BACKGROUND Chronic ankle instability commonly develops after lateral ankle sprain, but differences in movement patterns between subtypes of ankle instability have not been determined. We hypothesized mechanically and functionally unstable ankle subjects would demonstrate different kinematics and kinetics compared to a group of ankle sprain injury "copers". METHODS Sixty-three recreational athletes, 21 in each of 3 groups (11 males, 10 females) matched for gender, age, height, mass, and limb dominance were tested. Knee and ankle sagittal and frontal plane motion, peak ground reaction forces, and time to peak ground reaction forces were measured during 5 tasks (walk, step down, run, drop jump, and stop jump). RESULTS Using 5 one-way Analyses of Variance with Tukey post hoc testing at alpha=0.05, the mechanically unstable group displayed at least one difference in sagittal and/or frontal plane ankle motion at initial contact, maximum, and/or in displacement than the functionally unstable and coper groups in each task except the run (P<0.05). The greatest number of differences occurred in the drop jump and stop jump tasks. The mechanically unstable group exhibited greater frontal plane displacement and maximum eversion, and decreased sagittal plane displacement and maximum plantar flexion in several tasks. CONCLUSIONS The mechanically unstable group demonstrated altered ankle motion patterns when compared to functionally unstable and coper groups. The landing pattern may help avoid ankle sprains, but may also increase risk of ankle joint degeneration. Unstable ankle groups exhibit different movement patterns and may require different treatment regimens.