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to become stiff (of muscles)
to get absorbed in; to develop a passion for; to devote oneself to; to become obsessed with; to get hooked on
to be elaborate; to be intricate; to be exquisite; to be particular about; to pay great attention to
「凝る」 has other readings with different meanings.
長時間のデスクワークで肩が凝ってしまった。
My shoulders got stiff from long hours of desk work.
彼は趣味にとことん凝る性格です。
He has a personality that gets deeply absorbed in hobbies.
彼は庭園のデザインに凝っている。
He is particular about the garden design.