Xin Yi (also called Xin Yi Quan) is the oldest of the ‘internal’ martial arts, and possibly something of a precursor to Taijiquan. It was certainly the precursor to XingYi Quan.
There are numerous different regional and family styles of Xin Yi. The basic exercise used to develop the body in the Dai family version of Xin Yi is known as Squatting Monkey. Here Xin Yi practitioner Steve Chan goes through the basics of the exercise. It’s a fascinating look into the fundamentals of ‘internal’ movement.