Analysis of Electromyographic Activities of Ankle Muscles at Different Levels of Instability of Unstable Surfaces

[Purpose] The purpose of the present study was to examine differences in ankle joint muscles' electromyographic activities at different instability levels of unstable surfaces in order to provide a scientific basis for unstable surface training. [Subjects and Method] Sixty healthy adult males and females participated in the present study, and they were equally and randomly assigned to a 2.0 psi group, a 1.5 psi group, and a 1.0 psi group. The subjects were instructed to take a standing posture on a 13 inch diameter rubber air disc inflated at different air pressure for 15 seconds. While the participants maintained their standing posture, the EMG signals of the tibialis anterior, the peroneus longus, and the medial gastrocnemius of the dominant lower limb were recorded to assess the muscle activities. [Results] The muscle activity of the tibialis anterior was the highest at 1.0 psi and showed significant differences from its muscle activities at 1.5 psi and 2.0 psi. The muscle activities of the peroneus longus and the medial gastrocnemius were highest at 1.0 psi and showed significant differences from the muscle activities at 2.0 psi. At all the pressures, the muscle activity of the tibialis anterior was the highest and that of the medial gastrocnemius was the lowest, although the differences were not statistically significant. [Conclusion] The results of the present study show that to trigger the co-contraction of all muscles involved in ankle joint stability, 1.0 psi or lower instability should be provided.