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with all one's strength; with might and main; as hard as one can
子供たちは校庭を力一杯走った。
The children ran across the schoolyard with all their might.
観客は選手に向かって力一杯声援を送った。
The spectators cheered the player as hard as they could.
彼は最後の試合で力一杯戦い抜いたのだった。
He fought through his final match with everything he had.