Recorded data at gait initiation.

<p>Centre of pressure (CoP, black line) and centre of mass (CoM, dashed line) displacement in a healthy subject (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g001" target="_blank">figure 1</a> left) and child with RTT (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g002" target="_blank">figure 2</a> left) with corresponding EMG activity of tibialis anterior (TA) and soleus muscles (SOL) of swing and stance foot (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g001" target="_blank">figure 1</a> right and 2 right). The dotted line (in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g001" target="_blank">figure 1</a> left and 2 left) shows CoP-CoM distance at toe-off of the swing foot (SWto). In <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g001" target="_blank">figure 1</a> (right), arrows indicate bilateral suppression of the tonic activity of SOL which, together with the subsequent activation of TA, is responsible for the backward displacement of the CoP. This synergistic activity of pairs of postural muscles (i.e. TA and SOL) is not present in RTT girls (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092736#pone-0092736-g002" target="_blank">figure 2</a> right). <i>Imbalance phase</i>, from the instant APAonset, at which the CoP start moving backward, to the instant of heel-off of the swing foot (SWho). <i>Unloading phase</i>, from SWho to toe-off of the swing foot (SWto). STto is the instant of toe-off of the stance foot.