Timing of activation of different inspiratory muscles during incremental inspiratory loading in healthy adults: A cross-sectional study.

To evaluate whether the onset and duration of electromyography (EMG) activity of different inspiratory muscles vary during an incremental inspiratory threshold loading (ITL) in healthy adults and whether it is associated with dyspnea and inspiratory mouth pressure (Pm) at task failure. Twelve healthy adults (30 ± 7 years, six females) performed incremental ITL starting at warm-up (7.6 ± 1.7 cmH2O), followed by 50 g increments every two minutes until task failure in this cross-sectional study. EMG onset (relative to inspiratory flow) and activity duration of the costal diaphragm/7th intercostal and extra-diaphragmatic inspiratory muscles (scalene, parasternal intercostal, sternocleidomastoid) were quantified using a validated algorithm. Ventilatory parameters, including Pm, were evaluated. With increasing ITL, Pm increased (p ≤ 0.033), accompanied by increased EMG activity of extra-diaphragmatic muscles (p ≤ 0.016). Critically, the EMG onset of the sternocleidomastoid (p < 0.001), parasternal intercostal (p = 0.002), and scalene (p = 0.002) occurred earlier relative to inspiratory flow at task failure compared to lower loads. Earlier EMG onsets of these muscles were correlated with higher Pm at task failure (sternocleidomastoid: r = -0.65; parasternal intercostal: r = -0.45; scalene: r = -0.29; p ≤ 0.034). Notably, earlier EMG onsets of scalene at low loads were associated with higher Pm at task failure (r ≤ -0.75; p ≤ 0.026). Furthermore, an earlier EMG onset of the parasternal intercostal (r = -0.67; p = 0.023) and sternocleidomastoid (r = -0.65; p = 0.023) at task failure was associated with greater dyspnea intensity. Appreciation of timing of inspiratory muscle EMG may provide further insight into understanding the contributors to ventilatory task failure and dyspnea.