Abstract
Occurrence of laryngeal constriction, i. e. air trapping, which is thought to stabilize the thorax for assisting upper limb movements, were investigated in human subjects by means of endoscopic and EMG recordings. Subjects employed in the experiments were two Japanese gymnasts; one won a silver medal at the 1984 Los Angeles Olympic, and the other one held the All Japan Gymnastics Championship (horizontal bar) title for five consecutive years. They were asked to perform back hip circles (BHC) on the horizontal bar. Laryngeal dynamic gestures were recorded by means of a specially designed endoscopic video system. EMGs were recorded for 9 muscles of the upper extremity, shoulder girdle, and trunk using surface electrodes. Vertical and horizontal components of reaction forces from the bar during BHC were measured by strain gages attached to the bar. Side view motion pictures were recorded by a video camera. Signals of the video frame (laryngeal gestures and motion pictures) and force curves were simultaneously recorded with the EMGs. Very little laryngeal closure was hardly observed during BHC with the body attached to the bar. Activity was not remarkable in the muscles acting on the shoulder joint. However, in the case of BHC with the body completely detached from the bar (bar-free BHC), strong laryngeal sphincteric actions were consistently recognized. In the latter case, a marked co-contraction of the triceps brachii long head and posterior portion of the deltoid occurred in the first half, and that of the biceps barchii long head and anterior portion of the deltoid in the latter half, of the laryngeal closure. A remarkable increase in strain values from the bar also coincided with the closure. Dynamically, extension torques are required in the shoulder joint in the first half, and flexion torques in the latter half of the bar-free BHC. The muscle activities observed above appear to cope with these torques. From these results, it has been strongly suggested that laryngeal sphincteric actions are required to assist the shoulder extensors and flexors in compensating for the increased load imposed on the shoulder joint.
