Ankle inversions have been studied extensively during standing conditions. However, inversion traumas occur during more dynamic conditions, like walking. Therefore, in this study sudden ankle inversions were elicited in twelve healthy subjects who stepped on a trap door while walking on a treadmill. First, ten control trials (no inversion) were presented. Then, twenty stimulus (25° of rotation) and control trials were presented randomly. EMG recordings were made of six lower leg muscles. All muscles showed a short latency response (SLR) of ~ 40 ms and a late latency response (LLR) of ~ 90 ms. The peroneal muscles, showed the largest amplitudes in both responses. The functionally more important larger and more consistent LLR response was too late to resist the induced stretch during the inversion. The functional relevance of this response must lie after the inversion. During the first trial a widespread startle-like coactivation of the LLR was observed. The last trials showed only a recruitement of the peroneal muscles and to a lesser extend the gastrocnemius lateralis which is seen as a switch from reactive control to more proactive adaptive strategies. These proactive strategies were investigated separately by comparing trials in which inversion was expected (but not occurred) with those in which subjects knew that no inversion would occur. Anticipation of a possible inversion was expressed in decreased Tibialis Anterior activity prior to initial contact, consistent with a more cautious and stable foot placement. Furthermore, immediately after landing, the peroneal muscles were activated to counteract the upcoming stretch.