Breath-by-breath parasternal intercostal muscle stiffening tracks inspiratory effort during graded inspiratory threshold loading in humans

Inspiratory effort is associated with the recruitment of extra-diaphragmatic muscles, including parasternal intercostal muscles (PIMs). The mechanical behavior of PIM and its relationship to indices of inspiratory effort in humans remains poorly characterized. We investigated whether PIM stiffening, assessed using ultrafast ultrasound shear wave elastography (SWE), tracks inspiratory effort breath-by-breath during graded inspiratory loading. Fourteen healthy adults (9 men and 5 women) were studied during quiet breathing and inspiratory threshold loading at 10%, 20%, 30%, and 40% of maximal inspiratory pressure. Inspiratory changes in PIM shear modulus (Δµ) were quantified from SWE and related to esophageal (ΔP̄es) and transdiaphragmatic (ΔP̄di) pressure swings, pressure-time products (PTP̄es and PTP̄di), and crural diaphragm electromyography (EAdi). The ability of Δµ to detect increased inspiratory effort was evaluated per breath using receiver operating characteristic (ROC) analysis. Inspiratory loading induced progressive increases in PIM stiffening (Δµ: 10.2 ± 7.4 to 24.0 ± 15.9 kPa, P < 0.001). Δµ showed strong within-subject correlations with ΔP̄es and ΔP̄di (R = 0.72 and R = 0.68, respectively; both P < 0.0001) and moderate correlations with PTP̄es and PTP̄di (R = 0.50 and R = 0.49, respectively; both P < 0.001). Δµ was also moderately correlated with EAdi (R = 0.45, P < 0.001). Breath-by-breath Δµ discriminated breaths with increased inspiratory effort with a pooled ROC area under the curve of 0.78. PIM stiffening increased proportionally with inspiratory load and was correlated with established pressure- and activation-based indices of inspiratory effort. These findings define the mechanical behavior of the PIM during loaded breathing and identify PIM stiffening as a noninvasive mechanical readout of increased inspiratory effort in humans.NEW & NOTEWORTHY Using ultrasound shear wave elastography, we tracked parasternal intercostal muscle stiffness breath-by-breath during quiet breathing and graded inspiratory threshold loading. Changes in parasternal intercostal muscle stiffness increased progressively with load and were closely associated with indices of inspiratory effort from pressure recordings and diaphragm electrical activity. Stiffness measurements identified breaths with elevated inspiratory effort and may serve as a practical, noninvasive mechanical readout of global inspiratory effort, with potential relevance for respiratory disease assessment and assisted-ventilation monitoring.