You learn the most, when you are most tired.Chen Yingjun

Chen Yingjun told me that when i was learning Taijiquan (Tai Chi) from him in 2005. When you are the most tired, your body will try to resort to its ingrained, hard-wired ways of moving and generating power – among other reasons, because the body’s old ways use muscle groups that are more resistant to fatigue, precisely because those are used to prolonged effort. When we try to change the body’s way of moving to a new one, as in Taijiquan (Tai Chi), the muscles are used in a new and different way, and weaker muscle groups must be strengthened up to the standards of the stronger groups.
Only by practicing Taijiquan (Tai Chi) to the point when fatigue arrives is it possible to simulate the demands imposed by pushing with a strong opponent. This means that when you push hands with someone who’s strong, your body will feel and react the same way it does at the end of a very tiring Taijiquan (Tai Chi) class. That’s when it will try to revert to old movement and power patterns, the old patterns that you are trying to abandon and make better. Only if you keep practicing when you’re very tired will you be able to resist this tendency to revert back.
The catch here is that you must be very aware of what you are doing, and you shouldn’t demand more from your body than it is capable of delivering. When we are exhausted it’s much easier to make mistakes and get hurt, in any sport or physical training.