Centrifugation as a countermeasure during bed rest and dry immersion: What has been learned?

OBJECTIVES: We review the studies that have evaluated intermittent short-radius centrifugation as a potential countermeasure for cardiovascular, musculoskeletal, and sensorimotor deconditioning in simulated weightlessness.
METHODS: The findings from 18 experimental protocols that have used bed rest and dry immersion for comparing the protective effects of centrifugation versus standing upright or walking, and the effects of continuous vs. periodic exposure to centrifugation are discussed.
RESULTS: Centrifugation for as little as 30 min per day was found to be effective in mitigating orthostatic intolerance and strength in postural muscle after 5 days of bed rest, but it was not effective in mitigating plasma volume loss.
CONCLUSION: To determine the optimal prescription for centrifugation as a countermeasure, we recommend further studies using (a) bed rest of longer duration, (b) individualized prescriptions of centrifugation combined with exercise, and (c) functional performance tests.

Introduction – The gravity of the situation

Deconditioning of the cardiovascular and musculoskeletal systems has been observed during space flight1-5, head-down bed rest4,6-8, and dry immersion9. Evidence comes from reduced plasma volume, reduced exercise capabilities, and increased orthostatic intolerance, as well as muscle weakening and bone loss. The cause of this deconditioning is mostly attributed to the lack of both the static G force along the longitudinal body axis (z-axis) and the body’s exertion against this Gz force during movement and locomotion10.

Artificial gravity generated by centrifugation has the potential to mitigate this deconditioning by mimicking a constant Gz stimulus equivalent to the one experienced on Earth. A constant 1 Gz stimulation elicited by spinning the whole spacecraft would be the most effective, but this solution requires additional costs in term of mass, power, and controls. A more affordable solution is periodic Gz stimulation of individual crewmembers using an onboard short-radius centrifuge11-13.

To date, only two human-rated short-radius centrifuges have flown in space, on board the Space Shuttle in 1992 and 1998. However, the primary objective of these experiments was to investigate not artificial gravity but spatial orientation (eye movements, motion perception) in subjects exposed to transient linear acceleration in space14-15. In the first experiment, four astronauts were positioned on a rotating chair so that their head and feet were off center by a few cm, generating -0.22 Gz at the head level and a centripetal force of +0.36 Gz at the feet. Duration of rotation was 1 minute every other day of a 7-d mission. None of the subjects perceived any sense of tilt relative to the +Gz stimulus during the in-flight tests14.

During the second experiment, subjects sitting upright or lying supine on a flight centrifuge were exposed to +1 Gy, +0.5 Gz, and +1 Gz for about 10 min per day during a 16-d mission. They felt tilted relative to the direction of the G stimulus, so centrifugation was actually perceived as artificial gravity by the crewmembers15. No sign of altered vestibular responses or orthostatic intolerance was observed during postflight tilt tests in any of the four crewmembers exposed to in-flight centrifugation. The other three crewmembers on that mission had orthostatic intolerance. Based on the result that 64% of astronauts experienced severe orthostatic intolerance after Space Shuttle missions1, the probability that four crewmembers on the same flight would not exhibit orthostatic intolerance by chance is about 1 in 6016.

So, except for this latter study, little is known about the effects of centrifugation on cardiovascular function in space. Head-down bed rest (HDBR) is a valuable analog for simulating some of the effects of space flight on this function17. HDBR is characterized by inactivity, confinement, and suppression of the +Gz gravitational stimulus. Unloading the body’s upright weight reduces proprioceptive stimulation and eliminates the need for musculoskeletal force to work against gravity, thus reducing the body’s energy requirements. The upward fluid shift during HDBR, by acting on central volume receptors, induces a reduction in plasma volume that leads to orthostatic intolerance during head-up tilt and upright standing after HDBR. Multiple factors influence this orthostatic intolerance; they include decreased blood volume, decreased baroreceptor sensitivity, increased venous distensibility, decreased heart muscle strength, and altered autonomic function. In addition, bone resorption is increased by HDBR, leading to a sustained negative bone balance. Body weight, muscle strength, exercise endurance capacity, and aerobic power are also reduced in a manner similar to what happens during space flight. Over the past 20 years, HDBR has proved its usefulness as a reliable simulation model for most of the physiological effects of space flight6.

Dry immersion (DI) consists of immersing a subject covered with an elastic waterproof fabric in thermo-neutral water. As a result, the immersed subject, who is virtually buoyant, remains dry. Russian investigators have reported that DI leads to the same changes as HDBR, but after a relatively shorter duration of exposure, presumably because of the lack of perceived body weight9.

We have identified 18 experimental protocols that, in the past 50 years, have investigated the benefits and side effects of a +Gz stimulation during HDBR and DI. This article summarizes what has been learned during these ground-based studies and recommends further research.

Physiological effects of Gz stimulation during bed rest and dry immersion

In these experimental protocols the +Gz stimulus was periodically provided during HDBR and DI using four different methods: (a) short-radius centrifugation (SRC); (b) standing upright; (c) walking or running on a treadmill; and (d) physical exercise simulating locomotion (Table 1). For SRC the centrifuges had a short outer radius (1.5-2.5 m), and consequently the +Gz stimulus at the feet was larger than at the heart. Note, however, that on Earth, the centrifugal force combines with the gravitational force (along the Gx axis in a supine subject), and the resultant force is larger than 1 G. The smallest Gz level tested was 0.38 Gz at the heart, corresponding to the gravity level on the surface of Mars. The largest level tested was 2 Gz at the heart, coupled with cycling. In average, studies utilized 1 Gz at the heart (SD 0.4 Gz; median 1 Gz). This Gz level was presumably chosen because the most obvious countermeasure in space would be to provide a 1 Gz artificial-gravity environment.

Abbreviations

AGEG	Artificial gravity expert group
BR-AG1	First bed rest and artificial gravity study
CON	Control condition
DI	Dry immersion
ESA	European space agency
FT	Field test
FTT	Functional task test
HDBR	Head-down bed rest
ISS	International space station
LRT	Locomotion replacement training
MVC	Maximum voluntary contraction
SAG	Simulated artificial gravity
SRC	Short-radius centrifugation
STA	Standing

The duration of the DI protocols (mean 9.7 d; SD 9.8 d; median 5.5 d) was shorter than the HDBR protocols (mean 11.4 d; SD 11.3 d; median 5 d). Overall both of these studies had a relatively short duration (mean 10.8 d; SD 10.5 d; median: 5 d). Short-duration analog studies have the advantage of being both practical and cost effective, especially in the case of a crossover experimental design in which the same individuals can be tested repeatedly using various +Gz stimuli more frequently, and these treatments can be randomized. In addition, previous studies have demonstrated that orthostatic intolerance occurs within a few hours of HDBR or DI9,37, maximal exercise capacity is reduced after 24 h, bone resorption starts to increase on the second day of bed rest38, and diuresis occurs mostly during the first 48 h39-41. Although plasma volume is somewhat reduced by 24 h, this reduction is essentially maximal by 3 days42.

The duration of exposure to the +Gz stimulus during HDBR ranged from 25 min to 4 h per day (mean 1.0 h; SD 0.9 h; median 0.7 h). The SRC sessions were an average of 60% longer during the DI protocols (mean 1.6 h; SD 0.5 h; median 1.5 h). However, the SRC sessions were not performed every day (see notes in Table 1), so overall the duration of Gz exposure for both HDBR and DI studies was comparable. This duration was presumably chosen for the purpose of comparing the effects of artificial gravity with the effects of traditional countermeasures, such as physical exercise that crewmembers in orbit also perform for about 1-1.5 h per day43-44. This comparison is difficult, however, because some of the HDBR and DI studies using SRC also had the volunteers perform aerobic exercise.

Details on the study protocols 1-15 listed in Table 1, including number and gender of subjects, and experimental design (crossover or with a control group), can be found in Kaderka19. Kaderka also performed a meta-analysis on the main results obtained with these protocols in terms of cardiovascular performance (orthostatic tolerance time, plasma volume, hematocrit measurement, stroke volume, heart rate, total peripheral resistance, VO2 max), muscle alteration (soleus and vastus lateralis cross-sectional area, muscle volume, knee extensor maximum voluntary contraction) and bone changes (bone mineral density on the lumbar spine, femoral neck, trochanter and total hip, bone resorption markers, bone formation markers, calcium in the urine, and serum).

Despite vast differences between these study protocols in terms of objectives, durations and measured physiological parameters, results have shown that a periodic circa 1 Gz stimulus at the heart during HDBR and DI does the following: (a) improves post-HDBR and post-DI orthostatic tolerance time32,34,36,45-47; (b) reduces the exaggerated responses to head-up tilt after the interventions, such as elevated heart rate and increased muscle sympathetic nerve activity27,48-49; (c) attenuates plasma volume loss when SRC is combined with exercise29-30,50; and (d) maintains exercise capacity31,51. These benefits are not surprising given that cardiac performance and baroreceptor sensitivity are presumably optimized for functioning in a 1 Gz environment. On Earth we spend about 8 h per day exposed to a 1 Gx or 1 Gy stimulus when sleeping and 16 h per day exposed to a 1 Gz stimulus when sitting or standing (or more during locomotion). What is surprising is how little +Gz exposure the human body needs per day to maintain adequate exercise capacity and orthostatic responses10.

Is shorter, more frequent Gz stimulation best?

Only three studies have attempted to answer the question of how often +Gz stimulation is needed to maintain normal physiological functions. These studies have compared the effects of generating the same duration of +Gz stimulus during HDBR and DI in two, three, six, eight or sixteen daily sessions in the same subjects. It was hypothesized that several shorter centrifugation periods with rest in between would not only be better tolerated by the subjects, but also prove more efficient as a countermeasure. Support for this hypothesis comes from studies on hind-limb suspension in rats, and in-orbit exercises in astronauts that showed that repetitive short-duration, high-load exercise training was more effective in mitigating musculoskeletal deconditioning than longer, less intense sessions.

Vil-Viliams & Shulzhenko26 compared SRC-generated +Gz stimulation for 60 min twice a day and 40 min three times a day in subjects otherwise immersed in water. Both +Gz treatments were equally effective, as shown by the same mitigating effects on orthostatic intolerance after water immersion.

During the first ESA First Bed Rest and Artificial Gravity (BR-AG1) study, Linnarsson et al.36 compared daily SRC sessions generating 1 Gz at the heart for 30 min continuously (1 x 30 min) and for 6 bouts of 5 min (6 x 5 min) separated by 3 min of rest. HDBR without SRC was used as a control condition (CON). The effects of the two +Gz treatments and the control condition were investigated in a crossover study design in which eleven subjects were each tested during three campaigns of 5-d HDBR, in a random order. The results of the various investigations using this protocol are summarized in Table 2.

Table 1

List of the various experimental protocols used for investigating the effect of +Gz stimulation using short-radius centrifugation (SRC) or standing/walking/running during bed rest (BR) and dry immersion (DI) in the past 50 years. LRT: locomotion replacement training (see text for details). Adapted from Clément & Pavy-Le Traon18 and Kaderka19.

Study	Intervention	Days	+Gz at heart	Number of daily sessions	Session duration (min)
1. White et al.20	BR + SRC	41	1.4	4	7.5
2. Nyberg et al.21	BR + SRC	10	1.8	4	20
3. Kamenskiy et al.22	DI + SRC	3	0.5, 0.6	1	60
4. Gale et al.23	DI + SRC	4	0.5, 0.6	1	60
5. Grigoriev et al.24	DI + SRC	13	0.6	1[a]	60, 90
6. Vil-Viliams & Shulzhenko25	DI + SRC	3	1.0	3	40
7. Vil-Viliams & Shulzhenko26	DI + SRC	28	0.8, 1.2, 1.6	2[b] 3[b]	60 40
8. Yajima et al.27	BR + SRC	4	2.0	1	60
9. Vil-Viliams28	DI + SRC + Cycling	7	0.8, 1.2, 1.6	1	120
10. Vernikos et al.10	BR + Standing + Walking	4	1.0	8 16	15 15
11. Lee et al.29	BR + Running	5	1.0	1	30
12. Iwasaki et al.30	BR + SRC	4	2.0	2	30
13. Katayama et al.31	BR + SRC+ Cycling	20	0.4, 0.8, 1.4	1[c]	40
14. Iwase32	BR + SRC + Cycling	14	1.2	1[c]	30
15. Young & Paloski33	BR + SRC	21	1.0	1	60
16. Yang et al.34	BR + SRC + Cycling	4	0.4, 0.7	1	30
17. Mulder et al.35	BR + Standing + LRT	5	1.0	1	25
18. Linnarsson et al.36	BR + SRC	5	1.0	1 6	30 5

Notes:

SRC was used on DI days 8-13 only.

SRC was used on DI days 9-14 and 23-28 only.

SRC was used during 3-4 days per week only.

The 6 x 5 min +Gz treatment was found to be the most effective in preserving orthostatic tolerance after HDBR, and appeared equivalent to a continuous 60-min exposure to +Gz stimulation in other studies36. However, neither the 6 x 5 min nor the 1 x 30 min +Gz treatment attenuated plasma volume loss54. The interpretation for the observation that centrifugation has a beneficial effect on orthostatic tolerance without mitigating plasma volume loss is the following: first, the centrifugal force pushes the blood “down” to the feet and the venous return in the legs pushes it back to the heart. This reaction might take only a short period of time on the centrifuge because reflexes are usually reinforced with rather small time periods. The second effect of centrifugation is an attempt to maintain plasma volume, as well as muscle and bone integrity, by mimicking the continuous presence of Earth gravity, and this process is more time-consuming. The decrease in plasma volume is mostly due to urinary excretion in response to the fluid shift to the upper body that occurs during head-down tilt and spaceflight. This fluid shift is interrupted temporarily during SRC, but 30-60 min per day might not be sufficient. Nevertheless, if SRC were not sufficient, space travelers could perform fluid loading to compensate for plasma volume loss, as is currently done on the International Space Station (ISS) prior to returning on Earth1.

The subjects reported fewer neurovestibular symptoms during the 6 x 5 min than during the 1 x 30 min +Gz treatment53. The conclusion that a 6 x 5 min +Gz treatment was less stressful was also supported by the subjects’ neuroendocrine responses. Indeed, Choukèr et al.55 found that the 6 x 5 min +Gz treatment was associated with lower adrenocortical stress responses than the 1 x 30 min +Gz treatment in the same subjects.

The 6 x 5 min +Gz treatment also increased the maximal voluntary contraction (MVC) capability in the knee extensor and plantar flexor muscles, which was not the case for the 1 x 30 min +Gz treatment49. On the other hand there were no significant differences between the two +Gz treatments in aerobic power (peak VO2) after HDBR compared with the control condition36.

Serum levels of bone formation markers decreased and serum levels of bone resorption markers increased towards the end of HDBR in control subjects, and these changes were attenuated in centrifuged subjects for both +Gz treatments52. A decrease in vertical jump height after bed rest with no countermeasure was also prevented by both the 6 x 5 min and the 1 x 30 min +Gz treatments49.

In yet another study Vernikos et al.10 used a crossover design for testing nine subjects across four treatment conditions and one control condition during 5 HDBR campaigns of 4 days each. The treatment conditions included passive (standing still) or active (walking at 3 mph on a treadmill) +Gz stimulation for 8 times 15 min or 16 times 15 min. The interval between two successive sessions was 1 h. When comparing these four treatment conditions and the control condition for which no intervention was used, the investigators showed that periodic +1 Gz as low as 2 h per day was effective in mitigating cardiovascular deconditioning during HDBR.

However, 1 Gz standing was found to be more effective for protecting against orthostatic intolerance and decrease in plasma volume, whereas 1 Gz walking was found to better mitigate the decreased peak VO2 and the increased urinary calcium excretion during bed rest. The investigators suggest “that passive upright standing imposes a greater orthostatic challenge to maintenance of cardiac output and cerebral perfusion than walking since the contraction of leg muscles during walking, in combination with competent venous valves, contributes to venous return via the skeletal muscle pump”10. They further recommend that a combination of standing and walking should prove the most effective Gz prescription. Also supporting this view are the results of studies by Yajima et al.56 and Iwase32, which demonstrated that daily SRC combined with light leg exercise prevented most of the plasma volume loss during 4- and 14-day HDBR, respectively. Therefore, it appears that exercise is a more effective method of preserving plasma volume during HDBR than centrifugation without exercise.

The above results were recently confirmed in another series of 5-d HDBR during the ESA Simulated Artificial Gravity (SAG) study35. This crossover design study was performed under the following conditions with 10 male subjects during 6° HDBR: (a) with no countermeasure; (b) while standing upright for 25 min per day (STA); and (c) during a locomotion-replacement training (LRT) including a combination of heel raising, squatting, and hopping exercise for 25 min per day. The results of the investigations utilizing this protocol are summarized in Table 3. The LRT treatment was found to be more effective than STA for maintaining knee extensor and plantar flexor muscles’ integrity. LRT also increased the maximal voluntary contraction capability for the knee extensor muscles after HDBR57. STA and LRT Gz treatments were equally useful for preserving postural stability after HDBR58. However, neither countermeasure protected against metabolic59, cardiovascular35, or bone49 deconditioning induced by HDBR. The differences between this study and Vernikos’ study10 suggest that +Gz stimulation for 25 min per day applied continuously may be too short to be effective as a countermeasure.

Table 2

Summary of the results of the ESA First Bed Rest and Artificial Gravity (BR-AG1) study performed at MEDES in Toulouse in 2010. BR-AG1 consisted in a series of three 5-d HDBR campaigns in which 10 male subjects were not centrifuged (CON), or were exposed to short-radius centrifugation (SRC) generating 1 Gz at the heart for one single session of 30 min (1 x 30 min) per day, or for 6 sessions of 5 min (6 x 5 min) per day. Data from Kos et al.52; Clément et al.53; Linnarsson et al.36; and Rittweger et al.49.

Measures	CON	1 x 30 min SRC	6 x 5 min SRC
Metabolism
- Nitrogen balance	Decreased	Same as CON	No changes
- Urinary adrenaline	No changes	Same as CON	Increased
Cardiovascular
- Plasma volume	Decreased	Same as CON	Same as CON
- Exercise capacity	Decreased	Same as CON	Same as CON
- Orthostatic tolerance	Decreased	Same as CON	Decreased less
- Heart rate	Increased	Same as CON	Same as CON
Sensorimotor
- Postural instability	No changes	Same as CON	Same as CON
- Gait	No changes	Same as CON	Same as CON
- Vestibular symptoms		Moderate	None
- Subjective efficiency		Yes	Yes
Muscle
- Knee extensor MVC	No changes	No changes	Increased
- Knee flexor MVC	No changes	Increased	Increased
- Plantar extensor MVC	No changes	Same as CON	Same as CON
- Plantar flexor MVC	No changes	Same as CON	Increased
- Elbow extensor MVC	No changes	Same as CON	Same as CON
- Elbow flexor MVC	No changes	Same as CON	Same as CON
- Maximum jump height	Decreased	No changes	No changes
Bone
- Bone resorption	Increased	Increased less	Increased less
- Bone formation	Decreased	No changes	Decreased less
- Calcium level	Increased	Same as CON	Same as CON

Limitations and lessons learned

Centrifugation along the Gz axis in supine subjects not only restores the reduced orthostatic intolerance that occurs after HDBR or DI deconditioning, but also redistributes and retains blood in the venous system of the lower extremities similar to the effect of standing. In fact, most subjects perceive themselves to be standing upright when they are exposed to 1 Gz at heart level, i.e. close to their body’s center of mass60. As discussed above, significant benefits of a 1-Gz stimulation at the heart for as little as 30 min per day were observed for muscle maximum contraction, jump performance, and changes in levels of markers for bone homeostasis during HDBR and DI. A repeated, shorter exposure (6 x 5 min) was more effective than a continuous, longer exposure (1 x 30 min) and was also better tolerated by the subjects. With the shorter exposure, subjects complained of less discomfort due to the prolonged straining caused by high +Gz at the feet. The severity of the neurovestibular symptoms reported by the subjects during SRC was relatively low, with the highest score (13 on a scale from 0 to 45, with 45 being the most severe) reached during the first SRC session53. In addition, subjects reported that their perceived rate of recovery after HDBR was faster with SRC than without it.

As pointed out by Kaderka19 “an important consideration that must be realized when comparing different countermeasure groups is the variation in intent of treatment protocol.” Some AG protocols in Table 1 were created specifically to counteract a particular physiological deconditioning, e.g. muscle atrophy or bone loss. Only recently, starting with Young & Paloski33, HDBR studies have investigated the mitigating effects of AG across several physiological systems. The aim of the protocol is not to benefit any specific physiological system, but rather to evaluate the efficiency of a particular AG prescription across a large range of physiological and psychological responses.

Because both the SAG and the BR-AG1 studies used the same HDBR duration (5 d), +Gz stimulus duration (25-30 min), standardized bed rest core data measures, and a crossover study design, a direct comparison could be made between the effectiveness of intermittent standing, walking-like, and SRC. A qualitative comparison between the changes reported in Tables 2 and 3 indicates that SRC has a better protective effect than standing or walking in terms of metabolism, cardiovascular performance and bone marker changes after HDBR.

The challenge of a crossover study design is to determine the period of time needed between two consecutive HDBR campaigns, so that the effects of the first HDBR have completely washed out before the second HDBR begins. The longer this interval, the better; however, it is difficult to find volunteers who are available for very long periods. Both the SAG and BR-AG1 studies used a crossover design. During the SAG study the interval between the first and second HDBR campaigns was 65 days and the interval between the second and third HDBR campaigns was 114 days. During the BR-AG1 study, the interval between the three HDBR campaigns was 32 days. This 32-d interval was too short, as some of the sensorimotor and musculoskeletal responses had not completely returned to baseline between HDBRs35,49,52. For example, bone loss tends to continue for about 30 days after bed rest lasting 35-90 days61-63, and the exact nature of the bone loss during this recovery period is unclear.

A decrease in serum levels of markers for bone formation (CD200) and an increase in serum levels of markers for bone resorption (CD200R1) were observed after a few days of HDBR, and these changes were attenuated by SRC52. Nevertheless, a few days is too short for actually assessing structural changes in bone64. Smith et al.65 did not find significant differences in bone mineral density during SRC compared to controls after a 21-d bed rest. Bed rest studies that have shown a protective effect of exercise on bone were of much longer duration; e.g., 56 days63 to 117 days66.

No bed rest studies combined with intermittent centrifugation have examined the structural integrity of muscle fibers (i.e., cross-sectional area and distribution by fiber type) after deconditioning, although this test has been performed in many of the traditional countermeasure studies7,67. Future artificial gravity studies on skeletal muscle deconditioning should therefore focus on the analysis of global muscle parameters, such as muscle volume and endurance, but also on individual muscle fibers by fiber type.

Testing the effectiveness of centrifugation as a countermeasure for sensorimotor deconditioning is rendered difficult by the fact that small changes in sensorimotor functions are generally observed after HDBR. A recent systematic study of sensorimotor behavior after long-duration (42-63 days) HDBR demonstrated changes in postural reflexes and functional mobility, but no changes in balance control68. The investigators suggested that changes in postural reflexes and functional mobility result from ascending somatosensory changes caused by postural muscle and plantar surface unloading during HDBR. By contrast, postural equilibrium would not be affected by HDBR because the vestibular system is still receiving normal graviceptive inputs even when one is recumbent. When testing postural equilibrium during dynamic head movements, though, Mulder et al.58 found larger postural instability after a 5-d HDBR, which was mitigated by daily 25-min sessions of standing or locomotion-like exercise. In addition, Moore et al.69 found that the error in the subjective visual vertical was significantly different from zero in a centrifuged group of subjects and not different in a control group after a 21-day HDBR. The ability to perceive verticality depends on input from visual, vestibular, and somatosensory systems. The abnormal subjective tilt after HDBR may therefore be caused by ascending somatosensory changes through prolonged unloading. Also, because abnormal subjective tilt and postural instability during dynamic head movements are commonly observed in astronauts returning from space70, we recommend that these two measurements be included in the battery of standardized sensorimotor tests after HDBR or DI.

Recommendations for future studies

Protocol duration

The European Space Agency (ESA) funded the 5-d HDBR SAG and BR-AG1 studies for a first screening of the potential benefits of intermittent SRC as a countermeasure for mitigating the physiological deconditioning induced by (simulated) weightlessness. As discussed above, these short-duration studies have demonstrated that intermittent SRC was more effective than intermittent standing or walking for mitigating orthostatic intolerance, but longer durations studies are needed to determine the actual effects of SRC on muscle and bone strength. One option is to repeat medium-duration (e.g. 21-d) campaigns, possibly with crossover design to minimize inter-subject variability, for determining the optimal AG prescription. Once the initial beneficial effects are verified during these medium-duration studies, then the duration of the studies would be extended.

Another option is to proceed with 60-d campaigns. For all intents and purposes, the effort and cost of performing a 60-d parallel group study is about the same as for three 21-d crossover design studies. A 60-d intervention also induces larger deconditioning effects, making it easier to characterize the efficiency of the countermeasure on muscle and bone. These long-duration studies would allow a better comparison of the effects of SRC combined or not with exercise, since pilot studies have clearly shown that exercise can complement SRC for mitigating plasma volume loss, as well as muscle and bone loss71-75. For exercise during centrifugation, Kaderka19 suggests adopting the protocol used by many traditional countermeasure studies for preserving leg muscle and bone. This protocol includes a combination of squat/calf presses and cycling in a two-day cycle alternating aerobic and resistive exercise.

Another argument in favor of testing AG during long-duration studies as soon as possible is related to the time limitations of the space program. Indeed, the ultimate goal of these studies is to determine whether AG delivered by SRC can effectively protect crew health and performance during long-duration missions. For a human Mars mission scheduled to launch in 2030, the mission vehicle and habitat designers will need the AG requirements in terms of gravity level and rotation rate several years before, i.e. presumably around 2022. Consequently, there is barely enough time between now and then to conduct at least five long-duration campaigns.

AG prescription

The primary objective of the recommended long-duration studies is to determine the optimal countermeasure prescription in terms of +Gz stimulation amplitude, duration, and frequency on the physiological functions that are affected by exposure to weightlessness. A +Gz acceleration increases the weight of blood and thus the hydrostatic pressure gradient from head to foot. Although the hydrostatic effects on the arterial side of the circulation become important only at high acceleration, even moderate acceleration has relatively large effects on the low-pressure side of blood circulation, i.e. the venous circulation. Venous return is compromised and cardiac output to regions above the heart is reduced. Healthy subjects can tolerate 3-4 Gz at the feet for 90 min76. However, deconditioned space travelers and bed rest volunteers may not be able to tolerate these levels of acceleration. In fact, in previous studies using SRC during HDBR or DI, the acceleration at the feet did not exceed 3 Gz. Given this limitation and the gravity gradient, the range of Gz stimulus that can be applied at the heart in supine subjects is constrained to 0.38-1 Gz. Only a small protective effect of 0.38 Gz at the heart was observed when intensive cycling exercise was used13,16. Therefore, a logical path forward is to use HDBR and DI studies to determine the effects of duration and frequency of SRC while keeping a 1 Gz stimulus at the heart.

Another path forward might be the following: rather than imposing a level, duration, and frequency for +Gz stimulation using SRC, each subject will decide what +Gz stimulus they can tolerate on any particular day. By analogy with the individualized prescription used in sports medicine77, in the proposed studies the individualized Gz prescription will be tailored to a subject’s (or crewmember’s) specific goals, needs, and abilities. During each daily session, the subject will decide on the duration and frequency of Gz stimulation, as well as its intensity (by adjusting the rotation rate of the centrifuge), within some guidelines specified by a fitness or rehabilitation specialist. A similar approach is currently used for the exercise regime of the astronauts on board the ISS. An individualized prescription motivates the participant to comply with it, thus better achieving the goal of the countermeasure. Unlike athletes, who receive feedback frequently during training and competition, bed rest subjects and space flight crewmembers do not have the benefit of feedback until the HDBR study or flight is completed. However, the assumption is that the more intense the Gz stimulus, the more efficient the countermeasure. The rationale for allowing centrifuged subjects to set the Gz stimulus is based on the desire to achieve optimal Gz loads. When HDBR subjects are exposed to a tilt test or LBNP, they often detect the onset of syncope more quickly than the medical monitor. The same is expected during centrifuge runs. Allowing subjects to set the Gz stimulus will likely bring them closer to their tolerance limit.

Another advantage of an individualization of the AG protocol is that one AG protocol may not work for all, as shown by recent findings of gender differences in response to AG training78. One drawback of the personalization of Gz level though, is that comparison with fixed protocols with fixed gravitational force where subjects stand could introduce bias in interpretation of the results.

Subjects

All the 18 studies listed in Table 1 were conducted on male subjects. Despite the fact that female crewmembers comprise only 11% of the individuals who have flown in space79, and that only two female crewmembers will visit the ISS between May 2015 and May 2018 (vs. 32 male crewmembers, i.e. 6.3%), it is likely that the crew of the human Mars mission will be a mixed gender crew. A recent study indicates that men and women demonstrate different mechanisms for regulating their cardiovascular responses to orthostatic tolerance limit tests following 90 min of AG and 90 min of HDBR. Women appeared to regulate blood pressure while men did not80. It is therefore important that AG protocols examine the effectiveness of protocols across gender.

An emphasis should also be placed on documenting the user’s point of view in a more systematic manner. In addition to the standardized questionnaire on neurovestibular symptoms, the individuals should provide subjective rating of comfort/discomfort, perceived exhaustion, perceived benefits, and any other physiological or psychological issues associated with the Gz prescription81.

Finally, the goal of an operational countermeasure is not only to maintain physiological functions within reasonable limits, but also to ensure that individuals can perform nominally after flight82. For testing the effectiveness of the Gz prescription, it is necessary to also include some tests of individual functional performance before and after the HDBR. These tests could be based on NASA’s Functional Task Test (FTT) or Field Test (FT), which are performed on astronauts immediately after they return from the International Space Station. These simple tests evaluate the crewmembers’ ability to stand up from a seated position, recover from falling, walk and step over obstacles without assistance, and see clearly while moving83-84.

Study design

Although the crossover study design for a 5-d HDBR was time and cost-effective for a quick-look assessment, a longer duration HDBR is more suitable to test countermeasure efficacy. However, a longer HDBR would require a longer washout period, which makes crossover study design impractical from both a time and cost perspective. Also, it is more difficult to recruit volunteers for long-duration HDBR with a crossover design. Therefore the recommendation is to use long-duration HDBR in a randomized, controlled parallel group design. A potential design could be the following: (a) one group of subjects is exposed to HDBR with daily SRC exposure combined with exercise (e.g., squatting, hopping, cycling) on the centrifuge; (b) a second group of subjects serves as a control for the combined effects of HDBR and the superimposed countermeasure. Subjects in this group are exposed to HDBR and perform the same daily exercise in a supine position; (c) a third group of subjects could also be exposed to HDBR except when they stand up and perform the same daily exercise as the other subject groups on the centrifuge. The difference between this third group and the group of subjects who exercise while on the centrifuge should allow a direct comparison of the effects of a Gz stimulus provided by gravitational force and the effects of a stimulus provided by centrifugal force.

With a parallel group design it is imperative that both groups are as homogeneous as possible. For example, in addition to the standardized selection criteria used for previous studies, subjects should be screened for motion sickness susceptibility85-86 before they are included in a study. Indeed, the single subject who had to withdraw during the BR-AG1 study had a history of high susceptibility to motion sickness. It is also recommended to expose subjects to several SRC sessions with progressively increasing rotation rates during an ambulatory period prior to the HDBR study to ensure they all have a similar tolerance to centrifugation.

Table 3

Summary of the results of the ESA Simulated Artificial Gravity (SAG) study performed at DLR in Cologne in 2010-2011. SAG consisted of a series of three 5-d HDBR campaigns in which 10 male subjects stayed supine (CON), stood upright by the bed (STA) for 25 min per day, or performed an upright locomotion replacement training (LRT). Data from Mulder et al.35,57-58.

Measures	CON	25 min STA	25 min LRT
Metabolism
- Body mass	Decreased	Same as CON	Same as CON
- 24-h urine volume	Increased	Same as CON	Same as CON
- Nitrogen balance	Decreased	Same as CON	Same as CON
Cardiovascular
- Plasma volume	Decreased	Same as CON	Same as CON
- Exercise capacity	Decreased	Same as CON	Same as CON
- Orthostatic tolerance	Decreased	Same as CON	Same as CON
- Heart rate		Increased	Increased
Sensorimotor
- Postural instability	Increased	No changes	No changes
- Gait	No changes	Same as CON	Same as CON
Muscle
- Knee extensor CSA	Decreased	Decreased	No changes
- Plantar flexor CSA	Decreased	Decreased	No changes
- Knee extensor MVC	Decreased	No changes	Increased
- Plantar flexor MVC	No changes	Same as CON	Same as CON
- Maximum jump height	Decreased	Same as CON	Same as CON
- Neural activation	No changes	Same as CON	Same as CON
- Fatigability	No changes	Same as CON	Same as CON
Bone
- Bone resorption	Increased	Same as CON	Same as CON
- Bone formation	Increased	Same as CON	Same as CON