4 October 2018 A brief manual of biomechanics (Part 1)

Stress, strain, relaxation, elastic modulus, viscoelasticity, dissipation… The feeling of hearing “blah blah blah” when you hear about biomechanics? No stress! We are decoding for you the physics terms important to the understanding of the properties of biological materials.

Each month, find an illustrated fundamental notion of biomechanics. This month we start with a brief glossary.

Definitions of biomechanical terms :

• Anisotropy: It can be defined as a difference, when measured along different axes, as opposed to isotropy
• Strain : It is defined as the deformation of a solid due to stress (unit-less).
• Stiffness : It is the extent to which an object resists deformation in response to an applied external force (measured in N/m).
• Compliance (or flexibility) : It is the opposite of stiffness. The more compliant the object is, the easier it is to deform (measured in N/m).
• Stress : It can be defined as the ratio of applied force F to a cross section area (measured in Pa or Nm-2).
• Creep : It is the tendency of a solid material to slowly move or deform under the influences of mechanical stresses.
• Elasticity : It is the ability of a material to resist a force and to return to its original shape and size when force causing the deformation are removed (measured in Pa).
• Extensibility (or breaking strain) : It is the maximum value to strain before breakage of the material (unit-less).
• Hysteresis (or energy dissipation) : It is the non linear relationship whereby a force causes a change, but as the force is removed, the reversal in change is not as much as the initial.
• Viscosity : It can be defined as a measure of the resistance of a fluid to deformation under shear stress. It describes a fluid’s internal resistance to flow (measured in Pa.s).
• Viscoelasticity : It is the property of a material of exhibiting both elastic and viscous behaviour, the application of stress causing temporary deformation if the stress is quickly removed but permanent deformation if it is maintained.