Lordosis is used for sway back.
Lordosis is the medical term meaning swayback.Lordosis also known as swaybacklordosisinclination or sagginglordosislordosisLordosis
Lordosis is the abnormal condition of the lumbar spine that is also known as swayback. It causes an excessive inward curvature of the spine, leading to a swayback appearance.
Swayback
I think the term you are looking for is Swayback.
a horse goes swayback when it is attempted to be saddle trained too early (1 years old) or when a horse is ridden to hard at an old age causing it's back to be dipped or swayed that's where the term SWAYBACK comes from.
Swayback -- aka swayback posture -- is one of several types of posture ... Also, maintaining a strong core is vital to a healthy upright posture.
look on google
allograft
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onychia