Triangularis sterni muscle use during eupnea in humans: effect of posture.

The electromyograms of the triangularis sterni (transversus thoracis), of the abdominal external oblique, and of the deeper abdominal muscle layer (internal oblique or transversus abdominis) were recorded with concentric needle electrodes in twenty normal naive subjects breathing quietly in the supine and the standing posture. The triangularis sterni and abdominal muscles were usually silent in the supine posture. In contrast, all subjects in the standing posture showed activity in the abdominal muscles, and sixteen subjects also had activity in the triangularis sterni. The abdominal muscle activity was principally tonic, unrelated to the phases of the breathing cycle, whereas in fifteen subjects the triangularis sterni activity was confined to expiration. Expiratory activation of the triangularis sterni was more frequently observed in the older than in the younger subjects of the study. These observations indicate that unlike in the supine posture, most normal subjects when breathing at rest in the standing posture recruit both the triangularis sterni and the abdominal muscles. This recruitment may compensate for the adverse effects of the standing posture on the diaphragm and rib cage inspiratory muscles.