Adaptation of the gait initiation process for stepping on to a new level using a single step

During the gait initiation in level walking, the anticipatory postural adju stments (APA) which precede heel off consist of a forward fall of the whole body and their duration depends on the intended gait velocity related to t he step length. The present study examines the adaptation of the gait initi ation process for stepping on to a new level. Five subjects performed a sin gle step at natural speed in five experimental conditions. The first condit ion (C1) was a level walking task whereas the other (stair) conditions requ ired stepping on to a new level (from 8 to 32 cm). The horizontal step leng th was the same under all conditions. Results showed that the center of mas s (CM) forward velocity at the end of the APA, and also until foot contact of the leading limb, decreased from C1 to the stair conditions whereas the peak of forward velocity was similar under all conditions. Moreover, the CM forward displacement up to foot contact was smaller in the stair condition s than in C1. These re suits suggest the use of a sequential mode of contro l for the organization of the CM forward dynamics during the stair conditio ns. This adaptation of the gait initiation process for stepping up is exami ned mainly from the result that the majority of body lift, which occurred o nly from the beginning of the double-stance phase, involved a larger CM for ward translation than in level walking. As the horizontal step length was t he same in all conditions, it can be suggested that the CNS had to reduce t he CM forward displacement up to foot contact in the stair conditions, in o rder to take into account the subsequent greater forward translation.