An apparatus for testing stiffness characteristics such as modulus of elasticity E of an article such as a piece of lumber moving in a conveying direction transverse to the testing axis, comprises a bearing unit capable of contacting the piece of lumber at two spaced apart portions thereof. There is provided a first deflecting unit including a first working element being normally disposed in a first static position relative to the conveying path for applying a first thrust against a loaded area on the article at an intermediary portion thereof located between the spaced apart portions, to produce an article deflection of a first magnitude. Each working element defines a loading surface extending substantially parallel to the article-conveying path. There is further provided a second deflecting unit including a second working element being normally disposed in a second static position relative to the conveying path for applying a second thrust against a loaded area of the article intermediary portion, to produce an article deflection of a second magnitude, wherein the second position differs from the first position relative to the conveying path by a differential value. The apparatus further comprises load cells coupled to the bearing unit for generating signals indicative of respective magnitudes the first and second thrusts as applied by first and second deflecting units, and a computer for deriving from such signals and differential value an indication of the stiffness of the article, such as modulus of elasticity E.