It is called THE exercise or as we read often the “aspirin of art of riding” or “mother of all side movements”, because it is a cornerstone in horse training and helps to straighten up the horse.
It was first mentioned by François Robichon de la Guérinière an old French riding master who lived from 1688 to 1751. We see already many videos, books, and explanations of it all over to help us to increase our knowledge.
Lately, I have been reading the new book ‘Academic Groundwork’ by Bent Branderup and Celina Skogan, which is one part of a whole series.
Very interesting to read the following on this topic:
”The challenge of shoulder-in is not stepping sideways with the inside hind foot, but lifting the outside shoulder. For this reason, Bent would prefer to rename shoulder-in into ‘shoulder-up’. Associated with it is the art of finding just the right amount of sideways and downward for the body of the individual horse in this shape and at this moment. After all, the horse will fall on the outside shoulder if the hind foot does not step under.”
Personally, I like to think of shoulder-up while practising as it clarifies the true essence and automatically, I have a different picture in my mind.