Jamieson, Fausset and Brown on 2 Samuel 7:18

18. Then went king David in, and sat before the Lord--Sitting was anciently an attitude for worship (Ex 17:12; 1Sa 4:13; 1Ki 19:4). As to the particular attitude David sat, most probably, upon his heels. It was the posture of the ancient Egyptians before the shrines; it is the posture of deepest respect before a superior in the East. Persons of highest dignity sit thus when they do sit in the presence of kings and it is the only sitting attitude assumed by the modern Mohammedans in their places and rites of devotion.