Mule owners have long struggled with saddle fit due to the unique shape of mule's backs. Traditionally riders have had to resort to breast collars, britching or cruppers to keep their saddles in place. The truth is that if your saddle fits it doesn't move. If you have to hold your saddle in place by the use of straps and collars, then you have saddle fit problems. Impact or pressure between the tree and the animal is causing the saddle to move. By locking the saddle in place there's no way for the mule to get relief.
Here's our favorite mule rider, James Weese, rock crawling the canyons of New Mexico on his good mule, Minnie. He's using a Y girth, a wool saddle blanket and his Evolution Saddle. James reported that the saddle "stuck like glue" and that Minnie seemed more comfortable and moved more freely without being encumbered by the britching and breast collar.
Evolution Saddles LLC
The Next Generation of Fit