Principles underlying the organization of movement initiation from quiet stance

The purpose of this study was to determine common principles underlying the programming of movement initiation from quiet stance. Subjects were asked to initiate gait, step over a ruler, or to step over a 10 cm high obstacle at a self-paced speed and as fast as possible. The independent variables we re initiation condition (gait initiation, stepping over a ruler or obstacle ) and initiation speed (self-paced and as fast as possible). The dependent measures for the stance limb only were the latency between postural soleus (S-1) EMG inhibition and tibialis anterior (TA) EMG onset, the duration of both TA and soleus (S-2) activity following TA, duration and slope, impulse , and peak forces of the anterior-posterior (Fx) ground reaction force. Sel ected timing events were also monitored. Analysis of variance was used to d etermine main and interaction effects. The following results were obtained. (1) The interval from the inhibition of S-1 postural activity to the onset of TA remained invariant between all conditions. (2) The duration of TA in creased and S-2 decreased with an increase in speed of initiation. There wa s no difference in TA and S-2 duration between the initiation conditions. ( 3) Time to heel-off remained invariant for all conditions. (4) Prior to hee l-off all force variables increased with initiation speed but were similar between initiation conditions. After heel-off force variables were differen t between speeds and conditions being greater for fast speed and stepping o ver the obstacle. Two conclusions may be drawn from this study. First, the results indicate that gait initiation consists of two, highly coordinated m otor programs. Heel-off of the stance limb is the division between these tw o programs. Second, our findings also suggest that gait initiation and step ping are governed by the same motor programs. (C) 1999 Elsevier Science B.V . All rights reserved.