, where they maintain that a horse should be ridden "in position" (a slight shoulder fore at all times).
Others think that it will tempt riders to pull on the inside rein, which actually breaks the horse
up at the base of the neck. When this happens, the straightness is gone, and with it permeability (durchlässigkeit/throughness) and lightness.
However, no horse is completely straight and will bulge at least a little bit with one shoulder and
not step in and under with the opposite hind. This causes the Stiff vs. Hollow side.
"Flexed-straight" refers to the horse's functional straightness as opposed to linear
straightness. On round lines the horse is functionally straight when he adjusts his spine along the curvature of the circle, volte, corner, etc. It simply means that the hind legs follow in the
footprints of the front legs, stepping towards the centre of gravity. That's why the horse is "straight" in an equestrian sense, even though (or, rather, because) he is bent.
The thing about straightness on a straight line is that the horse is widest at the hips, slightly
narrower at the shoulders and then obviously narrowest at the head. If you ride with the head and neck straight out in front of you then the probability is that the inside hind leg will not fully engage.
By having the horse slightly flexed at the poll to the inside you can ride the horse such that the inside hind steps correctly under. This can particularly be seen at the canter
Hope this helps.