Occupational Physical Requirements for Astronauts

Comprehensive Effect of Deconditioning

A comprehensive study by Mulavara et al. aimed to determine how spaceflight influences the performance of representative functional tests of critical exploration mission tasks, as well as to identify physiological factors related to the cardiovascular, neuromuscular, and sensorimotor systems that could limit performances.¹⁹ For this purpose, a battery of tests representing the critical occupational tasks of astronauts and various physiological tests were conducted on long-duration mission astronauts and subjects undergoing a 70-d HDBR, before and after exposure to actual or simulated microgravity. The specific tests are detailed in Table III.

Briefly, a significant decrease in performance was observed following microgravity exposition in the recovery from fall to stand test and in the seat egress and walk test in astronauts (11 men and 2 women, aged 47 ± 5 yr; +66% and +31%, respectively), in HDBR control subjects (10 men, aged 38 ± 7 yr; +54% and +38%, respectively) and HDBR exercisers (9 men, aged 34 ± 6 yr; +42% and +23%, respectively). There was a noteworthy increase in completion time in both bed-rest groups during the ladder climb test (P < 0.008). In contrast, ISS crewmembers demonstrated a modest rise in completion time (P = 0.009). Object translation and jump down test performances were affected by both HDBR (+26% for both tests) and spaceflight (+58% for both test); however, exercising during HDBR mitigated the detrimental alterations, with HDBR exercisers showing no significant change in performance for either test. Performance in the activity board and hatch-opening tests did not exhibit significant alterations following spaceflight or HDBR. Astronauts experienced an approximate 8% reduction in the maximum power output of their lower body, while HDBR control subjects had a 14% reduction; similarly, total lower body work decreased by 10% for astronauts and 19% for HDBR control subjects only.

The following sections will discuss outcome variables from other studies related to cardiovascular and metabolic, neuromuscular, and sensorimotor systems that may restrict performance.

Cardiovascular and Metabolic Requirements

Ade et al. suggested that running critical speed and upper limb $\dot{\mathrm{V}}$O₂ peak would be better parameters than running $\dot{\mathrm{V}}$O₂ max and upper limb $\dot{\mathrm{V}}$O₂ peak for predicting physical performance during planetary abilities field test and extravehicular activity (EVA) walkback.¹⁴ Ade et al. recruited 71 subjects (40 men and 31 women, aged 23 ± 5 yr) and administered two tests associated with the physical demands of astronaut tasks; a continuous circuit consisting of 6 tests (12-ft ladder climb, cone agility test, 4.5-m stair climb, lateral wall climb, box lifts, and 40-cm step-over task) and a 10-km walk-back test. Subjects were unaware of the distance and number of completed laps to prevent pacing. The relationship between the two occupational tests and laboratory-based physical performance measures were evaluated. Running at critical speed (11.9 ± 2.2 km · h⁻¹) was the best single predictor of 10-km walk-back time in men (59.2 ± 12.3 min, r = −0.88, P < 0.001) and women (63.3 ± 13.6 min, r = −0.82, P < 0.001).

For the physical abilities field test, the best predictors of time completion were running at critical speed (12.4 ± 2.3 and 11.2 ± 2.0 km · h⁻¹, r = −0.82 and r = −0.78, men and women, respectively, P < 0.001) and arm-cranking $\dot{\mathrm{V}}$O₂ peak (28.7 ± 6.0 ml · kg · min⁻¹ and 24.4 ± 5.0 ml · kg · min⁻¹, r = −0.69 and −0.52, P < 0.001 and P < 0.05, men and women, respectively). In this context, critical speed serves as a proxy for the maximum sustainable aerobic metabolic rate, representing 58.1 ± 7.3 and 61.4 ± 8.7% of $\dot{\mathrm{V}}$O₂ max in men and women, respectively.

Ade et al. recruited 70 subjects and administered 2 tests derived from tasks identified by the NASA Human Research Program in 2009; a material transport field test involving loading, transporting, and unloading geological samples; and a device operations field test comprising tasks related to equipment setup and the manipulation of controls and valves.¹⁵ This study aimed to assess how standard aerobic fitness and muscular strength tests relate to performance in these mission-critical upper-body activities. Only 24 subjects underwent the second test. The average duration of the material transport field test was 36.0 ± 9.2 min, ranging from 25.8–79.7 min. Arm cranking critical power (62.3 ± 22.9 W, r = −0.66, P < 0.001), running critical speed (11.8 ± 2.2 km · h⁻¹, r = –0.56, P < 0.001) and arm-cranking $\dot{\mathrm{V}}$O₂ peak (26.9 ± 5.9 ml · kg · min⁻¹, r = −0.54, P < 0.001) were the parameters with the strongest relationship to predicted performance during the material transport test. A critical power of ≤39.8 Watts during the arm cranking test indicates a high risk for falling into the 4^th quartile of material transport field test times. The average duration of the device operations field test was 32.2 ± 6.4 min, with a range of 19.5–47.9 min. Arm cranking critical power (r = −0.70, P < 0.001), running critical speed (r = −0.62, P < 0.001), number of completed push-ups (r = −0.59, P < 0.001) and arm cranking peak power output (PPO) (r = −0.56, P < 0.05) exhibited the strongest correlation with the device operations field test time.

Astronauts must perform an unaided emergency exit through the space capsule’s top hatch as quickly as possible upon re-entry to Earth.²⁴ Alexander et al. enlisted 15 individuals (5 men and 10 women, aged 47 ± 4 yr), and conducted an egress test in an Orion capsule mock-up.¹⁶ Additionally, subjects were mandated to undergo two incremental exercise tests until exhaustion, one on a cycle ergometer and one on a rowing ergometer. Egress completion time was 54.9 ± 19.4 s, ranging from 34–114 s, and was not correlated to age, body mass, or height. There was a negative correlation (r = −0.60, P = 0.03) between egress time and rowing PPO normalized to body mass. Subjects’ peak $\dot{\mathrm{V}}$O₂ (mean cycling 24.0 ± 4.8 ml · kg · min⁻¹ and mean rowing 25.0 ± 4.4 ml · kg · min⁻¹) during the egress test reached 72 ± 25% of relative $\dot{\mathrm{V}}$O₂ peak. Handling the 5-kg bags elicited the larger increase in ventilation, from 19.31 ± 9.88 l · min⁻¹ at baseline to 31.68 ± 9.30 l · min⁻¹.

Taylor et al. recruited eight subjects (five men and three women; average age 34.88 ± 3.69 yr) and subjected them to six simulated astronaut tasks: hatch opening, hand drilling, construction wrenching, incline walking, samples collection, and emergency crewmember drag.²² Tasks are detailed in Table III. As for the cardiovascular and metabolic requirements, notable variations in aerobic metabolism and fuel utilization were observed, but there were no significant differences in heart rate or perceived intensity. For instance, the highest respiratory exchange ratio ($\mathrm{RER}\mathrm{or}\dot{\mathrm{V}}$CO₂/$\dot{\mathrm{V}}$O2) was observed during the task of hatch opening, reaching 1.44 ± 0.21. The task with the highest oxygen uptake demands was walking 800 m on an inclined treadmill with a 48-kg weighted suit, replicating the NDX-2 spacesuit weight distribution, with values of 32.28 ± 6.57 ml · kg · min⁻¹.

Neuromuscular Requirements

Ryder et al. studied task performance by adjusting strength, power, and endurance to body weight (BW) ratio using a weighted suit.²⁰ The goal was to determine the muscle performance thresholds required for tasks that replicate astronaut work. Subjects (10 men and 10 women, aged 36 ± 10 yr) engaged in 6 astronaut tasks (seat egress and walk, rise from fall, hatch opening, ladder climb, object carry, and construction board activity) as well as traditional laboratory tests, including leg press, bench press, and knee extension performance assessments. The astronaut data supported the resulting models. For lower body performance, the best predictor for the upright and supine seat egress and walk test, object carry, ladder treadmill, and rise from fall tasks was determined to be leg press maximal isometric force per BW (LPMIF/BW), with corresponding R² values of 0.6, 0.68, 0.52, 0.6, and 0.54, respectively. The highest threshold for LPMIF/BW performance was observed during upright seat egress task at 17.8 N ⋅ kg⁻¹. Consequently, any increase in load from the astronaut’s body weight gain or the inclusion of equipment like the spacesuit, without a concomitant increase in strength, would impact this ratio, potentially prolonging the time required to complete these tasks. Overall, the upright seat egress and walk task showed the highest thresholds for lower body measures: isokinetic knee extension per BW (1.9 Nm ⋅ kg⁻¹; R² = 0.44), knee extension maximal isometric force per BW (5.9 N ⋅ kg⁻¹; R² = 0.43), LPMIF/BW (17.8 N ⋅ kg⁻¹; R² = 0.6), leg press power per BW (17.6 W ⋅ kg⁻¹; R² = 0.41), and leg press work per BW (78.8 J · kg⁻¹; R² = 0.41).

Regarding upper body performance, the most significant predictor for hatch opening was the bench press work per BW (BPW/BW), yielding an R² of 0.74, with a threshold estimated at 18.3 J · kg⁻¹. For object carry and construction activity board tasks, the bench press maximal isometric force per BW (BPMIF/BW) was the top predictor, with R² values of 0.34 and 0.07, respectively. Relative strength thresholds for upper body performance were also investigated.²⁰ The highest threshold for upper body measures was hatch opening: BPMIF/BW (13 N ⋅ kg⁻¹; R² = 0.62), bench press power per BW (10.8 W ⋅ kg⁻¹; R² = 0.63), and BPW/BW (46 J · kg⁻¹; R² = 0.74).

Similarly, Ryder et al. enlisted 60 subjects (32 men and 28 women; average age 37 ± 7 yr) to investigate whether indicators of muscle strength and physical capacity could predict subjects’ ability to complete various astronaut tasks within an acceptable timeframe.²¹ Alongside traditional laboratory tests evaluating aerobic, anaerobic, and neuromuscular capacity, subjects performed simulations of four astronaut planetary EVA tasks: capsule egress, ambulation and supply transfer, rescue drag, and hill climb and descent. For all tasks, the threshold values remained consistent across predictor variables. Isometric midthigh pull, isokinetic knee extension work, bench press work, and aerobic capacity emerged as the most reliable indicators for predicting performance in capsule egress, ambulation and supply transfer, rescue drag, and hill climb and descent [receiver operator characteristics (ROC) area under the curve = 0.88, 0.89, 0.9, and 0.92, respectively]. For example, concentric isokinetic knee extension ranged from 1.83–1.86 Nm ⋅ kg⁻¹, with isokinetic knee extension work from 22.84–23.34 J · kg⁻¹. Bench press work varied from 25.08–26.50 J · kg⁻¹, isometric midthigh pull from 18.3–18.53 N · kg⁻¹, and aerobic capacity from 27.4–27.57 ml · kg · min⁻¹ across all tasks.

Building upon the Ryder et al. research, Taylor et al. investigated whether NASA’s lower body strength testing could predict astronaut occupational task performance and if occupational upper body strength test could improve predictions.²² They enlisted eight healthy individuals (five men and three women; aged 34.88 ± 3.69 yr) and conducted six occupational tasks (hatch opening, hand drilling, construction wrenching, incline walking, sample collection, and emergency crewmember drag), alongside NASA’s standard upper leg strength measures (knee extension-flexion strength). The total time to complete all tasks varied from 20.2–44.5 min. NASA’s standard strength measurements contributed 61.5% of the variability. Adding hand-drilling and wrenching performance to NASA standard measurements explained 99.6% of the variability in time-to-completion (P = 0.15). In men, the exertion of wrenching away and toward produced forces of 262 ± 71 N and 301 ± 111 N, respectively. Hand drilling away and toward yielded forces of 36 ± 12 N and 39 ± 12 N, respectively. In women, wrenching forces were 160 ± 40 N and 198 ± 17 N and hand drilling generated strengths of 34 ± 10 N and 37 ± 1 N for away and toward actions, respectively. Gender comparisons revealed differences in knee flexion peak torque (P = 0.006), overall work (P = 0.016), and knee extension total work (P = 0.002).

Partial gravity, as on the Moon and Mars, may help astronauts with some occupational tasks. In a study by Volkova et al., a positive correlation (P = 0.02) was found between endurance time in upper extremity tasks and gravity level, with negative coefficients for both men and women in static tasks.²³ The study involved 32 subjects (18 men and 14 women; aged 34.88 ± 3.69 yr) who performed 4 upper extremity tasks in 1 G and underwater (1/3 G and 1/6 G) to simulate Martian and lunar gravity. The tasks varied in intensity, with weights from 0.5–9 kg, adjusted to match the respective gravitational levels. Endurance task time increased 3.54-fold for women and 3.14-fold for men under lunar gravity compared to Earth’s gravity.

Sensorimotor Requirements

Sensorimotor alterations, highlighted by Miller et al. using data from spaceflight and simulated microgravity, may impact astronauts’ performance on operational tasks upon landing on a planetary surface.¹⁸ Balance control system evaluations challenging mission-critical tasks were conducted on astronauts before and after missions of varying durations. Assessments were also performed on subjects undergoing 70 d of continuous HDBR. Subjects completed a seated egress and walking test to simulate emergency evacuation tasks, an object translation test for manipulating an item on a planetary surface, a recovery from fall/stand test, and a jump down test. Control group subjects confined to bed (10 men, with an average age of 37.7 ± 7.2 yr) exhibited decreased performance immediately after the bed rest period. Bedridden individuals who exercised (nine men, aged 33.8 ± 5.5 yr) displayed comparable changes to those who did not exercise, with no significant difference between the two groups following bed rest. Similarly, individuals returning from extended space missions (11 men and 2 women, with an average age of 46.6 ± 4.5 yr) demonstrated reduced performance across all parameters. Specific performance changes are shown graphically in the article, limiting their applicability in future research. Notable findings include an increase in egress portal time (seated and reclined: P = 4.5e-06 and 1.1e-09, respectively), a decrease in continuous equilibrium score (P = 7.6e-13), and increases in fall recovery parameters such as postural settling time and mean sway speed (P = 5.1e-09 and 1.7e-29, respectively). Significant alterations were also noted in jump down settling time (P = 7.2e-09), object translation time (P = 1.1e-10), and tandem walk parameter (P = 2.3e-21). The decreases were more pronounced and required longer recovery compared to astronauts on short-duration missions (four men and two women, aged 43.0 ± 5.7 yr).

Mulavara et al. highlighted that occupational tasks requiring enhanced body coordination and postural stability, such as seat egress and walk, recovery from fall/stand, object translation, jump down, and ladder climb tests, are particularly affected by prolonged spaceflight and long-duration HDBR.¹⁹ In contrast, tasks demanding less postural stability, like the hatch opening and activity board test, displayed no performance impairment following real or simulated microgravity.

Astronauts’ performance may be restricted by spatial disorientation and motion sickness during spacecraft re-entry. Cowings et al. evaluated whether Autogenic-Feedback Training Exercise (AFTE), a physiological training method, could alleviate these issues for the Orion spacecraft.¹⁷ A group of 20 subjects (14 men with an average age of 35.5 ± 2.3 yr and 6 women with an average age of 35.5 ± 6.22 yr) underwent a simulated re-entry, occurring after the deployment of the drogue parachute. The simulation involved Coriolis acceleration in a rotating chair. Severe discomfort occurred in 60% of the subjects when the chair speed mimicked the parachute deployment phase. Subjects who completed 2 h of AFTE displayed reduced adverse symptoms, while those with 4 and 6 h of training had fewer symptoms and more consistent performance compared to the control group. This suggests that longer AFTE durations are more effective for subjects responding to this form of treatment.

Cardiovascular and Metabolic Requirements

According to NASA Constellation Program’s EVA Systems Project Office and the Health and Human Performance Directorates’ EVA Physiology, Systems, and Performance Project, rovers will not travel more than 10 km from base.²⁵ Consequently, astronauts should be prepared to walk up to 10 km in a spacesuit if the rover fails, matching the distances recorded in the identified studies (Table IV).

Although $\dot{\mathrm{V}}$O₂ max is significantly linked to astronauts’ occupational tasks, Ade et al. suggest that arm-cranking critical power and running critical speed—representing the highest sustainable aerobic workload—might be better performance predictors than $\dot{\mathrm{V}}$O₂ max or $\dot{\mathrm{V}}$O₂ peak for metabolically demanding tasks, such as the 10-km walk-back.^14,15 This could be attributed to the combined influence of the aerobic energy systems’ capacity and the subject’s functional metabolic efficiency, meaning the amount of energy spent per unit of velocity.²⁶ This last parameter is more sensitive between subjects with the same $\dot{\mathrm{V}}$O₂ max since the metabolic cost for a task is highly variable. Thus, the relationship between performance and $\dot{\mathrm{V}}$O₂ max tends to weaken with increasing duration of effort compared to critical speed or critical power. Critical speed and critical power might be more accurate for predicting astronaut performance during EVAs, which often span several hours, than $\dot{\mathrm{V}}$O₂ max, which remains the standard for ISS missions. These findings suggest developing PES with relevant measurements for the astronaut environment. Ventilatory threshold, calculated from $\dot{\mathrm{V}}$O₂ max performed on the CEVIS, could serve as an alternative to critical power.^27,28 Despite the lack of direct applicability to predicting astronaut performance thresholds due to a lack of contextualization, Ade et al. support shifting the assessment focus to better evaluate astronauts’ physical capabilities for long-duration missions.^14,15

Nevertheless, caution is warranted as studies on astronauts’ occupational tasks often overlook the distinct gravitational conditions, challenging environments (terrain and temperature), and altered metabolic costs associated with suited ambulation (Table IV). Norcross et al. showed that in lunar analog conditions, walking in suited conditions had a submaximal metabolic cost but was still higher than on Earth.²⁹ In contrast, brisk walking on Mars in a suit requires near-maximal physiological effort, emphasizing the importance of these factors in evaluating astronauts’ fitness tasks. The metabolic impact of the spacesuit, regardless of its weight, can be evaluated by comparing suited trials to unsuited weight-matched controls. This difference is about 8.0 ml · kg · min⁻¹, representing roughly 18% of the subjects’ average $\dot{\mathrm{V}}$O₂ peak. For example, Ade et al. found that individuals completed the 10-km walk-back on an indoor track in 61.0 ± 12.9 min without spacesuit,¹⁴ compared to 95.8 ± 13.0 min recorded by Norcross et al. while suited and on simulated lunar gravity.²⁹

Exploring key performance thresholds for astronauts is essential. This involves examining factors like space deconditioning,¹⁹ challenging terrain, the metabolic demands of the spacesuit, and variations in gravitational forces. Ryder et al. studied the impact of various weights, simulating the spacesuit, on astronaut EVA tasks.²¹ Aerobic capacity was the main predictor for performance in hill climb and descent. Among the studies, the task with the highest oxygen uptake was inclined walking while wearing a 48-kg weighted suit mimicking the NDX-2 spacesuit weight distribution, registering values of 32.28 ± 6.57 ml · kg · min⁻¹.²² This aligns with NASA’s preliminary fitness criteria, which aim to mitigate the physical performance decline due to reduced fitness.¹ The $\dot{\mathrm{V}}$O₂ max, as determined by either direct or indirect measures, should be maintained at levels no lower than 32.9 ml · kg · min⁻¹ for missions involving microgravity EVAs and 36.5 ml · kg · min⁻¹ for missions with EVAs on celestial surfaces. Given the historical declines of 15–25% decrease in $\dot{\mathrm{V}}$O₂ max during spaceflight, pre-mission $\dot{\mathrm{V}}$O₂ max guidelines are set at 38.7 and 43.8 ml · kg · min⁻¹, respectively. In-mission aerobic capacity must be sustained at or above 80% of the pre-mission capacity, achieved through either countermeasures or work performance. NASA guidelines rely on either flight data or analog study findings and would adequately accommodate the average peak EVA $\dot{\mathrm{V}}$O₂ of 19.4 ml · kg · min⁻¹ and even the average of the top 10 EVA peak $\dot{\mathrm{V}}$O₂ values, which stood at 32.3 ml · kg · min⁻¹. However, factors such as challenging terrains,^30,31 spacesuit movement restrictions, and their metabolic consequences may affect performance.

Assessing maximum oxygen consumption during spaceflight necessitates an onboard pulmonary gas exchange system, ideally a breath-by-breath metabolic cart. Historically, the ISS has supported such health-monitoring, but upcoming flight systems (e.g., Orion space capsule) may face challenges due to constraints such as limited space, weight, and technical restrictions.³² Besides maximal oxygen uptake, future exercise physiologists may take a closer look at what happens to oxygen uptake kinetics and ventilatory threshold in space.²⁷ Ventilatory threshold may be a stronger predictor of human performance in orbit than maximal oxygen uptake itself.

Unaided top hatch emergency exit, as exemplified by Alexander et al., represents a unique task.¹⁶ This is especially crucial because, in the event of an emergency, it must be performed quickly under Earth’s gravity following a mission that may cause deconditioning and a considerable decline in physical capabilities. As individuals with relatively low PPO and $\dot{\mathrm{V}}$O₂ peak below 20 ml · kg · min⁻¹ were still able to complete the egress mock-up test in under 2 min, $\dot{\mathrm{V}}$O₂ peak and PPO may not serve as reliable discriminators of astronauts’ conditions for emergency egress performance. Nevertheless, considerations such as the motion sickness or vestibular impairment induced by a quick return to gravity should be noted. Since NASA anticipates ocean landings, the completion time for this test may increase compared to the same egress test conducted on a flat surface with controlled conditions. Therefore, it is essential to assess these tests and performance benchmarks under conditions similar to those astronauts might encounter, potentially deconditioned, upon returning to Earth.

The deconditioning effects of spaceflight can significantly impact certain astronaut tasks. According to Mulavara et al., the decline in plasma volume observed after microgravity was linked to decreased performance in the seat egress and walk test, as well as the ladder climb.¹⁹ Simultaneously, alterations in heart rate were positively correlated with performance declines in functional tasks that strain the cardiovascular system, particularly those involving prolonged upright posture like seat egress and walk, recovery from fall to stand, object translation, ladder climb, and hatch opening.¹⁹ This indicates that accounting for plasma volume and heart rate adaptations to spaceflight is crucial in shaping countermeasures and establishing performance thresholds for Moon or Mars missions, especially for tasks requiring extended periods of whole-body upright physical activity. Furthermore, Taylor et al. found that performing tasks in a weighted suit made the hatch opening task particularly challenging, as evidenced by the peak RER.²²

A factor scarcely addressed in the studies is neurovestibular deconditioning resulting from extended exposure to microgravity.¹⁹ This issue could potentially impair performance during tasks and pose safety risks to astronauts. Research from NASA’s Human Physiology, Performance, Protection & Operations Laboratory has highlighted a notable increase in post-mission time required for unassisted capsule egress.³³ Hence, establishing a minimum requirement for this task also necessitates considering the landing terrain type and accounting for expected deconditioning based on the mission duration and implemented countermeasures. On Earth, using a virtual reality set-up to mimic neurovestibular impairment might be a viable option for preparation before performing a physically challenging task.³⁴

Neuromuscular Requirements

Research on the relationship between neuromuscular capacities assessed through traditional laboratory tests and performance in occupational tasks has yielded inconsistent findings. The strength of associations varies, with the most robust correlation reported by Ryder et al., indicating a high R² value of 0.9 for bench press work and crewmember rescue drag performance.²¹ Building on this, Taylor et al. demonstrated that incorporating occupational task performance, in addition to NASA standard measurements, accounted for 99.6% of the variability in the overall time-to-completion for a test battery.²² This was a major improvement compared to the 61.5% variability explained by NASA standard measurements alone.

As stated earlier, the deconditioning effects of spaceflight can pose important risks. Mulavara et al. demonstrated that certain tasks, such as seat egress and walk, recovery from fall to stand, and object translation, were notably associated with declines in lower-body neuromuscular performance metrics.¹⁹ Conversely, upper-body neuromuscular performance seems to remain relatively preserved during HDBR and spaceflight, as there was no clear association between changes in upper-body neuromuscular function and task performance.

Ryder et al. highlighted that defined muscle performance thresholds are critical for executing occupational tasks, with performance diminishing when these thresholds are not met.^20,21 In establishing muscle function thresholds for astronauts, it is essential to account for individual body weight, suit weight, and the effects of partial gravity.^20,21 The added weight of the suit can significantly affect performance. For example, a 20% and 40% decrease in power relative to body weight below 17.6 W · kg⁻¹ in the upright seat egress tasks results in an additional 2.8 s and 6.7 s in task duration, respectively, which could pose safety risks.²⁰ In line with the aforementioned lack of association with maximum oxygen consumption, relative leg press maximal isometric force, with a threshold of 17.8 N · kg⁻¹, emerges as a reliable predictor for short-duration tasks (<50 s) involving an ambulatory component. For longer-duration tasks, the threshold falls within the range of 17.3–17.65 N · kg ⁻¹.²¹ Due to the task-specific nature of thresholds, bench press performance metrics, such as relative bench press work at 46 and 25.08 J · kg⁻¹, are effective predictors for the hatch opening task²⁰ and crewmember rescue, respectively.²¹ Overall, these studies suggest that strength requirements for unaided egress are greater upon returning to Earth than for planetary surface, and the microgravity environment can ease the execution of certain tasks.²³

When predicting astronaut performance based on ground-based tests, it is crucial to address the lack of an astronaut-specific context. The microgravity environment and the pressurized spacesuit increase physical demands, leading to higher metabolic cost and reduced performance.²⁵ NASA’s current pre-mission neuromuscular fitness evaluations primarily focus on traditional lower-body strength and endurance laboratory tests, which may overlook the specific fitness requirements of the upper extremities in occupational tasks.²²

NASA’s preliminary fitness criteria specify that astronauts should meet pre-mission muscle strength requirements (deadlift and bench press) and maintain at least 80% of these baseline strength values during the mission.²⁴ The minimum standards are set at 1.0 times body weight for deadlift and 0.7 times body weight for bench press. For EVAs on celestial surfaces, higher thresholds are suggested: 1.6 times body weight for deadlift and 1.0 times bodyweight for bench press. Currently, there are no explicit recommendations for missions to the Moon or Martian surfaces, and the rationale behind these benchmarks remains unknown. NASA notes that “EVA suit design (i.e., how the suit design affects human performance) must be taken into account and may necessitate adjustments to these values.”¹

Sensorimotor Requirements

Maintained cardiorespiratory capacity, muscular strength, and endurance do not guarantee optimal execution of functional tasks after spaceflight. Cowings et al. provides evidence supporting that spatial disorientation and motion sickness may impact the performance during and after spacecraft re-entry.¹⁷ Thus, these factors must be considered when establishing acceptable performance thresholds. However, Cowings et al. used a simulation that does not fully replicate real-life conditions. Further research is needed to involve subjects in tasks that closely align with astronauts’ responsibilities during re-entry and account for deconditioning factors.

Astronauts performance during occupational tasks might be limited by sensorimotor impairment, as spaceflight can affect postural equilibrium.^35,36 This finding is corroborated by Miller et al., who indicates that deconditioning during bed rest markedly influences functional performance and balance control during simulated activities, such as seated egress in emergencies, object manipulation, recovery from fall/stand, and jumping down.¹⁸ Additionally, the extent of functional performance deficits was directly related to mission duration, and countermeasures were insufficient to counteract these effects, even though bed-rest exercise regimen enabled faster recovery.

Changes in fine motor control constrained tasks involving reaching, grasping, and object manipulation, such as the object translation and activity board tests.¹⁹ Most exercises, including those performed on the ISS, are performed on machines⁵ that may restrict the neuromuscular coordination required for tasks. For extended missions such as a journey to Mars, a countermeasure must address sensorimotor changes to efficiently reduce postflight postural impairments. Nevertheless, specific occupational tests need to be designed to capture these aspects.

Stress during spaceflight is well documented³⁷ and must be considered when establishing performance thresholds for occupational and critical tasks. During extended space missions, sleep deprivation, disruptions to circadian rhythms, and work overload are prevalent, contributing to diminished performance, concentration, and alertness.³⁸ As a result, the likelihood of errors and accidents increases, and the execution of critical tasks may be impaired.

In the studies reviewed, participants underwent varying degrees of familiarization, from none to four practice sessions. However, details about these practice trials and familiarization periods are seldom thoroughly reported in the methods sections. Given that astronauts undergo extensive training to prepare for various scenarios during space missions, it is crucial for simulation protocols to incorporate adequate familiarization. Since some tasks involved the use of weighted suits and unfamiliar equipment, additional practice sessions should be considered to ensure that participants are tested under conditions more closely resembling those faced by astronauts.

The limited number of studies meeting this review’s criteria, combined with significant variability in tasks and contexts, impedes the current establishment of definitive standards. Nevertheless, this review identifies the occupational tasks that should serve as the foundation for developing standards in future astronaut PES research. A limitation of this review is the exclusion of conference papers or organization reports from the synthesis, though these may be addressed in the discussion. Such documents often lack the detailed information necessary to evaluate the quality and relevance of occupational assessments effectively. The review is also restricted to French and English due to the authors’ language proficiency. However, we believe this does not hinder the identification of articles pertinent to this review on astronaut occupational tasks.

In summary, this review highlights the variety of muscle performance metrics available for predicting success in astronaut mission tasks and explores the physical demands of astronaut tasks, offering new insights into this area of research. Relying solely on standard laboratory tests to establish performance thresholds may overlook critical factors. It is recommended to base thresholds on the actual physical demands of astronaut tasks, irrespective of age or gender. Currently, to the best of the authors’ knowledge, no task-specific standards for astronauts have been officially set by NASA or other space agencies. While training to mitigate microgravity effects is well-documented, understanding operational performance in microgravity, especially in deconditioned astronauts, remains limited. This review advocates a shift in fitness assessment to develop tailored PES for astronauts, viewed as tactical athletes.