Interaction between postural and respiratory activities of the human rib cage muscler during thoracic rotation

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The parasternal intercostals muscles (PARA) and triangularis sterni muscle (TS) are recognized as the primary inspiratory and expiratory muscles of the chest wall, respectively. However there is scanty information on their postural or respiratory activity during thoracic rotation. We examined the electromyographic (EMG) activity of the PARA and TS using fine-wire electrodes in 20 healthy, awake, seated subjects during resting breathing. Fine-wire electrodes were inserted into the right PARA and TS using high-resolution ultrasound. With subjects breathing through a mouthpiece, airflow, raw and moving averaged EMG signals were recorded using a computer, during resting breathing through a series of maneuvers: 1)relaxed position, followed by voluntaiy thoracic rotation while holding the breath, 2)voluntary maintenance of thoracic rotation, 3)passive maintenance of thoracic rotation, and 4) voluntary isometric rotation against a counter force at a rotational angle of zero degrees (without r … More otation). During' the study, the EMG signals were recorded at zero degrees, and at rotational angles of 30 and 45 degrees to both the right and left. During resting breathing without rotation, the TS showed phasic expiratory EMG activity in all subjects in a seated posture. The right PARA showed remarkable tonic EMG activity in all subjects, during rightward voluntary thoracic rotation while holding the breath, but the right TS showed remarkable tonic EMG activity in all subjects during leftward voluntary thoracic rotation. With maintenance of rightward voluntary thoracic rotation during resting breathing, phasic EMG of the right PARA increased significantly but with lefttward voluntaiy thoracic rotation it decreased significantly. In contrast, phasic EMG of the right TS increased significantly with maintenance of lefiward voluntary thoracic rotation during resting breathing, but it decreased significantly with rightward voluntary thoracic rotation. Interestingly, phasic EMG activity of the right PARA with maintenance of rightward voluntary thoracic rotation is significantly greater than that with maintenance of rightward passive rotation, and there was no significant difference between passive rotation and during resting breathing without rotation. That is the same for the TS. EMG activity of the right TS with maintenance of leftward voluntary thoracic rotation is significantly greater than that with maintenance of leftward passive rotation, and there was no significant difference between passive rotation and during resting breathing without rotation. During leftward voluntaiy isometric rotation against the counter force at a rotational angle of zero degrees, phasic EMG activity of the right TS was significantly greater than that without any rotational force. However, phasic EMG activity of the right, is during rightward isometric rotation was smaller than that without any rotational force. it is concluded that 1)TS has more respiratory activity than previously reported, 2)PARA and TS have a postural function as well as respiratory function like other rib cage muscles, 3)There is a definite interaction between postural and respiratory control of the human PARA and TS, and 4)The interaction can not be explained by spinal reflex but higher brain center function. Less