To reach a level of effectiveness in self-defense, all responsive movement must be internalized to the point where it is executed automatically. If you have to think about the movement, it is too late.
Because Taiji combines co-ordination, balance and the internalization of complex motor programs, advanced practitioners would provide excellent subjects to further investigate how the brain is involved in motor programming, exploring connections between the Prefrontal cortex (responsible for initiation and programming of motor sequences) the dorsal and ventral stratum of the basal ganglia (responsible for habit formation), and the cerebellum (responsible for balance, co-ordination and control of voluntary movement).
The way movement is controlled in the brain suggests the basal ganglia region is ‘exercised’ in Taijiquan. On a movement basis, the motor cortex sends info to the basal ganglia and cerebellum, and both send info back via the thalamus.
The overall effect of the basal ganglia on the thalamus is inhibitory, while the overall effect of the cerebellum is excitatory.
An important function of the basal ganglia is to stop (put the brakes on) unwanted movement. Taiji movement strives for perfect efficiency with absolutely no extraneous or purposeless movement – as the classics say, “no excess, no deficiency”.
Smooth, coordinated, efficient movement is a function of the balance between the inhibitory (Yin) basal ganglia and the excitatory (Yang) cerebellum systems and their communication with the motor cortex. The balanced interplay of Yin and Yang is the very definition of Taiji.