Did Siddartha have a back problem? in Week 3 Mindfulness of Emotions an online course at:http://www.insightmeditationcenter.org/books-articles/articles/introduction-to-meditation-transcripts/all/1/

Gil Fronsdal says "... the Buddha had a bad back. You wouldn’t believe it with his good posture, but he had a bad back and sometimes he couldn’t give his Dharma talk, so he’d lie down instead and have one of his senior disciples give a talk."

Anyone know where he might have got this information from?
So the Blessed One, putting on his robe and taking up his bowl &amp; outer
  robe, went together with a community of monks to the reception hall.
  On arrival he washed his feet, entered the hall, and sat with his back
  to the central post, facing east. The community of monks washed their
  feet, entered the hall, and sat with their backs to the western wall,
  facing east, ranged around the Blessed One. The Kapilavatthu Sakyans
  washed their feet, entered the hall, and sat with their backs to the
  eastern wall, facing west, ranged around the Blessed One. Then the
  Blessed One — having spent most of the night instructing, urging,
  rousing, &amp; encouraging the Kapilavatthu Sakyans with a Dhamma talk —
  said to Ven. Ananda, "Ananda, speak to the Kapilavatthu Sakyans about
  the person who follows the practice for one in training. [2] My back
  aches. I will rest it." - Sekha-patipada Sutta : MN 53


http://www.accesstoinsight.org/tipitaka/mn/mn.053.than.html