It is an arch from behind the withers to the croup. Can cause the saddle to slip forward. If caused by the bone structure it can be a strong back but if caused by muscle spasm it is a sign of serious pain and weakness. Needs a vet to decide which. The latter is not present as a baby though. Hope that helps.
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If caused by the bone structure it can be a strong back
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I thought it caused the back to lack suppleness? Also can mean the quarters aren't set on right and lead to other complications? I always thought this was quite a bad conformation fault - surely if roach backs were 'strong' backs they would be desired and not a fault?
If not bad in a youngster just backed it can muscle up as a proper back. It is bad conformation but just because there is less flexion does not always equal weak. The opposite a sway back though is forced to always go hollow whereas the roach can round.
Take a look at this - Fugly horse has nearly all the conformation pictures/answers you might need! In fact -- there is an article on sway backs & roach backs...