I don't disagree that the position of ones heart is very important.
Kneeling and praying is important and taught through example in the Bible many times.
Daniel 6:10 "Now when Daniel knew that the writing was signed, he went into his house; and his windows being open in his chamber toward Jerusalem, he kneeled upon his knees three times a day, and prayed, and gave thanks before his God, as he did aforetime."
Luke 22:41 "And he was withdrawn from them about a stone's cast, and kneeled down, and prayed."
Acts 9:40 "But Peter put them all forth, and kneeled down, and prayed; and turning him to the body said, Tabitha, arise. And she opened her eyes: and when she saw Peter, she sat up."
Acts 20:36 "And when he had thus spoken, he kneeled down, and prayed with them all."
Acts 21:5 "And when we had accomplished those days, we departed and went our way; and they all brought us on our way, with wives and children, till we were out of the city: and we kneeled down on the shore, and prayed."
I believe we find one answer as to why kneeling is important in 2 Chronicles 7:14 "If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways; then will I hear from heaven, and will forgive their sin, and will heal their land."
We show emotions as humans. We cry when we are sad, in mourning, or something deeply touches us. We hang our head down sometimes. We jump up and rejoice when happy. And may I add that we kneel down before our God when we humble ourselves and our heart and call out to Him?