## 2 Materials and Methods
### Experimental Protocol  
Forceplate assessments were conducted to measure shifts in CoG standing balance, Limits of Stability (LoS), and center of pressure (CoP) for the various standing conditions (eyes-open/closed wide stance and eyes-open/closed tandem stance while being unsuited or donning the xEMU Vest, or HUT); this is summarized in Figure 1. Within the UDC CBRE, the Tekscan Forceplate Walkway (Tekscan Inc., Northwood, MA, USA) was used to acquire force data at 100 Hz for 10 s trials, tied to the CoP (seen in Figure 3A,B). At NASA JSC ABF, AMTI forceplates (Advanced Mechanical Technology, Inc., Watertown, MA, USA) were used to acquire data at 1000 Hz (seen in Figure 3C).  
Figure 2: **(A)** Exploration Extravehicular Mobility Unit (xEMU) Vest used to simulate superior shift in CoG; **(B)** 3D printed xEMU hard upper body torso (HUT) (left and middle) and xEMU spacesuit (right).  
The unsuited CoG was recorded. To determine their CoG, participants laid face down across the forceplate, resting their heads on their arms which were folded beneath them, with their toes at the edge of the forceplate (seen in Figure 3A). The participants' CoP time series was recorded for a 10 s trial. The average CoP location was determined and was equivalent to the average location of the CoG. The CoP, while the participant laid on the forceplate, was equivalent to the CoG. At the ABF, a NASA-created instrument named \"CoG Board\" was used to determine the participants' CoG location (Figure 3D) using the same methodology. The CoG board consisted of 4 high-capacity force transducers, an 80/20 aluminum frame, an Arduino microcontroller, an acrylic sheet, LED strips, and a portable power source. The LED strips ran parallel to a measuring tape on the aluminum frame, which zeroes at the true center of the board, allowing measurements of the CoG shifts for the test configurations. The CoG board can be seen in Figure 3D. Within both the UDC CBRE and NASA ABF, forceplates were calibrated prior to each session. After the CoG was determined, the LoS and standing balance trials followed.  
The LoS were acquired for unsuited, the xEMU Vest, and the HUT. LoS [17] involved how far the participant could lean in each direction (forward, backward, left, right) without losing their balance and without lifting any part of their foot off the ground. The participants first started in a forward-facing stationary position with feet shoulder-width apart and with arms at their sides and looking straight ahead. From this neutral position, within the 10 s trial, they then leaned as far forward as they could and held the position while the forceplate data were acquired. The same methodology was repeated for backward, right, and left directions. An example of a participant undergoing the LoS test can be seen in Figure 4. The LoS for each trial was determined by subtracting the maximum and minimum displacements, thereby obtaining the range of the CoP displacement. Standing balance assessments were conducted next. For the standing balance assessments, the participants were asked to stand with either their eyes opened or eyes closed, in a wide stance (feet  
Figure 3: Experimental setups for CoG data collection: **(A)** Use of Tekscan forceplate for standing balance and CoG data collection (example of CoG data collection shown) and **(B)** Tekscan 7.8 software during CoG data collection at UDC. **(C)** AMTI forceplates and **(D)** CoG board at NASA.  
shoulder width apart) or tandem stance (heel to toe) foot placements with their arms down by their sides, chin up, and facing forward. This led to four quiet stance test conditions (i.e., eyes-open/wide stance, eyes-closed/wide stance, eyes-open/tandem stance, and eyes-closed/tandem stance). The participant held this stance for 10 s while the CoP trial was recorded, two trials per test condition; this was conducted for each configuration, as summarized in Figure 1.