Effect of end-inspiratory lung volume and breathing pattern on neural activation of the diaphragm and extra-diaphragmatic inspiratory muscles in healthy adults

Abstract

This study examined the effect of changes in end-inspiratory lung volume (EILV) and breathing pattern on neural activation of the crural diaphragm (EMG_DIA) and of the sternocleidomastoid (EMG_SCM), scalene (EMG_SCA), and external intercostal muscles (EMG_INT) at constant ventilation (V̇_E). Twelve healthy adults performed a series of 30-s breathing trials at a constant V̇_E 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% (EILV_65%), 75% (EILV_75%), 85% (EILV_85%), and 95% (EILV_95%) of each participant's inspired vital capacity, whereas breathing frequency (f_R) was targeted at 15, 35, and 50 breaths/min using a metronome. The tidal volume needed for a participant to maintain V̇_E constant across trials was achieved via changes in end-expiratory lung volume. A multipair esophageal electrode catheter was used to record EMG_DIA, whereas surface electrodes were used to record EMG_SCM, EMG_SCA, and EMG_INT. On average, EMG_DIA, EMG_SCM, EMG_SCA, and EMG_INT increased as a function of increasing EILV at constant V̇_E, independent of changes in breathing pattern. The magnitudes of these increases were particularly notable in the transition from EILV_85% to EILV_95%, especially for EMG_SCM and EMG_SCA. 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 V̇_E, 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.