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to bend; to warp; to curve
体を後ろに反らすストレッチは腰に良いらしいよ。
Stretches that bend the body backward are apparently good for the back.
猫は背中を反らして伸びをする仕草が可愛いものだ。
It's cute when cats arch their backs and stretch.
胸を反らして立っている。
She is standing with her chest out.