Sensei Nishiyama used to start every class with checking our posture, he knew that all other elements of our technique depend on posture and our potential can be maximize by good posture or limited by bad posture.
In karate when we talk about posture most important is the head relationship to the neck and torso and the alignment of the center or the sacrum and pelvic.
In regards to the center we say "Upper stomach (at belly button) back, low stomach forward and sacrum straight to floor".
Sacrum straight to floor is more of a feel and prevention of forward tilt since the sacrum should be at about 30 degrees.
Belly button back help activate the inner unit muscles to stabilize the spine and prevent over extension of the lumbar.
Low stomach forward is not an actual movement but direction, and prevention of forward tilt and lumbar over extension, it allows the muscles to be at optimal length for function.
We constantly talk about moving and making power from the center, but if our pelvic and sacrum are not aligned properly, all the hard training and repetitions will take us in the wrong directions.
Looking at it from another angle, the sacrum, the feet and the cervical spine are the richest areas in proprioceptors, which give us feedback as we move and interact with gravity, momentum and ground reactions. If the muscles in those areas are out of balance, proprioceptive feedback will not be accurate, which will reduce performance and cause injuries.
This is when the lumbar lordosis is exaggerated, lower back is arched and butt stick out.
Hip flexion while standing erect can be the result of weak or over lengthened external oblique or lower rectus abdominis muscles and or short and stiff hip flexor muscles.
When the hip flexor is short relative to the abdominal muscles, instead of compensation in hip motion, there may be exaggerated anterior pelvic tilt and increased lumbar extension.
In most exercise programs more attention is given to stretching the hamstring muscles and little attention to maintaining the length of the hip flexor muscles or keeping the strength and optimal length of the abdominal muscles that prevent anterior pelvic tilt.
In gait in every step require maximum excursion of the hip flexors but minimal excursion of the hamstring muscles.
Maintaining optimal hip flexors length and optimal performance of the abdominal muscles is important in preventing increased lumbar lordosis and excessive anterior pelvic tilt, particularly under dynamic conditions such as walking, running or karate.
Because of the above, I believe that in our karate training we always have to include corrective exercises so we don’t keep on getting stronger in an imbalanced way.