Unlike speaking, singing often requires large lung volumes at the beginning of a phrase. Therefore, singers must learn to balance this passive expiratory force from muscular forces to sing dynamics such as, for example, pianissimi at high lung volumes, since when the lungs are full of air, this passive force generates high pressure.
To do this, singers must contract the main inspiratory muscles (i.e. the diaphragm and the external intercostals). With a decrease in lung volumes as the musical phrase progresses, this need progressively diminishes until passive expiratory forces generate a subglottic pressure (Psub) lower than that necessary for the tone produced.
From that moment on, the strategy will be to activate the expiratory muscles in order to maintain sufficient thoracic pressure to push the air out of the lungs and maintain the vibration of the vocal cords.
In summary, to sing a pianissimo, singers must recruit a compensatory activation of the inspiratory muscles, while singing a fortissimo the normal expiratory process may suffice, especially when the lungs are at full volume.
Gravity is another parameter that influences Psub. In the upright position, gravity acts as an inspiratory force pulling the diaphragm down. In the supine position, however, gravity acts as a force on the abdominal contents, facilitating the return of the diaphragm into the thoracic cavity. A supine body position increases awareness of the contraction of the diaphragm and encourages a less forceful contraction of the abdominal wall, thus providing a possible strategy for singers who tend to excessively contract the abdominal wall during singing.
During inhalation in a standing position, this gravitational force also exerts a mechanical force on the larynx, tracheal traction. As the diaphragm lowers and stretches the lung tissue downward, the trachea also lowers. Being inserted into the lower portion of the larynx at the bottom of the cricoid cartilage, the larynx is necessarily pushed down with it. As a result, tracheal traction exerts its force on the vocal folds, opening them slightly.
Posture has a huge influence on lung volume and indirectly on Psub. This, in one respect, explains why good posture has commonly been called an important asset in a singing performance.
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