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

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.