Dressage makes particular requirements of the horse's whole spine because of requirements for:

• Long contact times for limbs (deliberate tempo) have special requirements for alignment:

• Even "bend" (lateral positioning) from poll to tail in terms of overall posture, while the lateral swings of each gait are added to the primary posture.
• Lateral bending is emphasized in dressage competitions, but, as Denoix's functional anatomical analysis of equine spines shows, some degree of simultaneous axial rotation is "built-in" to movements of vertebrae. Faber's colleagues made direct measurements on vertebrae that confirm Denoix's analysis and point to complex swings of the horse's spine, repeated during each stride of a gait (symmetric as in walk & trot or asymmetric as in canter).
• "Straightness" (minimal or no right/left "tilting" of whole body or body sections during movement).

• Engagement as connection of limb mechanics from poll to tail.
• Throughness as substitution of balance and fluent coordination for resistances arising from leaning on "locked" body segments.

This movie below shows alternating (side to side) movement of the horse's back from the front. Max is working on a square volte and performing haunches-in on four tracks (right rein). The haunches-in movie begins opposite the biomechanical convention of beginning a stride sequence with left hind toe-down because the movement here is performed on the right rein where the right hind toe down moment is fully exposed to the camera. The walk spinal motion data set at the bottom of the page uses the left hind toe-down biomechanical convention to begin a stride cycle.

Max is supporting himself in collected walk by placing the hind leg under his inside hip. This makes the stride look short, but in fact sets up the gymnastic difficulty of the collected walk where diagonal pairs reach mid-stance and almost the same moment, which establishes subtly important adjustments in timing (phasing) among legs. Max's tempo is about 50 strides per minute. Collected walk demands complete engagement during long limb contact, while sustaining consistent lateral bend, straightness and throughness. Lack of throughness, especially in the base of the neck/shoulder leads to mistimed foreleg timing, usually seen as a a "lateral" walk. The exercise of haunches-in exaggerates the demands of lateral bending in the body and axial rotation in the hindquarters. As Faber and her coauthors state "For [axial rotations], rotations of the thoracolumbar vertebrae were linked to the pelvic AR, and subsequently, to hind limb placement..." (emphasis is mine)

After the movie plays, step through it with the controller to examine how Max's hips move up and down with each walk stride: this lets you see axial rotation affecting the lumbar spine. The first frame occurs just after the two right legs make a "vee" and the belly swing (lateral bend) is concave to the right and convex to the left. As is usual for back motions, the lateral bend is accompanied by some axial rotation (see data from direct recordings). Max also makes subtle chewing motions that readjust important muscle operations in his bottom line (connections to the sternum/hyoid apparatus/tongue). The movie of haunches-in on four tracks from behind on the left rein is HERE. Max is ridden in a treeless saddle specially built for Morgans.

TABLE BELOW:
Selected still frames from the movie ABOVE illustrating the movements of the spine coordinated with limb positions.The collected walk here is about 50 strides per minute, near the slow end of tempos for collected walk, piaffe and passage.
Use the RED LINE in the first six frames to examine the axial rotation (rise and fall) of the hindquarters/hip during one walk stride.
Distance from the back wall is minimal during the cycle of the right hind in Frames 1-6.
==>> ==>> Direction of action ==>> ==>>
Sequence of hoof beats RH-RF-LH-LF
Frames One, Three, Four, Six are separated by equal time intervals.(Frames Two, Five and Eight are for the purpose of comparing balance with a bend on the different diagonal pairs of a dressage gait.)

Frame 1: RH Toe-down (Hoof beat 1)	Frame 2: Balance on RH-LF diagonal pair	Frame 3: RF Toe-down (Hoof beat 2)	Frame 4: LH Toe-down (Hoof beat 3)
Frame 5: Balance on LH-RF diagonal pair	Frame 6: LF Toe-down (Hoof beat 4)	Frame 7: RH Toe-down (Hoof beat 1 again)	Frame 8: Balance on RH-LF diagonal pair again

Max's nose remains in the center of his chest throughout the movement. There is a slight motion between Frames Four and Five. Freedom of shoulders in a lateral position is essential for maintaining the pivot point action of the forelegs in maintaining the purity of the walk tempo.

Faber and her colleagues say that the motions begin in the hind end and travel forward. This observation fits other data about the highest proportion of propulsion being generated in the hind legs, although fore legs have an essential propulsive role as well. As in half pass, the propulsive action of legs is modified by the primary positioning required of lateral work (see the stroking trajectories of right and left hind legs in the movie).

During gait, each leg enters the same sequence of roles: braking at toe-down to mid-stance, supporting strut at mind stance and propulsion from mid-stance to toe-off. The velocity of a gait depends on the timing (phasing) to permit overlap of individual roles the four legs contribute to the overall smooth performance in a whole stride cycle. Below are the graphs from unmounted horses walking on a treadmill. Flexion/extension appears chiefly associated with head bobbing (coordinated with the flight of hind limbs). Axial rotation is most prominent in the lumbar and sacral spine. Lateral bending is associated with belly swing right or left (see also the Schusdziarra book)