On the origin of the mediolateral displacement of the centre of pressure during gait initiation.

OBJECTIVE: The critical action for gait initiation involves lifting of the swing leg while shifting the centre of mass (CoM) of the body over the stance leg. This action is made possible by the initial displacement of the centre of pressure (CoP) in the direction of the swing leg. We hypothesised that the CoP displacement requires a coordinated action of the swing-leg abductor muscles and of the dorsiflexor muscles of the stance leg.

MATERIAL/PATIENTS AND METHODS: Nine subjects initiated gait starting from three different initial stance widths (small, medium, wide). Optoelectronic, force platform and EMG measurements (ankle dorsi- and plantarflexors, and hip abductor muscles) were performed. The initial CoP and CoM displacements in opposite direction were accompanied by an early and moderate knee flexion of the stance leg. A linear relationship was found across stance widths between this knee flexion and CoP displacement. The concurrent dorsiflexor muscle activity of the stance leg was also greater with increasing step-width. This activity was almost simultaneous with that of the hip abductors of the swing leg.

RESULTS: We propose that the early CoP displacement in the direction of the swing leg during the preparation phase of gait initiation is caused not only by abduction of the swing leg. The flexion of the stance knee, favoured by the foot dorsiflexor muscles of the stance leg, also contributes to the unloading of the stance leg and the loading of the other, thereby producing the torque for CoM displacement toward the stance leg.

DISCUSSION-CONCLUSION: We suggest to monitor the activity of both hip abductor muscles and foot dorsiflexor muscles whenever gait initiation is impaired by brain injury or movement disorders, which may particularly affect the fine coordination between these muscles in their combined effect on to the mediolateral displacement of the CoP.