ABSTRACT
Changes in posture common in everyday life exert forces that, if uncompensated, would alter the operational length and, thereby, the contractile strength of respiratory muscles. The principal inspiratory muscle, the diaphragm, does the major part of the work during quiet breathing. The diaphragm is unusual among skeletal muscles in undergoing large changes in its resting length. In the supine human, the weight of the abdominal contents stretches the diaphragm slightly, allowing it to operate in an advantageous portion of its length-tension curve. When one assumes an upright posture, the weight of abdominal contents is removed from the diaphragm, resulting in shorter sarcomere operating length. If there were no compensatory changes in neural drive, this would result in a much smaller tidal volume, because the shortened muscle generates less force at any given neural drive (Pengelly et al., 1971; Eyzaguirre and Fidone, 1975; Kim et al., 1976). Quadriplegic patients, with C-1 lesions, whose diaphragms are electrically paced by the phrenic nerve provide a unique opportunity to study the function of the diaphragm with constant neural drive. When these patients are tilted from supine to upright, the tidal volume falls 50% or more, as predicted on the basis of muscle mechanics (Danon et al., 1979; Strohl et al., 1984; Fig. 1). This fall in tidal volume does not occur in the intact human because of reflex adjustments to respiratory muscle drive that are described in this chapter.
