According to Heschel’s daughter Dr. Susanna Heschel:
(http://www.dartmouth.edu/%7Evox/0405/0404/heschel.html)

"When he came home from Selma in 1965, my father wrote, 'For many of us the march from Selma to Montgomery was about protest and prayer. Legs are not lips and walking is not kneeling. And yet our legs uttered songs. Even without words, our march was worship. I felt my legs were praying.'"