The body is subject to pain, illness, disease and death. During practice, the body aches and gets tired, but the body is essentially the same as the Dharma body. The Dharma body is fundamentally pure, liberated and enlightened. You might ask, "Where is this pure and liberated Dharma body?" I would answer with a question, just like the master did in the story above: "What isn't already a pure, liberated Dharma body?"
There is no difference between the physical body and the Dharma body, but you have not realized it yet. Until you do, you will be bothered by pain and fatigue. Only when you discover that the physical body and Dharma body are identical will you be liberated from your problems.
During the T'ang Dynasty, there was a Ch'an master who was a highly accomplished practitioner. The Emperor heard about him, and requested that he come to the palace to try and cure one of his concubines, who was extremely ill. The Ch'an master refused to come. He said to the messenger, "I am not a physician. I am a practitioner. My purpose is to practice."
When the Emperor heard the master's reply, he spoke to his general. He said, "Go and ask this master to come. If he refuses, cut off his head."
The Ch'an master knew ahead of time of the general's intentions, so he left his temple and walked down the road to wait for him. Soon the general arrived, and when he saw the master, he asked, "Why are you waiting here?"
The Ch'an master replied, "I don't want my dirty blood to defile the temple floor, so I have come here to offer you my head."