Stiffness

Engineering definition

Stiffness, the amount of force required to cause a certain amount of movement, is often referred to as the spring constant, k. It is generally of constant magnitude and expressed using units of N/mm or lbf/in. The stiffness of a mechanical component can vary. For example, the coil spring supporting the wheels of an automobile will have a larger stiffness than the coil spring in a pen. The elastic modulus represents the stiffness of a material.

Medical definition

For clinicians, stiffness is associated with the resistance of a joint to move. A stiff joint may be swollen, requiring greater force to bend it and it may have a reduced range of motion. Clinicians will often use their hands to feel a joint to determine if it moves easily or is stiff.

Additional subdisciplines

Adding to the confusion are biomedical engineers and exercise scientists. In biomechanics, system identification methods can be used to determine the characteristics of an unknown system such as the human body. External perturbing force may be applied to a living person, and the body’s response is measured. The effective mass, damping, and stiffness can be determined by fitting the collected data to a mathematical model of the body (second order differential equation). Overall joint stiffness is composed of multiple subcomponents including passive tissue stiffness, reflexes, and voluntary control.