Leg stiffness: Comparison between unilateral and bilateral hopping tasks

Leg stiffness is a predictor of athletic performance and injury and typically evaluated during bilateral hopping. The contribution of each limb to bilateral leg stiffness, however, is not well understood. This study investigated leg stiffness during unilateral and bilateral hopping to address the following research questions: (1) does the magnitude and variability of leg stiffness differ between dominant and non-dominant legs? (2) Does unilateral leg stiffness differ from bilateral leg stiffness? and (3) Is bilateral leg stiffness determined by unilateral leg stiffness? Thirty-two physically active males performed repeated hopping tests on a force platform for each of the three conditions: bilateral hopping, unilateral hopping on the dominant leg, and unilateral hopping on the non-dominant leg. Leg stiffness was estimated as the ratio of the peak vertical force and the maximum displacement using a simple 1-D mass-spring model. Neither the magnitude nor variability of leg stiffness differed between dominant and non-dominant limbs. Unilateral leg stiffness was 24% lower than bilateral stiffness and showed less variability between consecutive hops and subjects. Unilateral leg stiffness explained 76% of the variance in bilateral leg stiffness. We conclude that leg stiffness estimates during unilateral hopping are preferable for intervention studies because of their low variability.