Contraction of the human diaphragm during rapid postural adjustments

TLDR

The study examined how the diaphragm responds to the postural perturbation caused by rapid shoulder flexion to a visual stimulus in standing subjects. Researchers recorded gastric, oesophageal, and transdiaphragmatic pressures, intramuscular and oesophageal diaphragm EMG, and ultrasonographic changes in diaphragm length to assess contraction mechanics. Diaphragm EMG preceded deltoid activity by ~20 ms, occurred regardless of breathing phase, coincided with transversus abdominis activation, and was followed by ~13.8 cmH₂O gastric and 13.5 cmH₂O transdiaphragmatic pressure increases 49 ms later, with ultrasonography showing costal diaphragm shortening and subsequent lengthening, demonstrating the diaphragm’s role in postural stability during sudden limb movement.

Abstract

The response of the diaphragm to the postural perturbation produced by rapid flexion of the shoulder to a visual stimulus was evaluated in standing subjects. Gastric, oesophageal and transdiaphragmatic pressures were measured together with intramuscular and oesophageal recordings of electromyographic activity (EMG) in the diaphragm. To assess the mechanics of contraction of the diaphragm, dynamic changes in the length of the diaphragm were measured with ultrasonography. With rapid flexion of the shoulder in response to a visual stimulus, EMG activity in the costal and crural diaphragm occurred about 20 ms prior to the onset of deltoid EMG. This anticipatory contraction occurred irrespective of the phase of respiration in which arm movement began. The onset of diaphragm EMG coincided with that of transverses abdominis. Gastric and transdiaphragmatic pressures increased in association with the rapid arm flexion by 13.8 ± 1.9 (mean ± s.e.m. ) and 13.5 ± 1.8 cmH 2 O, respectively. The increases occurred 49 ± 4 ms after the onset of diaphragm EMG, but preceded the onset of movement of the limb by 63 ± 7 ms. Ultrasonographic measurements revealed that the costal diaphragm shortened and then lengthened progressively during the increase in transdiaphragmatic pressure. This study provides definitive evidence that the human diaphragm is involved in the control of postural stability during sudden voluntary movement of the limbs.