Reliability of diaphragm voluntary activation measurements in healthy adults

Voluntary activation can be used to assess central fatigue of the diaphragm after tasks such as exercise or inspiratory muscle loading. Cervical magnetic stimulation (CMS) of the phrenic nerves elicits an involuntary contraction, or twitch, of the diaphragm. This twitch is quantified based on a measure of transdiaphragmatic pressure and can be used to evaluate diaphragm contractile function and diaphragm voluntary activation (diaphragm-VA). The test-retest reliability of diaphragm-VA using CMS is currently unknown. Thirteen participants (4 male, 9 female; aged 25 ± 3 years) performed a series of interpolated twitch manoeuvres, which included a maximal inspiratory effort against a semi-occluded mouthpiece and 2 CMS-stimuli, 1 during the inspiratory manoeuvre and 1 after when the participant returned to functional residual capacity to quantify diaphragm-VA. Intraclass correlation coefficients (ICCs) and standard error of measurement (SEM) measured between-day and within-session reliability of diaphragm-VA, respectively. Maximal diaphragm-VA values were 91% (SD: 6; SEM: 3.9) and 92% (SD: 5; SEM: 2.2) during visits 1 and 2 (p = 0.68), respectively, and displayed "good" between-day reliability (ICC: 0.88; 95% confidence interval: 0.67-0.95; SEM: 2.7). Our results suggest that assessing diaphragm-VA using CMS is reliable in young healthy adults. Measuring diaphragm-VA may provide additional insight into the consequences and mechanisms of diaphragm fatigue. Novelty: Magnetic stimulation of the phrenic nerves can reliably measure voluntary activation of the diaphragm. Diaphragm voluntary activation can be used to provide additional insight into fatigability of the diaphragm.