We measured the maximal isometric eversion moment developed under full weightbearing in 20 healthy adult men (age, 24.4 +/- 3.4 years; mean +/- SD) with their ankles in 15 degrees of inversion. Tests were performed at both 0 degree and 32 degrees of ankle plantar flexion in low- and in three-quarter-top shoes with and without adhesive athletic tape or one of three proprietary ankle orthoses. At 0 degree of ankle plantar flexion, the mean maximal voluntary resistance of the unprotected ankle to an inversion moment was 50 +/- 8 N-m; this increased by an average of 12% (or 6 N-m) when the subject wore a three-quarter-top basketball shoe. The maximal voluntary resistances to inversion moments developed with the ankles further protected by athletic tape or any of three orthoses were not significantly different. Biomechanical calculations suggest that at 15 degrees of inversion the fully active ankle evertor muscles isometrically developed a moment up to six times larger than that developed when an athlete wears a three-quarter-top shoe alone and more than three times larger than that developed passively when the athlete has tape or an orthosis worn inside a three-quarter-top shoe. We conclude that fully activated and strong ankle evertor muscles are the best protection for a near-maximally inverted ankle at footstrike.