In at least one implementation, a method for determining the stiffness of a structure may include connecting a test member to the structure, directing a beam of light at a measurement surface carried by at least one of the test member or the structure, impacting an impact surface carried by the test member with a known impact force, and determining the movement of the measurement surface that is caused by the impact. The movement of the measurement surface may be determined as a function of changes in the length of the beam of light and the stiffness of the structure may be a function of the magnitude of the measurement surface movement.