A horse that consistently stumbles under saddle but moves freely and balanced on the lunge has a rider-related balance problem — and saddle fit is the first place to look.
Saddle fit problems that shift a rider's weight forward load the horse's forehand beyond its ability to remain balanced. The horse may carry a correctly balanced rider without stumbling but be unable to compensate for the consistently forward weight distribution of a misfit saddle.
Lunge the horse without a rider in both directions at all three gaits. If the horse moves with a normal, balanced stride and no stumbling, the problem is rider or saddle-related rather than a physical soundness issue. If the horse stumbles or moves unbalanced on the lunge without a rider, the cause is physical and requires veterinary evaluation — hoof balance, limb soundness, neurological function, or vision.
While saddle fit is a common and underinvestigated cause of stumbling, do not overlook hoof balance. A horse with long toes and low heels breaks over late, producing a stumbling-like action that saddle fit cannot address. Evaluate hoof angles alongside saddle fit when stumbling is the presenting complaint.
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