1. 1. Introduction

A human mission to Mars will be physically demanding and will present a variety of risks to crew members. For a mission to be successful, crew members must remain healthy and able to complete a variety of tasks including launch and re-entry, system maintenance and repair, extravehicular activities, surface exploration, and flight-based and Mars surface-based experiments. Humans have evolved in and adapted to a terrestrial environment with a ubiquitous1-g gravitational acceleration and geomagnetic shielding from solar and cosmic radiation. The reduced gravitational loading and exposure to cosmic radiation that occur in long-duration space missions are known to have adverse effects on the human neuromuscular and musculoskeletal systems. Unless effective countermeasures are developed prior to a Mars mission, it is likely that crew members will experience debilitating losses of bone mass, muscle strength, cardiovascular fitness, and endurance; changes in sensorimotor functions (e.g. impaired balance control); swelling of vertebral disks; and an impaired ability to heal bone fractures and other injuries (Fig. 1, Table 1). Crucial gaps remain in our understanding of the biological mechanisms and variability of the physiological changes that occur during long-duration space missions, and further development and testing of potential countermeasures for each of these risks is needed to ensure mission success.

2. Bone Loss

The skeleton is a dynamic, living system that adapts to changing mechanical loads. Cells called osteoblasts produce new bone tissue when the level of daily mechanical loading exceeds normal physiological levels, and cells called osteoclasts remove existing bone tissue when daily mechanical loading falls below normal levels (Carter et al. 1987; Judex and Zernicke 2000; Manske et al. 2010). Because of bone’s "mechanosensitivity," exposure to microgravity causes changes in bone cell activity and results in a loss of bone (Rambaut et al. 1975; Whedon et al. 1976; Whedon et al. 1976; Rambaut and Johnston 1979; Rambaut et al. 1979; LeBlanc et al. 2000; Lang et al. 2004). The loss of bone is likely due to a combination of factors, including but not limited to a reduction in mechanical load stimulus, reduced fluid pressures in the legs, and altered nutritional intake and metabolic processing.

Dual-energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT), medical imaging modalities used to monitor bone mineral status, have been used to quantify changes in bone mineral density (BMD) that occur during space missions. Areal BMD, which is equal to the total mass of bone mineral in a region of interest (ROI) divided by the area of the ROI in a 2-D image, is measured with DXA. Volumetric BMD, which is the mass of bone mineral in an ROI divided by the volume of the ROI in a 3-D image, is measured by QCT and allows separate analysis of the cortical shells of bones and the spongy, trabecular bone that fills vertebral bodies and the proximal femur. Areal BMD measured with DXA decreased 1% per month at the lumbar spine and 1% to 1.6% per month at the hip in crew members of the Russian/MIR space station and ISS (LeBlanc et al. 2000). In the hips of ISS crew members analyzed with QCT before and after 4- to 6-month missions, cortical BMD decreased 0.4% to 0.5% per month, and trabecular BMD decreased 2.2% to 2.7% per month (Lang et al. 2004).

The losses in bone mineral experienced on ISS took place despite the crew members’ participation in a cardiovascular and resistance exercise program aimed at maintaining bone and muscle during missions. Measurements of forces on the foot during exercise aboard ISS are drastically reduced (25% reduction for walking and 46% lower for running) compared to those experienced on Earth, despite the use of a harness system to replace gravity loads on crew members while pulling them toward the treadmill running surface (Cavanagh et al. 2010). Animal studies provide encouraging evidence that, if muscle contractions are of high enough intensity, spaceflight-induced reductions in bone formation and in bone mineral content can be prevented (Swift et al. 2010). Improved harness systems used with current exercise devices or even entirely new exercise devices that provide for more effective loading of the musculoskeletal system may well be required to achieve the same success in human astronauts.

The decreases in BMD that occur during long-term exposure to microgravity may increase the risk of fracture during transit to and from Mars, landing on the Martian surface, extravehicular activities on Mars, and re-entry and landing after returning to Earth. A recent study combined QCT imaging and finite element analysis, a computational engineering tool used to predict the strength of complex objects and structures, to measure changes in bone strength that occurred during 4- to 6-month ISS missions (Keyak et al. 2009). The results of that study showed that the strength of the proximal femur decreased by 2.6% per month for stance loading, which is the loading configuration that exists under single-leg support as when taking a step (Fig. 2). Losses in bone strength during the flights from Earth to Mars, while on the surface of Mars, and during the return flight may therefore place crew members at an increased risk of fracture during the mission. Upon returning to Earth from the ISS, lost bone mineral is recovered slowly over a few years (Lang et al. 2006; Sibonga et al. 2007; Carpenter et al. 2010). It is unclear how the time spent on Mars, during which the gravitational load will be approximately 40% of that experienced on Earth, will affect bone strength. However, computer simulations suggest that decreases in bone strength can be expected (Carter et al. 1996). Because a Mars mission will span multiple years, age-related changes in BMD and bone structure may occur along with changes resulting from decreased gravitational loading (Carpenter et al. 2010). The combined effects of aging and microgravity may affect fracture risk both during the mission and upon return to Earth.

3. Muscle Atrophy

Like bones, muscles can also adapt to different levels of physical activity. Reduced muscle activity during space missions leads to muscle atrophy, a loss of muscle strength and power, and altered muscle physiology, especially in the lower limbs (Fitts et al. 2001). In a recent study, crew members experienced a 13% loss in calf muscle volume (2.2% loss per month on average) and a 32% decrease in peak muscle power (5.3% loss per month on average) after 6 months aboard the ISS despite the use of an exercise program incorporating treadmill running, cycling exercise, and resistance exercise (Trappe et al. 2009). A second recent study of ISS crew members demonstrated losses in muscle volume of 10% to 16% in the calf and 4% to 7% in the thigh after 6-month missions (Gopalakrishnan et al. 2010). Muscle strength was lost at the knee (decrease of 10% to 24%) and ankle (decrease of 4% to 22%). Gopalakrishnan et al. did not observe a significant change in muscle volume in the upper arm, but strength at the elbow decreased by 8% to 17%.

The loss of muscle size and strength in long-term space missions is also accompanied by the adaptation of both type I ("slow twitch") and type II ("fast twitch") muscle fibers to the microgravity environment. Type I fibers are primarily responsible for maintenance of posture and endurance activities, and type II fibers are responsible for shorter-duration bursts of speed and power. Living in a microgravity environment causes muscles made up of predominantly type I fibers to lose contractile proteins to a greater extent than muscles composed primarily of type II fibers, suggesting that muscle endurance may be particularly compromised during a long-duration space mission (Baldwin et al. 1990). A study of rats on a 16-day space shuttle mission suggested that the degradation of contractile proteins in muscle occurred through a ubiquitin-dependent proteolytic pathway (Ikemoto et al. 2001). The reduction in muscle protein synthesis observed during bed rest (a commonly used simulation of spaceflight) impairs the ability of muscle to replace or repair degraded contractile proteins (Kortebein et al. 2007). Additional studies are needed to fully characterize the cellular and molecular mechanisms responsible for muscle’s atrophic response to microgravity.

The large loss in muscle mass and strength that occur during spaceflight could have important implications in the success of a Mars mission. Fatigue of muscles is an important limiting issue in system maintenance, repairs, and extravehicular activities, and the large losses in muscle mass and preferential loss of contractile protein in type I fibers may amplify this problem. Because muscles are also the primary determinant of the mechanical forces applied to bones, decreased muscle strength would also likely exacerbate the loss of bone mineral that occurs due to the absence of ground reaction forces in a microgravity environment. Without the use of effective countermeasures, losses of muscle strength and endurance during the flight to Mars may impede crew members’ abilities to complete mission critical tasks.

4. Intervertebral Disc Expansion

Intervertebral discs help to support and cushion the compressive forces in the spinal column. These compressive forces, which result from a combination of ground reaction forces and the contraction of postural muscles during daily life in a 1-g environment, are drastically reduced during life in microgravity. Discs expand in microgravity, elongating the spinal column and leading to increases in crew member height of up to 6 cm (Wing et al. 1991; LeBlanc et al. 1994). At least half of crew members report lower back pain during flight and upon resuming normal function after landing (Wing et al. 1991). The expansion of intervertebral discs during spaceflight may be the cause of, or at least related to, the lower back pain experienced during spaceflight and upon return to Earth. While the change in disc size is reversible for relatively short missions, longer periods of spaceflight may result in longer lasting effects. Disc size returns to normal within a few days after five weeks of bed rest, while discs remained larger than normal six days after the end of a 90-day bed rest period (Holguin et al. 2009) and six weeks after the end of a 17-week bed rest period (LeBlanc et al. 1994). These persistent changes and their possible relationship with lower back pain will need to be addressed when planning a manned mission to Mars, because they may lead to discomfort and a decrease in crew members’ physical abilities.

5. Impaired Healing

In addition to the losses of bone and muscle that occur in spaceflight, these tissues and others also exhibit an impaired healing capacity in a reduced-gravity environment. Both ground-based and in-flight animal studies have shown impaired healing of fractures (Sweeney et al. 1985; Kaplansky et al. 1991; Kirchen et al. 1995; Midura et al. 2006). A delayed repair process in muscle crush injuries was also observed in an animal study aboard the Cosmos 2044 biosatellite (Stauber et al. 1992), and a shuttle-based study of abdominal incision wounds in rats demonstrated an abnormal healing process characterized by increased inflammatory response and formation of scar tissue (Bolton et al. 1997). Ground-based studies using rat and mouse hind limb unloading models, which disallow any weight bearing by the animals’ hind limbs and effectively simulate changes that occur during spaceflight, have demonstrated impaired healing in ligaments (Provenzano et al. 2003; Martinez et al. 2007), the cornea (Li et al. 2004), and the skin (Radek et al. 2008). It is still unclear what biological factors lead to impaired healing in spaceflight. While mechanical loading is known to be an essential part of successful healing in bones and ligaments in a clinical setting, the role of the mechanical environment in the healing of muscle, cornea, and skin is not well understood. It is possible that the effects of reduced gravity on blood vessels are an important factor affecting tissue healing (Vico 2007). The effects of cosmic radiation on tissue healing are also not yet fully understood. Discovering the mechanisms responsible for impaired healing in such a wide variety of tissues will be an important aspect of space medicine when planning a mission to Mars.

The delayed musculoskeletal healing response will need to be taken into account when planning a mission to Mars. The stock of medical supplies onboard the spacecraft and lander and the medical capabilities of the crew selected for the mission (e.g. whether to include a flight surgeon or other medical personnel) will need to be adjusted with longer healing times in mind. In the case of external wounds, a longer healing time will also translate to an increased possibility of infection, which could have important consequences for a trip that is expected to span multiple years.

6. Sensorimotor Function

Exposure to microgravity induces adaptive central reinterpretation of visual, vestibular and proprioceptive information. This microgravity adaptive state, however, is inappropriate for a 1-g environment, so astronauts must spend time readapting to Earth’s gravity following their return. During this readaptation period there are disturbances in perception, spatial orientation, posture, gait, manual control and eye-head coordination (Reschke et al. 1998). Astronauts have difficulty walking after returning from spaceflight due to alterations in multiple systems responsible for the control of locomotion including disruptions in leg muscle activations patterns, head-trunk coordination and spatial orientation (Bloomberg and Mulavara 2003). Functional mobility testing after long-duration spaceflight using an obstacle course has shown that recovery takes an average of 2 weeks after landing which is similar to that observed for recovery in postural equilibrium control after long-duration spaceflight (Mulavara et al. 2010). Following spaceflight, astronauts also experience changes in otolith-spinal reflex function (Reschke et al. 1984; Watt et al. 1986). These reflex mechanisms are essential for many pre-programmed motor responses such as those required to stabilize posture after a voluntary jump down from a platform and therefore astronauts experience disruption in their ability to maintain postural equilibrium when performing these tasks (Newman et al. 1997). Postural and gait disturbances have significant implications for performance of operational tasks that require ambulation immediately following landing on a planetary surface including rapid emergency egress from a landing vehicle.

Motor skills are modified in microgravity, leading to decrements in manual dexterity tasks such as tracking, pointing and grasping (Bock et al. 2003; Bock and Bloomberg 2010). Depending on the situation, movement speed, accuracy and/or the cognitive costs of performing the skill can be affected. The central reinterpretation of sensory inputs from the vestibular organs has adverse effects on gaze stabilization by the vestibulo-ocular reflex, and thus degrades eye-head coordination and visual target acquisition. Deficient gaze control experienced during periods of adaptive change during the first days of exposure to microgravity and re-exposure to a gravitational environment can cause blurred vision and decrements in dynamic visual acuity caused by a decreased ability to keep a visual target stabilized on the retina during head and body motion (Paloski et al. 2008). Decreased dynamic visual acuity coupled with changes in manual control poses a unique set of problems for astronauts especially during entry, approach, and landing on planetary surfaces.

7. Effects of Radiation

During a mission to Mars, crew members will be beyond the protection of geomagnetic shielding, which helps protect the ISS crew against galactic cosmic radiation and ions from solar flares (Wilson et al. 1995). Exposing mice to radiation levels comparable to those expected on a mission to Mars have been observed to produce adverse changes in bone macro- and micro-structure, bone strength, bone cell activity, and muscle fibers (Lloyd et al. 2008; Willey et al. 2008; Bandstra et al. 2009; Kondo et al. 2009). Any effects of space radiation on blood vessel formation, mesenchymal stem cell differentiation, or satellite cells in skeletal muscle would also affect musculoskeletal tissue regeneration. Additional study is therefore needed to determine the effects of radiation on the healing of fractures and other musculoskeletal injuries.

8. Countermeasures

To ensure the success of a manned mission to Mars, effective countermeasures must be developed and implemented for each of the problems identified above, including bone loss, muscle atrophy, muscle fatigue, intervertebral disc expansion, impaired healing, sensorimotor adaptation, and the adverse effects of radiation exposure. Exercise programs, nutrition supplements, pharmaceuticals, and mechanical and electrical stimulation devices are all being investigated, or are already in use, as countermeasures. Research in the coming years will provide quantitative evaluations of these countermeasures in order to select the most effective strategies, producing an integrated program of countermeasures and helping to ensure the success of long-duration space missions.

Crew members of the ISS currently use three different exercise systems in an attempt to maintain bone and muscle mass: a treadmill, a cycle ergometer, and a resistive exercise device. Despite the use of these countermeasures, losses in bone mass, muscle mass, and muscle tone occur during 4- to 6-month long missions (Lang et al. 2004; Trappe et al. 2009). Continued development of new exercise systems and training programs that aim to prevent these losses will be an essential part of space biomedical research as the space community prepares for a manned Mars mission. A balance training component designed to increase adaptability of sensorimotor function could be added to the existing battery of exercises used on ISS to facilitate adaptation to a novel gravitational environment. An exercise system that incorporates cardiovascular training, resistance training, and balance training in a single, compact footprint and potentially integrated with a short-range centrifuge for artificial gravitational loading would likely be the best choice for a mission to Mars due to space and mass limitations.

Dietary interventions, including nutritional supplementation, may also serve as a countermeasure for bone and muscle loss. Calcium and vitamin D supplementation were used in a 21-day mission aboard the Mir space station, but bone mineral loss was not reduced (Heer 2002). Introduction of vitamin K supplementation in one crew member halfway through a 6-month Mir mission produced an increase in osteocalcin and alkaline phosphatase (markers of new bone formation) during the second half of the mission (Vermeer et al. 1998). More research on calcium and vitamin supplementation, as well as adequate caloric intake in combination with other countermeasures such as exercise, is needed to determine their effectiveness in preventing bone loss. To help prevent muscle loss, amino acid supplementation also shows promise and is currently being studied (Paddon-Jones and Rasmussen 2009).

Pharmaceuticals developed to combat osteoporosis may also prove useful in reducing losses in bone mass in space. Antiresorptive therapies, such as bisphosphonates and newer antibody therapies (i.e. anti-RANKL), inhibit bone resorption and are in wide use for the treatment of osteoporosis. Bisphosphonates are currently being tested in a limited number of ISS crew members. These drugs may even help prevent bone loss due to radiation exposure in space travel far from Earth by reducing bone turnover, as evidenced by the success of recent tests of risedronate in mice irradiated with x-rays (Willey et al. 2009). There is a possibility of bisphosphonate treatment causing a slight delay (about a week) in fracture healing (Rozental et al. 2009), but the importance of preventing bone loss may outweigh this problem. Anabolics, such as intermittent parathyroid hormone treatment and emerging antibody therapies (i.e. anti-sclerostin), can increase bone formation and may be appropriate for testing in crew members or in ground-based flight analogs in the coming years. Clinical case reports suggest that intermittent parathyroid hormone treatment may in fact enhance fracture healing (Resmini and Iolascon 2007; Yu et al. 2008), but more research is needed to determine whether this would be the case for astronauts in a reduced-gravity environment. For prevention of muscle loss in spaceflight, pharmaceuticals being developed for treating sarcopenia in the elderly may prove effective. Angiotensin-converting enzyme inhibitors, for example, have been shown to be effective in slowing declines in muscle strength over the course of a 3-year trial in elderly subjects (Onder et al. 2002). Myostatin, a negative regulator of skeletal muscle mass, is a potential target for pharmacological inhibition. A study of mice on a 13-day Space Shuttle mission (STS-118) demonstrated that inhibiting myostatin not only prevented muscle loss, but also prevented bone loss (Ferguson et al. 2009). Bone fracture healing has also been shown to be enhanced in myostatin-deficient mice, which suggests another potential advantage to myostatin inhibition in humans (Kellum et al. 2009).These drugs have not yet been tested in humans in spaceflight or flight analogs. Other pharmaceutical countermeasures for both bone and muscle loss in spaceflight are still in early stages of testing to determine dose-response characteristics, the potential for combination therapies, and long-term effects (Cavanagh et al. 2005).

Although still in the very early stages of development, therapy using small interfering RNA (siRNA) may also offer the potential to prevent or reduce the deleterious effects of space travel on the skeleton. Although it is well-established that bone responds to its mechanical environment, the molecular mechanisms by which osteocytes (cells that live in bone tissue) and osteoblasts (cells that create new bone tissue) sense and respond to the mechanical environment are poorly understood. Genes such as SOST, fibroblast growth factor 23 (FGF-23), dentin matrix protein 1 (DMP1), and matrix extracellular phosphoglycoprotein (MEPE) may play important roles in the regulation of bone cell activity (Feng et al. 2006; Lorenz-Depiereux et al. 2006).

Basic research needs to be done to determine whether the mechanical environment regulates the expression of these genes and to identify the cellular and molecular mechanisms underlying such effects. Sclerostin, the product of the SOST gene, is an osteocyte-specific protein and recently has emerged as an important therapeutic target for bone diseases such as osteoporosis and osteopenia (Li et al. 2009; Veverka et al. 2009). Sclerostin expression is inhibited, in vivo and in vitro, by parathyroid hormone and mechanical forces (Keller and Kneissel 2005; Lin et al. 2009). Research is needed to understand the cellular and molecular mechanisms underlying SOST regulation. It is also important to determine whether osteoblasts are the sole targets of sclerostin or if sclerostin also acts directly on osteocytes. Understanding the mechanisms of mechano-transduction by osteocytes, at the cellular and molecular level, is the first step towards the development of novel therapeutics for skeletal diseases induced by microgravity.

Mechanical and electrical stimulation devices, which would be intended to restore some of the physical stimuli that are lost in a microgravity environment, are also being tested as potential countermeasures for bone loss, muscle loss, and intervertebral disc expansion. Providing low-level cyclic loads through the use of a vibrating foot plate has been shown to help prevent or reduce bone loss in postmenopausal women (Rubin et al. 2004) and disabled children (Ward et al. 2004). Low-level vibrations also increased bone mass and muscle mass in young women in a 12-month trial (Gilsanz et al. 2006) and reduced intervertebral expansion in a 90-day bed rest study (Holguin et al. 2009). High-frequency (50-Hz) electrical stimulation of muscles, in which electrodes are used to directly induce muscle contractions, has also been shown to attenuate bone loss in rats during four weeks of hind limb unloading (Lam and Qin 2008). Whether direct electrical stimulation or vibrating plate stimulation will be effective in helping prevent the loss of bone and muscle in human spaceflight remains to be determined.

9. Possible Mission Scenario

Based on the current state of knowledge summarized in this paper, we can propose a hypothetical picture of some of the effects that neuromuscular and musculoskeletal changes will have on a manned mission to Mars. We will focus on a mission that would involve a 6-month flight to Mars, a 1-Earth-year stay on the Martian surface, and a 6-month flight back to Earth. During the outbound and return flights, we will assume that rates of change in the human body are equal to the mean rates of change observed in long-duration (4 to 6 months) missions aboard ISS. Changes that will occur while on Mars are more speculative, but we can infer some overall changes based on results of computer simulations and by scaling the effects that occur in microgravity. A summary of the predicted changes in bone and muscle strength is provided in Fig. 3. It is important to note that all of these predicted changes are based on the assumption that mission will implement only those countermeasures (e.g. exercise, nutrition, etc.) that are currently being used in space missions. These predictions also do not account for the fact that the changes will most likely slow over time as the body adapts to its new environment. Therefore these predictions should be viewed as a worst-case scenario.

On the outbound flight, during which crew members will live in a microgravity environment, we expect crew members to experience losses in bone density, bone strength, muscle mass, muscle strength, and sensorimotor functions. The strength of the proximal femur will likely decrease by about 12%. Losses of muscle strength of approximately 15% at the knee and 10% at the ankle will likely occur. As a result of the changes in bone and muscle, crew members will be weaker and their bones more fragile when they land on Mars. They will also likely have impaired balance during the period of adaptation to Martian gravity and an increased incidence of back pain that will likely persist throughout the remainder of the mission.

After arriving on the Martian surface, crew members will live for 1 year under a gravitational acceleration that is 38% of that on Earth. It is possible that the crew’s reduced muscle strength and bone strength may in fact be adequate for safely performing tasks in the reduced-gravity Martian environment. However, they will likely continue to experience losses in bone and muscle strength due to reduced gravitational loading. Based on a computer simulation using established modeling methodology (Carter et al. 1996; Carpenter and Carter 2008), we expect crew members to lose an additional 9% of their bone strength while on Mars.

Assuming a rate of muscle loss 60% of that experienced aboard ISS, crew members will lose an additional 18% of muscle strength at their knee and 12% of muscle strength at the ankle. These estimates do not account for the fact that crew members will also have to wear space suits, which may help to attenuate some of the losses in bone and muscle strength due to the increased effort needed to move the extra mass of the suit.

During the return flight, initial rates of loss may resume, leading to changes similar to those seen on the outbound flight. In this case the strength of the proximal femur will decrease by an additional 12%, and losses of muscle strength of approximately 15% at the knee and 10% at the ankle will occur. Therefore, when the crew members arrive back on Earth, their hip bones will have lost approximately 33% of their fracture strength, and they will have lost approximately 48% of their muscle strength at the knee and 32% of their muscle strength at the ankle. The reader should remember that these are worst-case scenario estimates that do not take into account the possible development of effective countermeasures such as exercise programs, nutritional supplements, or pharmaceuticals to mitigate these losses. The effects of increased radiation exposure, which would likely exacerbate bone loss, are also not taken into account. However, these estimates point out the seriousness of the musculoskeletal problems faced by the crew members, engineers, and space medicine physicians and scientists who will plan and execute a manned mission to Mars.

10. Conclusions

A manned mission to Mars will be faced with a variety of health-related challenges. The health of the neuromuscular and musculoskeletal systems, which are crucial for the mobility of crew members, will be of great importance on a mission that will span multiple years. To ensure that crew members have the physical endurance, strength, and sensorimotor capacity needed during missions, it is important that the physiological effects of space travel are thoroughly understood and that effective countermeasures are in place. To avoid the increased risk of fracturing a bone during the mission, there is a need to develop countermeasures for bone loss that are more effective than those currently being used on ISS. A combination of new exercise equipment, individually tailored exercise programs, nutritional supplements, and traditional and emerging pharmaceuticals will likely be needed to combat microgravity-induced and radiation-induced bone and muscle loss. Sensorimotor adaptability training incorporated into the exercise regimen may also help reduce sensorimotor impairment during gravitational transitions, and mechanical and electrical stimulation devices may help to reduce bone loss, muscle loss, and intervertebral disc expansion. Together, these countermeasures will help reduce the effects of long-duration space travel on the neuromuscular and musculoskeletal systems and will help ensure the success of a future manned mission to Mars.