In biomechanics, which term describes internal resistance to deformation?

• A. Stress
• B. Strain
• C. Load
• D. Pressure

Answer: (A)

Internal resistance to deformation under load is described by stress. When a force acts on tissue, internal forces develop within the material to oppose the change in shape or length, and these internal forces per unit area define stress. Stress is quantified as force divided by area (for example, N/m^2, or pascals) and it captures how strongly the material resists being deformed.

Strain, in contrast, measures the amount of deformation itself—how much the material changes shape or length relative to its original size. Load refers to the external force applied to the body, not the tissue’s internal reaction. Pressure describes force per area typically in fluids or at contact surfaces, not the internal resistance inside a solid under load.

So, the internal resistance to deformation is best described by stress, which is the material’s internal response to the external load.