Shunryu Suzuki, from a dharma talk in Not Always So
"Zazen practice is a very subtle thing. When you practice zazen, you become aware of things you did not notice while you were working. Today I moved stones for a while, and I didn’t realize that my muscles were tired. But when I was calmly sitting zazen, I realized, “Oh! My muscles are in pretty bad condition.” I felt some pain in the various parts of my body. You might think you could practice zazen much better if you had no problem, but actually some problem is necessary. It doesn’t have to be a big one. Through the difficulty you have you can practice zazen. This is an especially meaningful point, which is why Dogen Zenji says, 'Practice and enlightenment are one.' Practice is something you do consciously, something you do with effort. There! Right there is enlightenment."