Fig. 2

Strain/time curves for elastic (spring), viscous (dash in pot), and viscoelastic (spring and dashpot) systems. An applied force (red arrows) generates a stress that results in a yield or strain once the force reaches critical opening pressure. Upper left: the spring models elasticity with a rapid increase in strain leading to a plateau strain, which is distinctive of that spring. Upper right: the dashpot models viscous strain, where movement of the dash progresses (dashed line) with flow of the fluid in the pot around the dash (brown arrows), which is distinctive of the viscosity of the fluid. Bottom: viscoelastic behavior is modeled by the spring and dashpot, where force transfer from the spring to the dash results in a time-dependent strain with an initial rapid change in strain (1–2 s), which becomes gradual over time (2–5 s). Lung strain follows this behavior (Fig. 3). Bottom left: an extended inspiratory time (HOLD) optimizes lung recruitment once critical opening pressure is reached. Bottom right: a short expiratory time (RELEASE) minimizes lung derecruitment if it is sufficiently fast to prevent reaching the critical collapse pressure