This study examined the effect of changes in end-inspiratory lung volume (EILV) and breathing pattern on neural activation of the crural diaphragm (EMGDIA) and of the sternocleidomastoid (EMGSCM), scalene (EMGSCA), and external intercostal muscles (EMGINT) at constant ventilation (V̇E). Twelve healthy adults performed a series of 30-s breathing trials at a constant V̇E corresponding to 15% of their maximum voluntary ventilation while 1) altering EILV at a constant breathing pattern and 2) altering breathing pattern at a constant EILV. Using a real-time visual display of each participant's spirogram, EILV was voluntarily targeted at 65% (EILV65%), 75% (EILV75%), 85% (EILV85%), and 95% (EILV95%) of each participant's inspired vital capacity, whereas breathing frequency (fR) was targeted at 15, 35, and 50 breaths/min using a metronome. The tidal volume needed for a participant to maintain V̇E constant across trials was achieved via changes in end-expiratory lung volume. A multipair esophageal electrode catheter was used to record EMGDIA, whereas surface electrodes were used to record EMGSCM, EMGSCA, and EMGINT. On average, EMGDIA, EMGSCM, EMGSCA, and EMGINT increased as a function of increasing EILV at constant V̇E, independent of changes in breathing pattern. The magnitudes of these increases were particularly notable in the transition from EILV85% to EILV95%, especially for EMGSCM and EMGSCA. In healthy adults, as EILV increases toward total lung capacity, progressive compensatory increases in neural activation of the diaphragm and extra-diaphragmatic inspiratory muscles are needed to support V̇E, independent of changes in breathing pattern.NEW & NOTEWORTHY We examined the effect of changes in end-inspiratory lung volume (EILV) and breathing pattern on neural activation of the inspiratory muscles in healthy adults. We found that, at a constant ventilation, neural activation of the crural diaphragm and of the extra-diaphragmatic inspiratory muscles (sternocleidomastoid, scalene, and seventh external intercostals) increased as a function of increasing EILV, independent of changes in breathing pattern. Our results point to a critical mechanistic role of EILV in determining the level of central inspiratory neural drive needed to support ventilation.
Keywords: diaphragm; electromyography; inspiratory; lung volume; muscle.