… I was interested in stretching because my knees had begun hurting in the second week of full time training. I had already asked Master Chen about that. He hadn’t been surprised.
“It is because of your kua and your feet,” he had said. “Never plant your foot.”
He had pointed to a tripod in the corner and said, “Taiji is about relationships. None of the feet are planted to the floor, but the tripod is very stable.”
— Source: Sore Knees | Chen Practical Method – Chen Zhonghua