A.T. Robertson Commentary Acts 20:36

"And when he had thus spoken, he kneeled down and prayed with them all." — Acts 20:36 (ASV)

He kneeled down (θεις τα γονατα αυτου). Second aorist active participle of τιθημ, to place. The very idiom used in 7:60 of Stephen. Not in ancient writers and only six times in the N.T. (Luke 22:41; Acts 7:60; Acts 9:40; Acts 20:36; Acts 21:5). Certainly kneeling in prayer is a fitting attitude (cf. Jesus, Luke 22:41), though not the only proper one (Matthew 6:5). Paul apparently prayed aloud (προσηυξατο).