풀리다 is the passive of 풀다 'untie, undo': knots, muscles, tension, hard weather and stuck situations all 풀리다. 하루 종일 하면 풀리거든 근육이 'do it all day and the muscles loosen up'; 오후 되면 풀린다, said of an anaesthetic wearing off. It very often follows -다 보면 'if you keep at it', with the sense that things work themselves out given time.