Chronic ankle instability, repetitive giving way of the ankle, commonly develops from an initial ankle sprain. Our purpose was to identify factors contributing to ankle sprain, and whether or not kinematic, kinetic, and surface electromyography differences existed between mechanically unstable (MAI), functionally unstable (FAI), and comparison groups of subjects performing five different tasks (walking, stepping up and over, running, drop jumping, and stop jumping). There were 11 male and 10 female subjects in each of the three groups, matched by gender, age, height, mass, and limb dominance. An electromagnetic tracking system, coupled with a forceplate and telemetered surface electromyography were used to collect data. Unstable ankle subjects reported repeated episodes of spraining, and MAI subjects displayed positive anterior drawer and/or talar tilt tests. Using estimates of adjusted means, 95% confidence intervals, and effect sizes, we noted the MAI group displayed a pattern across tasks of increased dorsiflexion and eversion, increased frontal plane displacement and decreased sagittal plane displacement, with slower time to peak anterior ground reaction force in comparison with the FAI and comparison group. The FAI group demonstrated increased tibialis anterior mean amplitude as a percentage of maximum voluntary isometric contraction, but decreased lateral gastrocnemius mean amplitude. The coefficient of variation and standard deviation (SD) were obtained from an ensemble curve of each variable from the 8 test trials. The unstable groups displayed greater loge SD in the ankle inversion-eversion motion than the comparison group. The MAI group demonstrated smaller SD values for each the tibialis anterior, peroneals, and lateral gastrocnemius in comparison to the FAI group. The altered movement pattern may be a coping mechanism designed to keep the ankle in a stable position, perhaps by relying on bony stability and not stressing the anterior talofibular ligament. The increased variability observed in the unstable groups may predispose them to experience "risky" joint positions, closer to the limits of injury, and the FAI group may not activate their leg muscles enough to sufficiently rely on the muscles as dynamic stabilizers. These findings provide an explanation for the pathomechanics of ankle instability and need to be considered in rehabilitation programs
