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 (PIM). 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, 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, PTP̄di), and crural diaphragm EMG (EAdi). The ability of Δµ to detect increased inspiratory effort was evaluated per breath using 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 AUC 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 non-invasive mechanical readout of increased inspiratory effort in humans.