Electromyographic activity of human triangularis sterni muscles during thoracic rotation

: The triangularis sterni muscles (TS) have been recognized as expiratory muscles, but there is no information on their postural activity or their phasic electromyographic (EMG) activity during thoracic rotation. We examined the EMG activity of the right TS using fine-wire electrodes, in 5 healthy male subjects. With subjects breathing through a mouthpiece, airflow, raw and moving-averaged EMG signals were sampled with a computer during thoracic rotation and resting, accompanied by breathing with or without voluntary thoracic rotation. Maximum EMG (EMGmax) was defined as the greatest moving-averaged EMG activity recorded from each subject during the slow expiratory maneuver from functional residual capacity to residual volume. With leftward thoracic rotation without breathing, the tonic EMG activity of the right TS was observed in all subjects. During resting breathing without thoracic rotation, there were respiration-related phasic activities on TS EMG during expiration in all subjects. With leftward thoracic rotation, the respiration-related phasic activities on TS EMG significantly increased from 8.2 +/- 3.5 (mean +/- SE) %EMGmax to 15.2 +/- 6.1 %EMGmax. On the other hand, with rightward thoracic rotation, it decreased to 3.0 +/- 1.0 %EMGmax. We concluded that 1) TS has a postural function like that of the intercostal muscles, and that 2) the respiratory function of TS is affected by thoracic rotation.