The heat of the body can release in the woods absorbed by nature but artificial environments cannot absorb the body heat; the body heat is trapped and cannot be released causing disease and heart attack. So you need to release the body heat or body will overheat burning up the internal organs (disease) by deep breathing moving the heat out of the body.
The Psoas muscle connects & moves the hips, spine & legs; all movement of Tai Chi Chi Kung comes from the Psoas muscle. The kidneys are connected to Psoas muscle & nerves are behind move it. The Psoas muscle is different from all other muscles, when you are relaxed it shortens & when you move it lengthens but problem is when you don't move it is shortens, and does not grow. Eventually it shortens until you cannot move it & your body tightens up.