A Nasty Cycle
Riders tend to fit a saddle that's just a tiny bit too narrow, because it gives a bit of room to manoeuvre, providing lots of clearance of the wither. The tightness of the saddle causes atrophy of the muscle. The saddle then drops due to the muscle having dropped away. The rider then can see that the saddle is too wide and fits a narrower gullet, which in turn causes increased atrophy of the muscle and the long downward spiral of atrophy begins.
This horse appears to have a shark's fin wither, a conformation defect, but really this horse doesn't have a shark's fin wither. What he does have is very prominent vertebrae, because of the loss of the muscle surrounding those vertebrae.
We can see the culprit saddle here and see how narrow it is. We can also see that the flocking in the saddle is uneven, causing even more problems for this horse.
Dr Gail Williams BA(Hons) PhD
Equine therapy and performance development