Albert Barnes Commentary Ezekiel 21:6

"Sigh therefore, thou son of man; with the breaking of thy loins and with bitterness shalt thou sigh before their eyes." — Ezekiel 21:6 (ASV)

The prophet was directed to let the people see him sighing and prostrate, as a sign of the sorrow and weakness about to come upon the people.

The breaking of thy loins - The prostration of strength; the loins being the seat of strength.