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to stiffen; to harden
「凝る」 has other readings with different meanings.
血液が凝って傷口を塞ぐ仕組みは興味深いものだ。
The mechanism by which blood coagulates to seal wounds is interesting.
長時間のデスクワークで肩がしこった。
My shoulders stiffened up from sitting at a desk for hours.
首の筋肉がずっとしこっている。
The muscles in my neck have been stiff.