Abstract In order to characterize the mechanical behaviour of human tendon in vitro, we have developed a testing apparatus which applies a small periodic displacement to a specimen pre-stretched at different extension ratios (1.005–1.05). The electropneumatic vibrator we have used in this device has a frequency range from 0.5 to 15 Hz. In the study, we measured the periodic displacements and stresses of flexor digitorum tendons of the human hand and calculated therefrom the dynamic tangential elastic modulus of this biological tissue. As expected, the tendon bahaves as a nonlinearly viscoelastic body, the influence of the initial elongation being more important than the influence of the frequency of the deformation. The data of the 15 tendons tested are compared by means of a t′ o-parameter linearization for the real and imaginary part of the dynamic tangential elastic modulus.