|When doing high reps of keriwaza, it is hard to keep good posture.|
I could go on promoting good posture but will rather just give two more `pros', which I have not covered... Proper shisei helps one to breath well and is good for the human body. As far as kokyu (breathing) goes, "slumping the body tends to cause one's breathing to be shallow" and, thus, is less efficient and unhealthy. If one thinks of the body as a machine, you can simply imagine that bad shisei is like setting up and/or using the machine incorrectly: the output/productivity will certainly be less. Insofar as health goes, good shisei eradicates the danger of 'uneven distribution of weight on the spine' and, indeed, and other joints. Everyone knows that bad posture is one of the prime culprits behind back pain and back/neck injuries as a whole, so I will leave that there.
More than physical gains, shisei also assists one psychologically. Standing tall from the waist up makes one feel `genki' (positive/energetic) and also conveys jisshin (self-confidence) to others. Conversely, while a 'bad shisei' won't necessarily mean that one is lacking confidence or is sad; or that they are tired; however, it certainly won't help a person feel otherwise. The bottom line is that bad posture, when one is not totally exhausted or facing illness, can be summarized as either laziness and/or lack of self-awareness: both of which, needless to say, are not in one’s best interest.