Schematic representation of active and passive mechanical inputs and active biological and passive material outputs, in the context of stiffness, force, and displacement. (a) Substrate stiffness (i) is a passive mechanical input. Applied stress or strain (ii) is an active mechanical input. Viscoelastic deformation and stress stiffening (iii) are passive material outputs. Changes in contractile behavior, cytoskeleton remodeling, and the activation of signaling pathways are active biological outputs. (b) Diagram highlighting major relationships (solid arrows) between input and output types. In the context of a cell, more complex relationships often occur (dashed arrows).