The purpose of this study was to analyze the effects of lubricin on tendon stiffness and viscoelasticity. A total of 36 mice were tested with 12 mice in each of the following groups: lubricin knock-out (-/-), heterozygous (+/-) and wild-type (+/+). A ramp test was used to determine the elastic modulus by pulling the fascicles to 2.5% strain amplitude at a rate of 0.05 mm/s. Then, followed by a relaxation test that pulled the fascicles to 5% strain amplitude at a rate of 2 mm/s. The fascicles were allowed to relax for 2 min at the maximum strain and a single-cycle relaxation ratio was used to characterize viscoelastic properties. There was no significant difference in the Young's modulus between the three groups (p > 0.05), but the knockout mice had a significantly (p < 0.05) lower relaxation ratio than the wild type mice. Based on these data, we concluded that lubricin expression has an effect on the viscoelastic properties of tendon fascicles. The clinical significance of this finding, if any, remains to be demonstrated. (C) 2011 Elsevier Inc. All rights reserved.