A device to simulate ankle motion associated with inversion ankle injury was constructed. This device consists of a trap door that can be tilted 30° from the horizontal plane. Surface EMG electrodes were placed over the peroneus brevis and peroneus longus muscles. The time measured from the tilting of the plate to the first muscular response on the EMG was defined as the reflex time. Twenty individuals with unilateral ankle joint instability were tested. The mechanical ankle joint stability was measured using standardized radiographic measurements, taking into account anterior talar translation and talar tilt. The mean anterior talar translation was 5.9 mm and the mean talar tilt was 3.2° in the stable ankles, compared to 12.7 mm and 10.5° for the unstable ankles. The mean reflex time was 68.8 ms (peroneus longus) and 69.2 ms (peroneus brevis) in the stable ankles as compared to 84.5 ms (peroneus longus) and 81.6 ms (peroneus brevis) in the unstable ankles. Important factors influencing functional instability are discussed.
J. Karlsson and L. Peterson are with the Dept. of Orthopaedics, East Hospital, Gothenburg University, Göteborg, Sweden. G. Andreasson and C. Högfors are with the Centre for Biomechanics, Chalmers University of Technology, Göteborg, Sweden.