We investigated the role of a muscular defense in the stabilization and protection of the ankle joint against sudden forced inversion. Ten volunteers with mechanically stable ankles were tested in different standing and walking situations using a trap door model and lower extremity electromyography and electrogoniometers. Peroneal electromyographic activity was observed 54 msec after the detection of ankle inversion. This latency was shorter when the ankle was already in inversion and longer with the ankle in eversion. Quadriceps and hamstring muscle electromyographic activity occurred 68 msec after the ankle inversion stimulus. Evidence of active eversion was seen 176 msec after sudden inversion. Active changes in knee and hip joint angles occurred even later. In contrast, the trap door rotated 30 degrees in approximately 80 msec. We conclude that the reflex reaction to sudden inversion is initiated at a peripheral level by the inversion motion followed by a reaction pattern mediated by spinal or cortical motor centers. Both peripheral and central reactions, however, seem too slow to protect the ankle in case of sudden inversion occurring at the time of heel contact.