What is the opposite of "arch" as a verb when talking about movement?

verbs phrases antonyms

Solution 1:

While it is commonly used in horses, swaybacked is exactly the term you're looking for, at least in extreme cases. The term is also applied to people who suffer from lordosis.

"having an abnormally hollow or sagging back :'a swaybacked mare'"

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Solution 2:

An inverted arch is an arch 'the other way up'. Both the technical and everyday usages of this compound / collocation are shown in this Wikipedia article, which is matter-of-fact to the point of being platitudinous:

An inverted arch is a civil engineering structure in the form of an inverted arch, inverted in comparison to the usual arch bridge....

Used as a verb, 'arch downwards' would be needed.

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