An important phase in the step cycle is foot contact. When the moment of foot contact differs from the one expected, a fast response is needed. Such a mismatch can be caused by hitting a support surface earlier or later than expected. To investigate this, experiments were performed with healthy young adults who walked on a platform that was unexpectedly at a lowered (5cm) or at a level height. Glasses blocked the lower visual field. In the unexpectedly lowered trials, the absence of expected heel contact triggered responses in the ipsilateral anti-gravity (MGi, RFi) and contralateral flexor muscles (TAc, BFc) with latencies of 47-69 ms. Following the delayed heel contact, enhanced activity was found in the MGi, RFi and TAc muscles. This specific muscle synergy was presumably activated to arrest the forward propulsion of the body. In contrast, when the surface was unexpectedly at level height, the subjects expected to step down and the leg briefly yielded. A muscle synergy was activated at 46- 81 ms that flexed the ipsilateral knee (TAi, BFi, RFi) and extended the contralateral one (MGc, BFc) in order to unload the perturbed leg and delay the contralateral swing phase. Both conditions triggered a fast functionally relevant muscle synergy due to a mismatch between the expected and actual sensory feedback at the moment of foot contact. The results are consistent with an internal model that compares the expected with the actual sensory feedback. The short latency of the response suggests a subcortical, possibly cerebellar pathway.