I recently worked on a preliminary event horse that has been stumbling behind. Since he has a visible hunter’s bump (see other posts I have written on this topic) I immediately went to work on the area around the sacroiliac joint, as well as the lumbar area. I found nothing out of the ordinary for a horse in rigorous training and showing, but I did find tightness in the illiacus. (I was not able to find a great diagram, but it is in the general area of Number 24 below). This muscle, which attaches to the pelvic crest, flexes the hip joint and rotates the thigh.
When tight, the illiacus can cause the hind leg to buckle. Massage can definitely help, but since this is a tough little muscle, several sessions may be required. This is one area that I can show clients to massage on their own horses.