From compliance to moduli: clarifying basic mechanical properties of biological tissues

In physiology education, terms such as compliance, distensibility, capacitance, elastance, elasticity, etc. refer to the mechanical properties of biological tissues but are often a source of confusion in teaching and learning. This article identifies three main causes underlying the confusion: 1) inconsistent interpretations of mathematically defined terms, 2) a gap between physiologic and material science terminology, and 3) a lack of illustration of the interrelationships among these terms. To address these problems, we divide these terms into three groups: terms measuring how easily a structure can be deformed, terms measuring the stiffness of a structure (i.e., how resistant a structure is to deformation), and other terms. Through clarifying the terms in groups and introducing two fundamental terms in material science (Young's modulus and bulk modulus) that are often missing in physiology education, the problems above are resolved and a relatively complete picture of the mechanical property-related terms is provided. This article serves as a critical resource for physiology educators, researchers, and clinicians and a robust foundation for improved teaching, research, and clinical applications of tissue mechanics.NEW & NOTEWORTHY This article advances physiology education by clarifying the basic mechanical property-related terms in physiology, introducing two fundamental terms in material science that are often missing in physiology education, and revealing the interrelationships among these terms. It serves as a handy tool kit for physiology educators, researchers, as well as clinicians to select and apply these terms appropriately based on their needs.