44 Men sit when speaking against others, they stand when they praise the Lord, to whom it is said: “Behold now, praise the Lord, all you servants of the Lord, you that stand in the house of the Lord.” He who sits, to speak of the bodily habit, is as it were loosened by ease, and relaxes the energy of his mind. But the careful watchman, the active searcher, the watchful guardian, who keeps the outposts of the camp, stands. The zealous warrior, too, who desires to anticipate the designs of the enemy, stands in array before he is expected.
Source: Letters (New Advent)