Objectives. To determine the inter-day reliability and concurrent validity of a test of unilateral triceps-surae stiffness.Design. Cross-sectional study.Setting. Human Performance Laboratory, University of Technology, Sydney.Participants. Twenty healthy males with no history of injury to the lower body musculature.Main outcome measures. Stiffness measures of the triceps-surae using varying loads of maximum isometric force, conducted approximately seven days apart to assess inter-day reliability. Drop, countermovement and static jump performance, and isometric force and rate of force development (RFD) were also assessed as measures of force production and athletic performance.Results. Intra-class correlation coefficients showed very good reliability for the test of ankle stiffness (r=0.87-0.94, P<0.01). Maximal stiffness was also significantly correlated to isometric RFD (P<0.05). Drop jump performance at various heights in compliant subjects was improved when compared to relatively stiff subjects (P<0.05-0.01).Conclusions. The current test of unilateral ankle stiffness is both a valid and reliable measure. Stiffness is an important determinant of a joint's force production capabilities and performance in stretch shortening cycle activities.