Posture
The term "posture" refers to the way the body is held (e.g., straight, slumped, etc) and its position when sitting or standing. For singing, it is best to stand rather than sit, as this allows the most open and unrestricted passage of air for both breathing and singing.
Of course, if you are accompanying yourself on piano you will have to sit. In this situation, it is essential to sit up straight but relaxed for the best sound. Compare the correct and incorrect singing postures as demonstrated the following diagrams.
Incorrect
The spine is not straight and the head and pelvis both tilt forward. In this position, it is not possible to move freely or produce the best sound.
Correct
The spine is comfortably straight and in line with the head, legs and pelvis. This position keeps the airways open and makes movement easy and comfortable.
If you think of a situation where a singer is performing with a band, it would look fairly dull if the singer stood straight in the one position all the time. Movement is a large part of any stage show. This means it is not always possible for the singer to maintain perfect posture.
However, it is possible to keep the pathway from the diaphragm to the mouth open, flexible and relaxed most of the time, which means it is still possible to sing well while moving around. Relaxation and flexibility are the keys to good posture regardless of standing or sitting position.
