The effectiveness of external ankle support in influencing the role of a primary ankle everter during the application of an ankle inverting torque was examined by monitoring the electromyographic (EMG) output of the peroneous longus. Six male intercollegiate athletes were randomly placed in a balanced latin squares design, and were tested under three treatment conditions: (a) wrapped, (b) taped, (c) no external support. The subjects performed a backward stepdown movement. During the experimental step, a trapdoor in the lower platform collapsed so as to form a potential ankle inverting angle of 30 degrees. Sixteen millimeter film was used to record each subject's stepping sequence. The film was in turn synchronized to the subject's recordings of EMG activity. Statistical analysis of the integrated EMG pulse counts showed a significant difference (P less than .05) between the normal (control) and ankle inverting (experimental) steps. No significant differences (P less than .05) were detected between treatments. It was concluded that the apparatus placed a measurable stress on the ankle and that preventive ankle strapping has no statistically significant (P less than .05) effect on the tension developed in the peroneous longus during a quick inversion (less than or equal to 30 degrees) of the ankle joint.