*3.3. Effect of PD and History of FOG on GI*

We observed significant alterations in the GI execution in both PDNF and PDF patients, with the latter group showing overall more severely altered APA measurements (Table 3, Figure 2).

The CoP displacement and velocity during IMB showed a progressive and significant reduction from HCs to PDNF to PDF groups along both the mediolateral and anteroposterior axes.

The UNL and stepping phases were also altered in PDNF and PDF patients (Table 3, Figure 2). Of most relevance, in PDF patients, the anteroposterior displacement of the CoP during UNL was backwards in most of the trials.

PDF patients had a significantly reduced first step length and both PD groups had a lower first step average velocity compared with HCs.

The CoM forward propulsion (velocity and acceleration) progressively decreased from HCs to PDNF to PDF.

**Figure 2.** Two-dimensional center of pressure and center of mass trajectories during gait initiation.

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Example of the pathway of the center of pressure (CoP, grey solid line) and center of mass (CoM, black dotted line) during a gait initiation trial of one healthy subject (left panel) and one Parkinsonian patient without (PDNF, central panel) and one patient with (PDF, right panel) a positive history of freezing of gait. We defined the imbalance (IMB) phase as the interval between the onset of the APAs (APAONSET) and the heel-off of the swing foot (HOSW), and the unloading phase (UNL) as the interval between the HOSW and the toe-off of the swing foot (TOSW). The black dashed line represents the CoP–CoM vector at the end of the unloading (UNL) phase. With respect to healthy controls, the CoP displacement during the IMB phase was reduced for both PD and PDF patients. The CoP displacement during the UNL phase was in most cases backwards for the PDF patients only. Please see Table 3 for further details. Abbreviations: APAs, anticipatory postural adjustments; AP, anterior–posterior; CoP, center of pressure; HC, healthy controls; HO, heel off; ML, mediolateral; PDF, Parkinson's disease with freezing of gait; PDNF, Parkinson's disease with no freezing of gait; TO, toe-off.
