Respiratory muscle activation and action during voluntary cough in healthy humans

Cough is a defensive airway reflex consisting of a modified respiratory act which involves the sequential activation of several laryngeal and respiratory muscles. The contraction of the latter results in thoraco-abdominal volume variations in order to provide enough amount of air available, the operating volume (OV), to be expelled. Because both posture and OV could influence muscular activation and thoraco-abdominal displacements during voluntary cough, we aimed to verify if and how they play a role during inspiratory (ICP) and expiratory (ECP) cough phases, in terms of flow, volumes and surface electromyography activity (sEMG). In 10 healthy subjects, we measured sEMG of 7 muscles (scalene, sternocleidomastoid, parasternal, intercostal, diaphragm (assessed at the 8^th intercostal space), external abdominal oblique and rectus abdominis) in supine and seated position during cough maneuvers performed at 4 different OV measured by opto-electronic plethismography: total lung capacity (TLC), functional residual capacity and two intermediate volumes. The amplitude of sEMG signals tended to be maximal at TLC (p < 0.005) during ICP in the neck and parasternal muscles and during ECP in abdominal muscles. Postures slightly affected only sEMG of the thoracic muscles. sEMG data were similar (p > 0.05) in the other OV, but cough peak flow increased with OV. Thoraco-abdominal volume variations during cough were unaffected by posture and OV as well, being predominantly thoracic (supine: 60 and 64%; seated: 68 and 69%, respectively during ICP and ECP). Our results suggest that voluntary cough OV or posture do not have an important effect on voluntary cough that seems more likely to be resulting from a motor mechanism that activates a synergetic antagonistic contraction of inspiratory and expiratory muscles leading to a specific thoraco-abdominal pattern, in which the rib cage is the predominant.