EMG study of respiratory muscles in humans immersed at different water temperatures

The electromyograms of the rectus abdominis (EMGra) and of the diaphragm (EMGdi) have been recorded on human subjects immersed at two bath temperatures (TW), 25 and 40 degrees C. The recordings were obtained during a calibrated isometric contraction sustained for 20 s against a closed stopcock at functional residual capacity (FRC) level for EMGra (expiratory effort) and at pulmonary volume greater than 90% vital capacity for EMGdi and EMGra (inspiratory effort). After eliminating the electrocardiographic artifact, the EMG signal was processed to obtain its root-mean-square (rms) value and three parameters of its frequency spectrum, total energy (Etot), centroid frequency (fc), and high-to-low ratio (H/L). The results show that EMGdi is not modified by TW. On the other hand rms and Etot of EMGra are always increased at TW = 25 degrees C compared with TW = 40 degrees C, whereas fc and H/L decrease with temperature during the expiratory effort at FRC level but do not vary during inspiratory effort at high pulmonary volume. These results, compared with those previously published for cooled limb muscles, show that TW can elicit EMG alterations on the superficial respiratory muscles through two mechanisms, an intrinsic mechanism due to the local variation in muscle temperature and an extrinsic mechanism acting upon the control system of the muscle contraction. Linked alterations of the muscular mechanical activity probably account for the observed effects of TW on the statics and the dynamics of the pulmonary volumes.