Please see the following links for expert tips on training, biomechanics, saddle fitting, and more:
Exercises and Advice from Robert Dover
A Horseman's Approach to Eventing (William Fox-Pitt)
Creating Harmony and Expression
Build a Solid Foundation for Success with Mary King
Lessons for Dressage Instructors
How Stretching can Benefit your Horse
Saddle Fitting the Asymmetric Back
Six Veterinary Approved Tips on Saddle Fit
Dressage-Position 101 With Shannon Peters
Scott Hassler: Collection with Throughness and Forward Desire
The Importance of Riding in Stretching Position
The following video provides excellent information on the mechanics of the horse's back. Our special thanks to Equitopia for allowing us to share it. The research done at UC Davis Veterinary School is particularly significant.
Please see the The Physics of Flexion, and The Physics of Collection & the ESM 2012 abstract for more information on the subject (Related Articles link).