Jerome supposes an allusion here to a custom common in Judea in his day, and which he thinks was known in the time of Zechariah. The young men were in the habit of lifting heavy stones for exercise, and for a display of strength. They lifted them to various heights, according to the weight of the stones and their own strength: to the knees, the breast, the top of the head, and even above the head, at arms’ length. Jerusalem is declared by the prophets to be such a “burdensome stone” that whosoever should undertake to lift it would be destroyed by its weight.
Most commentators have followed Jerome’s interpretation, though some prefer to think that the reference is made merely to a heavy stone used in building.