Method to deter crossover steps during training. Large foam blocks (indicated by the shaded areas) are placed in front of and behind the feet during the lateral-perturbation trials in the first two training sessions. Subjects cannot execute a crossover step because the foam prevents the swing foot from travelling or landing in front of or behind the stance foot (panel A). The training is intended to promote instead the use of a side-step sequence (panel B). Note that both of these step patterns involve an initial step with the foot that is unloaded as a result of the perturbation-induced body motion. The foam blocks also permit a lateral step with the loaded leg (panel C); however, this pattern of stepping tends to occur infrequently . Note that panels A-C depict stepping responses evoked by rightward platform translation, which causes the subject to fall to the left; this motion of the body leads to an initial increase in the loading of the left foot and an unloading of the right foot (panel D). The natural tendency is to step with the unloaded leg.