Investigating the Effects of Center of Gravity (CoG) Shift Due to a Simulated Exploration Extravehicular Mobility Unit (xEMU) Suit on Balance

Abstract

Maintaining balance is critical to minimizing astronauts’ risk of falling and reducing injury or suit damage. Previous studies involving spacesuits have not examined the effects of the superior shift of the center of gravity (CoG) on astronauts’ ability to balance. Here, the purpose of our study was to investigate the effects of CoG shift due to a simulated Extravehicular Mobility Unit (xEMU) on balance. Seventeen participants’ standing balance was examined for three test configurations: unsuited, weighted with an Extravehicular Mobility Unit (xEMU) vest, and xEMU hard upper body torso (HUT). Using a Tekscan forceplate walkway, the CoG locations were determined. Balance assessments were performed to determine the limits of stability and standing balance performance during wide or tandem stances with eyes open/closed. The center of pressure (CoP) time series was examined in terms of displacement, velocity, and frequency measures. During the eyes-open wide stance, the xEMU vest significantly increased the mediolateral balance parameters, while the HUT significantly increased the total displacement (TOTEX), mean velocity (MVELO), and mean frequency (MFREQ) of the CoP. In the eyes-closed wide stance, the HUT significantly increased these parameters. In the eyes-closed tandem stance, the xEMU vest significantly decreased the parameters. The xEMU vest significantly reduced the TOTEX, MVELO, and MFREQ (improved standing balance), while the HUT decreased standing balance ability, seen with significant increases in said parameters. By quantifying CoG’s effect on balance, our results form the basis for future balance and posture studies of xEMU spacesuits.

1. Introduction

Critical to the longer-term Mars and future lunar missions is the capacity for astronauts to mobilize (i.e., stand, walk, and perform functional tasks) while donning a cumbersome Extravehicular Activity (EVA) spacesuit [1]. The newest EVA spacesuit design is the Exploration Extravehicular Mobility Unit (xEMU) spacesuit. Overall, balance-related studies involving the new xEMU spacesuits and prior EVA spacesuits are few, if any. Thus, technological and knowledge gaps exist tied to crewmembers’ mobility, function, performance, injury-risk mitigation, and overall safety [2].

Donning the xEMU spacesuit affects their center of gravity (CoG) location. The CoG is the hypothetical point location at which the total weight of the body is concentrated. The ability of an object (or person) to balance is related to the position of the CoG [3]. While donning the xEMU spacesuit, one’s CoG is shifted superiorly on the torso and away from the body. This shift could potentially affect the ability of the suited crewmember to perform mission tasks [1]. However, balance and posture studies in the xEMU spacesuits have been nascent, and the shift in CoG effects on balance is left uncharacterized.

In one recent study [4], NASA researchers developed a computer-based model that estimated the true CoG location in a microgravity environment of a participant donning the xEMU spacesuit and allowed for predictions of CoP area shifts tied to balance. The CoP is the point location of the vertical ground reaction force vector, and it represents the weighted average of all pressures of the surface area in contact with the support surface (ground). Measurements were taken while the participants were in the Neutral Buoyancy Laboratory (NBL), which simulated planetary EVAs. Three “weighouts” (i.e., how the weight of the spacesuit and attached components are distributed) were investigated: a baseline weighout (derived from data collected from past microgravity runs at the NBL), an exaggerated weighout (a large amount of weight from the portable life support system (PLSS) moved to the waist belt), and a weighout that was derived from the computer-based model that altered the individuals’ CoG location [4]. Researchers tested the stability and reachability of the subject donning the spacesuit for each weighout while in the NBL [4]. Results were limited to an overlay comparison of CoP excursion areas for eyes-open and eyes-closed. The CoP displacement time series and parameters extracted from it, such as root-mean-square displacement, velocity, and frequency measures [5], can be used to interpret how a person balances. However, in this paper, these measures were not shown nor compared, and further, tests for the statistical significance of the changes in the CoP excursion area were not determined.

Previous studies did not investigate balance nor postural control while the individual was suited. However, there are several studies of spacesuit components performance, spacesuit ergonomics, and/or if the participants (astronauts) could complete certain tasks while donning the spacesuit. A previous study investigated the mobility of a suited subject versus an unsuited subject [6]. Agility during locomotion and functionality of the hip brief assembly were examined for the Mark III spacesuit architecture. It was determined that degrees of freedom were limited, the operator’s mobility and agility were restricted, and thus, increased hip joint torques were required [6]. Several studies investigated measurements in-suit and suit fit. There are multiple studies examining suit counter pressure for specific joints, such as the shoulder [7], the elbow [8], and the hands/gloves [9], with many more not listed here. Studies for injury mitigation in gloves [10] and suit padding [11] have been conducted. There was a study investigating energy expended and recovered during walking vs. skipping vs. running as the choice of locomotion on Earth and simulated lunar and Martian gravity [12]. Skipping under simulated lunar gravity consumes less energy than walking and may be the reason why astronauts favored skipping in the Apollo missions [12]. Further, previous motion studies with spacesuits focused on the energy consumption of astronauts and had been conducted several decades ago during the Apollo missions [13,14].

During Apollo 15 and 16, preliminary studies investigating balance (more so falls) were conducted, but measures were crude compared to today’s standards. The main focus was on fall and fall recovery as opposed to balance and gait [13,14,15]. Since then, the spacesuit has been redesigned and improved, leading to the current spacesuit (the xEMU), and new investigations are needed to fill the existing knowledge gaps tied to balance and gait while donning the xEMU spacesuit. Currently, there is inadequate data to predict acceptable mass and CoG combinations for the xEMU spacesuit [1], with a current goal being to develop an adjustable mass and CoG simulation [4]. NASA had developed virtual computer-based, 3D CoG spacesuit computer models that integrated the associated mass of spacesuit components [1]. Virtual manikins from individual body segments were aligned inside the CoG spacesuit model to provide an overall CoG location; this was projected onto a virtual forceplate which showed the shift of the CoG is superior on the torso. Model refinement and additional CoG validation testing are ongoing [1]. With the upcoming manned Artemis missions to the Moon and future missions to Mars, there is a need to study how these new xEMU spacesuits impact balance and functional performance [16].

Our study aimed to provide new knowledge on how altering one’s natural CoG location due to the added weight of a simulated xEMU spacesuit affects their ability to balance. To investigate our aim, we observed healthy adults with no added weight, with a weighted vest (or “xEMU” Vest) and a xEMU hard upper body torso (or “HUT”); the latter two were meant to simulate the xEMU spacesuit shift in CoG. Our main focus was to establish a baseline of characteristics of unsuited xEMU Vest and the HUT in earth’s gravity and then potentially compare them with simulated microgravity conditions. We hypothesized that the superior shift in the CoG caused by the xEMU Vest and/or the HUT would alter the balance observed by forceplate-derived center of pressure (CoP) parameters. This will assist NASA in developing new xEMU spacesuits for future use in upcoming missions.

2. Materials and Methods

An overview of our research study is shown in Figure 1.

2.1. Participants

Research experiments were conducted at the Center of Biomechanical & Rehabilitation Engineering (CBRE) at the University of the District of Columbia (UDC) and the Anthropometry & Biomechanics Facility (ABF) at the NASA Johnson Space Center (JSC). This research complied with the American Psychological Association Code of Ethics and was approved by the Institutional Review Board at the University of the District of Columbia (Protocol ID: 1837316-1). Informed consent was obtained from each participant who was recruited via flyer postings around the University campus. Twelve physically fit and healthy individuals (5 males and 7 females) aged 20–40 years old (26.91 ± 6.81 years old) participated. None of the participants had a previous injury or condition, nor were they taking any medications that affected their balance and/or gait. At the NASA JSC ABF, five participants (4 females and 1 male) aged 20–40 years old (25.6 ± 6.34 years old) were assessed. This particular group was used to assess the effects of the 3D-printed HUT in addition to the control and xEMU vest. Participants were selected based on their ability to fit in the HUT.

Both NASA ABF and UDC CBRE participants were tested unsuited (or control configuration) and donning the xEMU Vest (20 lbs (9.072 kg)) concentrated at the torso, as seen in Figure 2A. The purpose of the xEMU Vest was to raise the CoG of the participants, mimicking the superior shift of CoG due to the xEMU spacesuit.

Aside from the control/unsuited and xEMU Vest configurations, NASA participants also underwent testing using a 3D-printed hard upper body torso (HUT) shown in Figure 2B. The HUT (11 lbs or 4.98 kg) is a 3D-printed replica of the torso section from the xEMU spacesuit that restricts the range of motion in the upper body and alters the resting, standing posture of the individual, shifting the shoulders forward and hunched over (Figure 2B). This stance is unique to spacesuits and cannot be replicated with the xEMU Vest.

2.2. 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).

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 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.

2.3. Data Analysis

CoP in both the mediolateral (ML), side-to-side, and anterior-posterior (AP), front-to-back planes were used to quantify balance. The CoP time series is the point of location of the resultant ground reaction vertical force; it is a commonly used quantitative measurement to characterize one’s balance. From the CoP time series, displacement, velocity, and frequency characteristics were computed [5] using MATLAB (MathWorks, Inc., Natick, MA, USA, R2021b). Root-mean-squared (RMS) displacement and peak-to-peak range of displacement (maximum distance, or MAXD) were computed for ML and AP CoP. Further, combined parameters (total excursion (TOTEX), mean velocity (MVELO), and mean frequency (MFREQ)) were computed using the AP and ML CoP. The above parameters were computed using Equations (1)–(7), respectively [5,18]:

• Maximum distance (MAXD):

$$\mathrm{MAXD}=\max \left(\mathrm{x}(\mathrm{i})\right)-\min (\mathrm{x}(\mathrm{i}))$$

(1)

where x(i) is the COP data for the sample number “i”.

• Root-mean-square of trunk position (RMS):

$$\mathrm{RMS}=\sqrt{\frac{1}{\mathrm{N}}\sum _{\mathrm{i}=1}^{\mathrm{N}}{\left[\mathrm{x}(\mathrm{i})\right]}^2}$$

(2)

where x(i) is COP data for the sample number “i”, N = number of samples.

• Total Excursion (TOTEX)

$$\mathrm{TOTEX}=\sum _{\mathrm{n}=1}^{\mathrm{N}-1}\sqrt{(\mathrm{A}\mathrm{P}\left[\mathrm{n}+1\right]-\mathrm{A}\mathrm{P}\left[{\mathrm{n}]}^2\right)+(\mathrm{M}\mathrm{L}\left[\mathrm{n}+1\right]-\mathrm{M}\mathrm{L}[{\mathrm{n}]}^2})$$

(3)

Calculated as the respective distance between AP and ML directions of the CoP from the origin where N is the number of samples and n is the sample number.

• Mean Distance (MDIST)

$$\mathrm{RD} [\mathrm{n}]=\sqrt{{\mathrm{A}\mathrm{P}}_{\mathrm{n}}^2+{\mathrm{M}\mathrm{L}}_{\mathrm{n}}^2,} \mathrm{n} = 1, \dots , \mathrm{N}$$

(4)

MDIST is calculated as the distance between the CoP AP and ML direction from the origin where N is the number of samples, RD is the resultant distance from the origin, and n is the sample number.

• Mean Velocity (MVELO)

$$\mathrm{MVELO}=\frac{\mathrm{T}\mathrm{O}\mathrm{T}\mathrm{E}\mathbf{X}}{\mathrm{T}}$$

(5)

Calculated as TOTEX over total time (T).

• Root-mean-square distance (RDIST)

$$\mathrm{RDIST}=\sqrt{\frac{1}{\mathrm{N}}{\sum }_{\mathrm{n}=1}^{\mathrm{N}}\mathrm{R}\mathrm{D}[{\mathrm{n}]}^2}$$

(6)

Calculated as the square root of the sum of the squared MDIST.

• Mean Frequency (MFREQ)

$$\mathrm{MFREQ}=\frac{\mathrm{M}\mathrm{V}\mathrm{E}\mathrm{L}\mathrm{O}}{2\mathbf{\pi }\mathrm{M}\mathrm{D}\mathrm{I}\mathrm{S}\mathrm{T}}$$

(7)

Calculated as the ratio of the MVELO to the MDIST.

In terms of statistical analysis, the software RStudio version 4.2.1 was used. The LoS datasets were non-normally distributed. The LoS was analyzed using Wilcoxon (rank-sum) and the Kruskal–Wallis’ (one-way ANOVA) significance tests. The Holm–Bonferroni method was used to determine the adjusted p-values. All reported p-values are the adjusted p-values. Three groups were analyzed: unsuited, xEMU Vest, and HUT. The LoS data were not normally distributed. Therefore, the Wilcoxon rank-sum significance test was used to compare specific groups: (1) unsuited vs. xEMU Vest, (2) unsuited vs. HUT, and (3) xEMU Vest vs. HUT), while the Kruskal–Wallis was used for the overall group comparison. The standing balance data were analyzed using Wilcoxon and Kruskal–Wallis’ significant tests. The Holm–Bonferroni method was used to cross-check all significant findings to eliminate “chance significance”.

3. Results

3.1. Limits of Stability (LoS)

The xEMU Vest raised the CoG of the participants by 13.6 cm ± 10.9 cm. The HUT shift in the CoG location was not captured. The HUT is 3D-printed and fragile; participants were not permitted to lay on the HUT towards the measurement of the CoG location.

Table 1. Limit of Stability in each direction for each test configuration; averages and standard deviations are shown.

	Forward	Backward	Right	Left
Distance (cm)
Unsuited	11.90 ± 2.59	5.22 ± 2.18	15.20 ± 4.51	14.55 ± 5.20
xEMU Vest	11.19 ± 1.77	4.72 ± 2.05	15.47 ± 4.68	15.30 ± 5.46
HUT	10.64 ± 2.72	5.66 ± 1.74	20.07 ± 4.17	20.02 ± 4.43

shows the results from the pooled LoS data for each test configuration. To compare the LoS the Wilcoxon and Kruskal–Wallis’ significance tests were used (not shown here); there were no significant differences between the groups.

3.2. Standing Balance

Table 2. CoP parameters values across test conditions (eyes-open wide stance and eyes-closed wide stance) and CBRE eyes-closed tandem stance for the test configurations comparisons (unsuited vs. xEMU, unsuited vs. HUT, xEMU vs. HUT). The top value is for the first listed configuration; the bottom value is for the second listed configuration. They are color-coded to represent the degree of significance (see Table Key).

Config. Comparison	RMS ML COP	RMS AP COP	MAXD ML COP	MAXD AP COP	MDIST	RDIST	TOTEX	MVELO	MFREQ
Eyes-Open Wide Stance
Unsuited xEMU	0.154	0.361	0.774	1.512	0.342	0.204	22.543	2.254	2.086
	0.228	0.445	1.124	1.836	0.44	0.26	20.784	2.078	1.698
Unsuited HUT	0.154	0.361	0.774	1.512	0.342	0.204	22.543	2.254	2.086
	0.126	0.394	0.676	1.732	0.353	0.22	29.879	2.988	4.559
xEMU HUT	0.228	0.445	1.124	1.836	0.44	0.26	20.784	2.078	1.698
	0.126	0.394	0.676	1.732	0.353	0.22	29.879	2.988	4.559
Eyes-Closed Wide Stance
Unsuited xEMU	0.199	0.407	0.967	1.849	0.393	0.243	23.738	2.374	1.754
	0.226	0.519	1.087	2.192	0.495	0.307	21.372	2.137	1.481
Unsuited HUT	0.199	0.407	0.967	1.849	0.393	0.243	23.738	2.374	1.754
	0.186	0.468	0.736	2.341	0.423	0.29	31.205	3.12	3.315
xEMU HUT	0.226	0.5195	1.087	2.192	0.495	0.307	21.372	2.137	1.481
	0.186	0.468	0.736	2.341	0.423	0.29	31.205	3.12	3.315
CBRE Eyes-Closed Tandem Stance
Unsuited xEMU	0.899	0.812	4.359	5.12	1.012	0.676	270.236	27.0236	4.433
	1.014	0.822	4.852	4.758	1.095	0.723	67.468	6.747	1.171

Significance: p = 0.01 < 0.05, p = 0.001 < 0.01, p = 0.0001 < 0.001.

shows parameter values and the p-values (color-coded) for eyes-open and eyes-closed wide stance test conditions for all the CoP parameters compared across test configurations.

There were a total of 14 significant parameters for eyes-open and eyes-closed wide stance data. The Kruskal–Wallis’ test supports the findings of the Wilcoxon test with p-values for eyes-open wide stance: RMS ML and MAXD ML (p < 0.05); MDIST, TOTEX, MVELO (p < 0.01); MFREQ (p < 0.001). Figure 5A shows the ML and AP displacement CoP parameters for eyes-open and eyes-closed wide stance data for the combined CBRE and NASA data. Figure 5B shows combined (ML and AP) CoP displacement parameters, and Figure 5C shows combined velocity and frequency CoP parameters. In the dataset for eyes-open and eyes-closed wide stance data, Table 2, a majority of the significant differences come from comparing unsuited vs. HUT and xEMU Vest vs. HUT.

In Figure 5A, the CoP displacement measures for eyes-open RMS ML, significant differences were observed between the unsuited vs. xEMU Vest (p = 0.013) and the xEMU Vest vs. the HUT (p = 0.023). The xEMU Vest led to a significant increase in RMS ML and MAXD ML parameters when balancing with eyes-opened/wide stance, these observed increases may be an indication of decreased stability. The HUT for these same parameters shows decreasing trends; however, only the RMS ML has a significant decrease (p = 0.023). The xEMU Vest significantly increased movement on the ML plane in the RMS ML CoP, seen in Figure 5A and Table 2, whereas the HUT significantly decreased movement on the ML plane. The xEMU Vest also significantly increased the MAXD ML CoP, whereas the HUT did not.

Figure 5B shows TOTEX, and Figure 5C shows the mean velocity and frequency. For TOTEX, the HUT introduces a significant increase while the participants attempt to balance with their eyes-open and eyes-closed while donning the HUT compared to unsuited (eyes-open p = 0.04, eyes-closed p = 0.015) and compared to the xEMU Vest (eyes-open p = 0.0038, eyes-closed p = 0.00059). This indicates that the HUT introduces more instability; the participants are shifting and readjusting to maintain their balance more so with the HUT than with the other configurations. This also applies to MVELO and MFREQ, where the significant differences come from comparing the unsuited vs. HUT (eyes-open p = 0.04, eyes-closed p = 0.015; eyes-open p = 0.0045, eyes-closed p = 0.0035) and xEMU Vest vs. HUT (eyes-open p = 0.04, eyes-closed p = 0.00059; eyes-open p = 0.00081, eyes-closed p = 0.0012) for MVELO and MFREQ, respectively. The xEMU Vest significantly affected the ML parameters more than other parameters for the combined eyes-open and eyes-closed wide stance data. Whereas the HUT seemingly only significantly affected TOTEX, MVELO, and MFREQ parameters.

The tandem stance data for CBRE and NASA data were analyzed separately. Neither set was normally distributed; therefore, the Wilcoxon and the Kruskal–Wallis significant tests were used. There were no significant findings for the CBRE eyes-open/tandem stance. Table 2 shows all parameter values for CBRE eyes-closed/tandem stance data. As this is only a two-group comparison, unsuited vs. xEMU Vest, the Kruskal–Wallis’ significant test was not needed for the overall group comparison. The Holm-Bonferroni method was implemented to eliminate any significant findings by chance. There are three significant differences in the CBRE tandem data, and all three are from eyes-closed/tandem stance: TOTEX, MVELO, and MFREQ. The xEMU Vest significantly decreased TOTEX, MVELO, and MFREQ, therefore resulting in more stability compared to unsuited. The NASA tandem dataset did not show any significant findings eyes-open/closed tandem stance.

4. Discussion

In our study, we observed how CoG shifts, investigated here using the xEMU Vest and HUT, affected standing balance. We established a baseline investigation and metrics of how the superior shift in CoG, such as those caused by xEMU spacesuits, impacts an astronaut’s balance. The xEMU Vest raised the CoG of the participants superiorly on the torso, mirroring what is observed by the xEMU spacesuit. Our findings showed that when participants donned the xEMU Vest or the HUT, balance (quantified in terms of TOTEX, MVELO, and MFREQ) was the most affected. However, the xEMU Vest and the HUT had opposite effects; the xEMU Vest significantly decreased the amount while the HUT significantly increased the amount of TOTEX, MVELO, and MFREQ. The xEMU Vest results showed improved stability, in line with previous studies with weighted vests [19,20,21]. Table 2 shows the xEMU Vest had lower TOTEX, MVELO, and MFREQ compared to that of the unsuited and the HUT. This is the opposite of the HUT, the HUT increases the amount of TOTEX, MVELO, and MFREQ significantly when compared to unsuited and the xEMU Vest. Past research findings from the increased load on the torso (via military backpacks) resulted in increased postural instability [22,23], increased postural sway [22,24], and increased balanced impairment [22,25]. The results from TOTEX, MVELO, and MFREQ of the HUT all support the past literature findings. The HUT and potentially the xEMU spacesuits, which will exaggerate the CoG shift compared to the HUT, introduce instability in terms of TOTEX or the amount of sway (balance adjustments when standing), the speed of these adjustments (MVELO), and the rate of these adjustments (MFREQ). The HUT does not significantly affect ML or AP balance parameters but does significantly affect the ability of the wearer to maintain their balance while stationary. If astronauts are to perform manual tasks in the xEMU spacesuits, which will occur during the planned planetary EVAs in the upcoming Artemis missions and future Martian missions, there will be difficulties in maintaining a specific posture.

The amount of instability introduced by the HUT was enough to significantly affect the balance of participants when compared to unsuited and the xEMU Vest. Eyes-closed has been shown to increase instability [26] and can be seen in our data by the increase in data point values from eyes-open to eyes-closed. The inherent instability of the HUT and eyes closed causes greater instability than the HUT alone. The HUT only replicated the effect on the CoG of the participants to a certain extent; in astronauts donning a true xEMU spacesuit, the CoG location will be higher and farther away from the body of the astronaut than that when compared to the HUT. The xEMU spacesuit may weigh up to 160 lbs (72.57 kg), and the portable life support system may weigh up to the same amount or even higher [27]; therefore, these parameters (TOTEX, MVELO, MFREQ) will increase even more with an actual xEMU spacesuit, making the xEMU spacesuit inherently unstable.

Spacesuits introducing some level of balance instability is intuitive; however, now we are beginning to understand exactly how balance is impacted. This understanding is important because it will allow scientists and engineers to develop new spacesuits that can minimize the inherent instability and allow astronauts to perform actions and tasks without astronauts fighting to maintain their balance. Astronauts may become reluctant to perform a certain task if they are aware that they will be failing, possibly multiple times, to complete one task. Another possibility is the astronauts may become hesitant and try to be “safe” while performing the task, which can lead to the consumption of more resources than necessary; resources will be severely limited during planetary EVAs. As much as astronauts train in preparation for space travel, it is in human nature to hesitate or hold back when performing a task that can lead to an injury or performing a task that has already injured the person. Past research has shown that a fear of reinjury can increase the likelihood of reinjury [28]. This is seen in athletes [28], who train their bodies as rigorously, if not more so, than astronauts and non-athletes [29,30]. There is also the psychological impact if an injury is to occur to an astronaut during a planetary EVA; rehabilitation of an injury is impacted by the psychological state of the person injured [28,31], and becoming injured on a different planet entirely may have a severe effect on the astronaut’s psychological state.

These findings may not be a problem for low gravity, such as lunar gravity, which is roughly 1/6 Earth’s gravity, as we have seen during the Apollo missions back in the ’60s and ’70s [13,14,15]. However, one of the upcoming missions is to have a planetary EVA on the surface of Mars, and Martian gravity is roughly 3/8th that of Earth’s gravity. This increase in gravity may impact astronauts’ ability to perform tasks since it is a very lengthy voyage to the surface of Mars; it is estimated that the trip to Mars will take 400–600 days [32]. On a 180-day mission on board the International Space Station, despite in-flight exercise, an astronaut can lose 45% capacity for physical work, a 35% loss to produce force, a 20% loss in velocity, and a 15% loss in muscle volume [33]. The astronauts going to Mars will not have access to the exercise equipment aboard the International Space Station. We do not yet know what the full extent will be to the human body after such a lengthy journey in space because there has never been a mission in space for that extended period. The lengthy journey and increased gravity can become a significant factor, coinciding with the xEMU spacesuit instability, for astronauts’ ability to mobilize and perform the necessary tasks needed on the surface of Mars.

Standing balance for astronauts is important, and our study has scratched the surface of how the xEMU spacesuits affect astronauts’ standing balance. Testing with a fully-fledged xEMU spacesuit will be the next step in understanding how spacesuits affect balance ability, but it was not carried out here, in that will require tremendous resources and personnel. The cost of an actual xEMU suit is in the order of several millions of dollars. Via discussions with spacesuit experts at NASA JSC with first-hand experience developing a mock xEMU spacesuit, developing a mock xEMU spacesuit is a multi-year project; this was not the scope of our work. To develop a mock xEMU spacesuit at the CBRE that behaves and mimics an actual xEMU spacesuit would require a tremendous amount of effort, time, and resources. Therefore, a weighted vest that raises the CoG was considered the best option, in that it allowed for the widest range of eligible participants, cost-efficient, customizable weighout, and minimized injury/safety concerns. However, the xEMU Vest simulates a superior shift in CoG but does not simulate the full weight, the motion of, or the pressurization of the actual xEMU spacesuit. The HUT may be the best possible method to test an xEMU spacesuit without the limitations of the xEMU Vest. The HUT limits the range of motion for the head, shoulders, and arms slightly and also alters the wearer’s resting posture (as seen in Figure 2B). The HUT significantly affects the wearer’s ability to maintain their balance in terms of body sway (TOTEX), speed of body sway (MVELO), and rate of body sway (MFREQ) with a superiorly raised and away from the body CoG shift. This shift mimics what is seen in the xEMU spacesuits; however, the xEMU spacesuits shift the CoG is higher and further away from the body than the HUT, so parameters that were significantly affected (TOTEX, MVELO, and MFREQ) may see even greater increases, i.e., more instability.

Our baseline investigation and metrics presented here can be built upon by future studies tied to the xEMU spacesuit’s impact on astronaut balance.