Albert Barnes Commentary Psalms 18:36

"Thou hast enlarged my steps under me, And my feet have not slipped." — Psalms 18:36 (ASV)

Thou hast enlarged my steps under me - The idea here is, “You have made room for my feet, so that I have been enabled to walk without hindrance or obstruction.” So in (Psalms 31:8), “Thou hast set my feet in a large room.” The idea is that he was before restricted, compressed, hindered in his movements, but that now all obstacles had been taken out of the way, and he could walk freely.

That my feet did not slip - Margin, my ankles. The Hebrew word here rendered in the text feet, and in the margin ankles, means properly a joint; small joint; especially the ankle. The reference here is to the ankle, the joint that is so useful in walking, and that is so liable to be sprained or dislocated. The meaning is that he had been enabled to walk firmly; that he did not limp. Before, he had been like one whose ankles are weak or sprained; now he was able to tread firmly. The divine favor given to him was as if God had given strength to a lame man to walk firmly.