Scalene and sternomastoid muscle function.

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Abstract
The present study tests and proves the hypothesis that scalene muscles contract with increasing strength during upward shift in respiratory position and delineates factors related to onset of accessory muscle activity. The scalene electromyogram obtained through needle electrodes was electrically rectified and integrated to quantify isometric muscle strength expressed as percentage of maximum. Sternomastoid electromyogram, scalene electromyogram, and integral, intraesophageal pressure, mouth pressure, lung volume (bag-in-box system with wedge spirometer), and airflow (pneumotachograph) were recorded simultaneously. In all subjects scalene strength progressively increased as actively maintained, but stationary respiratory position approached full vital capacity. Comparison of scalene strength with intraesophageal pressure during Muller maneuvers showed scattered data; however, a relationship exists. Onset of muscle activity was found related to respiratory rate, end-expiratory position, rapidity of inspiration, and volume inspired. During maximum ventilatory efforts muscle onset preceded inspiration by up to 0.2 sec. The data delineate factors calling into play scalene and sternomastoid muscles and demonstrate a relationship between scalene force, intrathoracic pressure, and lung volume.