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Lack of standardized test procedures has resulted in considerable variation in reported values for bone strength. Such variation can be attributed in part to the type of instruments used to determine physical properties of bone, procedures used to prepare the bones for testing and equations used to calculate strength. If bone strength is to be used as a major criterion of response in mineral nutrition research, standardization of procedures for measuring and reporting bone strength is essential. Traits that describe the mechanical properties of bone as determined in the commonly used flexure test in which force is applied perpendicularly to the longitudinal axis are bending moment, stress, moment of inertia, strain and modulus of elasticity. Bending moment is a measure of the amount of force withstood by the bone, whereas stress is a measure of force per unit area of bone. Stress allows comparisons to be made between bones that differ in size and shape. The moment of inertia is a measure not only of the area over which the force is applied, but also of the shape in which the area is distributed. Strain is a measure of the amount of bending per unit of length that occurs as the bone is tested. The modulus of elasticity is a measure of the rigidity of the bone or, more simply, is the stress to strain ratio. Instruments that allow the researcher to control the rate of deformation as well as to record the force and deformation are important. Since the modulus of elasticity is affected by the rate of deformation, a standard rate of 5 mm/min is suggested.