Sixteen subjects with arthroscopically verified anterior cruciate ligament (ACL) deficiency and 16 subjects without knee problems participated in this study. The functional ability of the ACL-deficient subjects was assessed with the Noyes knee rating system. Each subject completed a landing task in which they hopped from a box 300mm in height to land on one leg. Vertical ground reaction forces and anterior tibial accelerations were recorded, while electrogoniometers measured angular displacement at the knee and ankle joints. Electromyographic (EMG) signals from the vasti lateralis and lateral hamstrings were also recorded. The active stiffness of the hamstring muscles was also assessed in the ACL-deficient subjects. No significant differences (p > 0.05) were observed between the ACL-deficient group and the normal knee group for the landing task. Nor were any significant correlations (p > 0.05) observed between the kinematic, electromyographic variables and the functional ability of the ACL-deficient subjects. However, the active stiffness of the hamstring muscles was positively correlated with functional ability (r = 0.73, p < 0.05), whereas it was negatively related (r = -0.83, p < 0.05) to the magnitude of the EMG activity observed during the landing task. These findings suggest that there is an interplay between the stiffness characteristics of the hamstring muscles and their activation by central processes. It was apparent that not all ACL-deficient subjects take on a landing strategy to minimize stress to their knee joints.