Why anticipatory postural adjustments in gait initiation need to be modified when stepping up onto a new level?

The study examined why anticipatory postural adjustments (APA) associated to gait initiation in a stepping up to a new level situation (SU) are reduced as compared to a level walking situation (LW), as previously reported. Five young adults performed gait initiation in both situations at normal and fast speed. Data from a force platform provided gait parameters related to the motion of the body's centre of mass (CM) on the anteroposterior (progression) and vertical axes. The electromyographic activity of the soleus of the stance limb (SOst) and the vastus lateralis of the swing limb (VLsw) were analyzed prior to and after the onset of the double stance phase. The results showed that APA and progression CM velocity at the time of foot contact were smaller in SU, whereas the peak of this velocity was similar in both situations. Thus, the change in progression velocity during the double stance phase had to be greater in SU than in LW. In both velocity conditions, the activity of SOst stopped after the time of foot contact in both situations, but clearly later in SU. So, this ankle plantar flexor muscle would be involved not only in the change of body lift but also in forward CM progression. The latter role of this muscle brought supporting evidence for the reduction of APA in SU, enabling the peak of progression velocity to be similar in both situations. Only in SU, the timing of activation of VLsw and deactivation of SOst strongly co-varied, showing the implementation of a motor synergy to fulfil the new requirements of the task, i.e. body lift.