## 2 Materials and Methods
### Instruments  
Posturographic tests were performed using an Equitest posturograph manufactured by NeuroCom International(r). The device consists of a dynamic force plate, visual surround, and a computer with software. Both the force plate and visual surround are moveable (\(\pm 10^{\circ}\) rotation for both and a maximum angular velocity of 50\({}^{\circ}\)/s and 15\({}^{\circ}\)/s, respectively). The study protocol included the following tests: Sensory Organization Test, Motor Control Test, and the Adaptation Test.  
The Sensory Organization Test (SOT) evaluates the usefulness of signals coming from the different senses involved in maintaining body balance. This test is performed using six sensory stimulation conditions, during which visual stimuli are changed and a rotation of the foot support platform, or movements of the visual surround, are introduced. During the first three tests (SOT1-SOT3) the foot support platform is stationary while the visual information is varied: SOT1--eyes open, SOT2--eyes closed, SOT3--moving visual surround. During these tests, analyzing postural stability determines the usefulness of the visual signal and the patient's ability to suppress visual stimuli which are contrary to reality. The next three tests (SOT4-SOT6) are performed with a moving foot support platform, which interferes with the information received by the proprioceptive system. As with the previous three tests, visual information is varied: SOT4--eyes open, SOT5--eyes closed, SOT6--moving visual surround. Two final trials evaluate the usefulness of the vestibular system, whose role increases significantly in the case of incorrect or missing stimuli from the remaining systems involved in posture control. During SOT, the Equilibrium Score (ES) that quantifies the COG sway or postural stability under each of the 3 trials of the 6 sensory conditions is evaluated. A score of 100 represents perfect balance (no sway) and a score of 0 indicates a fall. During SOT, Composite Equilibrium Score (CES)--a weighted average of all 6 individual scores ES--the body's COG displacement in the anterior-posterior direction, as well as the motor strategy (MS)--correctness of the selected postural strategy--is also assessed. A score near 100 indicates a full ankle strategy while a score near 0 indicates a full hip strategy with maximum shear force. In addition the Sensory Analysis (SRS) that determines a patient's ability to use input from the somatosensory (SOM), visual (VIS), vestibular (VES) system to maintain balance as well as the degree to which the patient relies on visual information whether it is correct or not (PREF) is assessed [19; 20].  
The Motor Control Test (MCT) is performed for 6 conditions using the foot support platform capable of forward and backward movements through small, medium, and large displacements. A series of three trials is performed for each condition during which the patient's ability to perform corrective movements is evaluated in response to unexpected changes in the foot support platform. The MCT analysis the Latency Response (L)--the time between start of platform movement and start of postural response--the Amplitude (A) of the postural response and the Weight Symmetry (WS)--a nondimensional quantity with a score of 100 indicating that weight is borne equally by the two legs. The WS score decreases to zero or increases to 200 when all the weight is borne by the left or right leg, respectively.  
In the Adaptation Test (ADT), the patient is subjected to two series of sudden platform perturbations, one of which causes dorsiflexion (ATU) while the other causes plantarflexion (ATD) in the ankles. Each series consists of five trials. During subsequent trials, the patient should maintain a vertical posture, minimizing with every subsequent trial the amount of energy required to rebalance the body. This sway energy (SE) is determined after each platform perturbation and indicates the amount of COG displacement during each trial.