Forward saddle migration is less common than backward but more immediately damaging — a saddle sliding forward puts the front bars directly on the moving shoulder blade with every stride.
A saddle migrates forward when placed too far back and gravity pulls it toward its natural position, when rigging pulls it forward, or when uphill conformation creates a natural forward slope. The result is shoulder restriction and front bar pressure on the shoulder blade.
Uphill conformation: A horse built higher at the withers than the hip creates a forward-sloping topline. Gravity pulls the saddle toward the shoulder. Manage through correct positioning and rigging selection.
Rigging too far forward: Full-position rigging can pull the front of the saddle down and forward as the cinch tightens. Horses with uphill builds often do better with 7/8 or center-fire rigging.
Saddle placed too far back: Counterintuitively, placing a saddle too far back can cause it to migrate forward to its natural balance point. If the saddle consistently ends up in the same forward position, that may actually be where it wants to sit — the question is whether that allows adequate shoulder clearance.
Stand at the shoulder and place two fingers between the front edge of the bar and the rear edge of the shoulder blade. You need at least two finger-widths. Have someone walk the horse forward while you watch the shoulder blade rotating back and upward with each stride. The front of the bars must not contact the scapula at any point in that rotation.
David Solum has been evaluating saddle fit problems for 40+ years. Call, text, or email him directly — he can advise on whether it's a fit issue, a tree problem, or a saddle you should replace.
See also: Free Saddle Tools · How to Fit a Western Saddle · Parts of a Western Saddle · How to Buy a Certified Used Saddle