simulation information in 2007 a report was published about an effort to develop a computer-based simulation named “the digital astronaut” to predict the loss of skeletal muscle mass and function in a microgravity environment or to predict the efficacy of countermeasures in experimental animals or humans. the digital astronaut was described as “an integrated, modular modeling and database system that will support space biomedical research and operations, enabling the identification and meaningful interpretation of the medical and physiological research required for human space exploration, and determining the effectiveness of specific individual human countermeasures in reducing risk and meeting health and performance goals on challenging exploration missions”. because of the difficulties in developing such a mathematical model based on the complexities and variables of human physiology operating in the unusual environment of microgravity, the utility of this approach, although reasonable, remains to be proven. future exploration missions the risks related to loss of skeletal muscle mass, strength, and endurance depends not only on the level of loss but also on the starting point and the relative physiologic expense required to successfully complete a requisite set of tasks within a fixed period. thus, a crewmember must be capable of completing a task before being exposed to microgravity, the amount of functional loss cannot be allowed to fall below the level needed to successfully complete all assigned tasks, and the physical performance requirements for completion of the tasks should be known. without information relating to the physical performance requirements of tasks, it is not possible to determine the risk of failure. additionally, if a task could not be completed by a crewmember before microgravity exposure, it can reasonably be stated that the risk of failure during a mission is 100%. however, even if the crewmember has the capability to complete every possible task, a composite of the tasks to be completed over a finite period presents an entirely different requirement because it might be possible to select a composite of tasks to be completed within a work period that exceeds the capabilities of a single crewmember or perhaps every crewmember. additionally, all possible contingencies that might arise must be considered, so that a crewmember will be able to deal with such off-nominal scenarios even near the end of a duty day. thus, even an approach as basic as thoughtful scheduling of daily tasks could serve to help mitigate risk. from the above discussion, several important items emerge that must be known with respect to the risks related to loss of skeletal muscle mass, strength, and endurance. these include: baseline level of crewmember functional performance (strength, endurance, level of functional performance, etc.) magnitude of functional loss from baseline at any point during the mission physiological demand of a task or set of tasks to be completed the time period in which the tasks should be performed all possible contingency events that could affect functional performance any other interfering conditions that could affect functional performance (such as nutritional and psychological status, eva suit specifications, equipment malfunction or failure, illness, injury, etc.) an indication of the importance of individual baseline performance is obtained from an illustrative example from the edomp program. losses in trunk flexor and extensor strength were greater for the crewmembers who exercised on the shuttle treadmill during flight than for the crewmembers who did not exercise during their mission (see figure 6-7). although at first this seems counterintuitive, simple logic provides the explanation. crewmembers who chose to exercise during flight did so because they exercised regularly as part of their daily routine before flight. since they were at a higher level of fitness than their non-exercising crewmember cohorts, they lost more strength during flight. however, what cannot be ascertained from % change data are absolute strength levels. for instance, exercising crewmembers who lost twice as much abdominal and back muscle strength as their non-exercising counterparts could still have greater strength in those muscles if they started off three times stronger than their non-exercising colleagues. lunar sortie missions with respect to future missions involving humans, lunar sortie missions will probably represent the lowest risk of the currently planned missions and will likely be no greater in risk than the apollo missions (at least with respect to skeletal muscle performance) unless unusual surface operations are planned that differ markedly from apollo lunar surface operations. the longest cumulative time of lunar surface eva by a crew during the apollo program was about 22 hours. (combined from 3 separate days) and the longest total duration of the crew on the lunar surface was about 75 hours during the sixth and final apollo mission (apollo 17). the answer to the question of whether exercise equipment should be available to crewmembers for short missions to the moon and back is actually an easy one and the answer is a resounding “yes.” during some of the apollo missions, a small, lightweight device called the “exer-genie,” which required no external power, was made available to crewmembers (see figure 6-1) and they were encouraged to use it. specific comments from the apollo crewmembers collected during the recent “apollo summit” are particularly relevant and can be summarized as follows: exercise is not necessary on a short trip and crews did not feel that they suffered noticeable deconditioning, but they did demand that exercise capability be available as much as possible for "rest and relaxation" for all phases of the mission. the exercise device was used by all crewmembers with varying amounts and intensities. lunar surface crews (maximum time spent on surface operations was 22 hours during a 75-hour stay for apollo 17) felt that their activities on the lunar surface provided enough exercise for a short-duration mission but would have welcomed a simple, robust device for stretching and forearm exercise. (note: the exer-genie remained on the command module with the command module pilot; it did not accompany the two astronauts who descended to the lunar surface in the lunar excursion module.) apollo crewmembers felt that crew surgeons and mission planners should not hard-schedule exercise prescriptions for such short-duration missions but allow the crew to perform them at their leisure. they stated that a more robust and lightweight piece of in-flight exercise equipment is needed than flown during apollo. the exer-genie was limited, its ropes were friable, and the device generated a lot of heat and smell, so an alternative exercise device is needed. most crewmembers felt that the pre-mission timeline should provide adequate time to maintain musculoskeletal strength and stamina. some astronauts attributed their capabilities on the lunar surface to pre-mission training because in some cases more force was needed on the lunar surface while wearing the eva suit than was needed in 1g on earth. the crew felt that exer-genie or an alternative was definitely needed, and because of a fear that they would break it, they actually tapered off from intense use to save it for use in reconditioning on the return trip before re-entry. the strongest comment was that "as many exercise capabilities as possible should be built into all future vehicles" because they will get used and the crew further felt that exercise capability throughout flight was critical and that a variety of exercises should be provided. lunar outpost missions lunar outpost missions will present a greater challenge than shorter “sortie” missions, but with respect to the current risk topic they probably represent risks similar to those experienced on the iss. lunar gravity, although about 1/6 that of earth gravity, is still more conducive to providing sufficient loading to maintain muscle mass and function than is microgravity. certainly exercise regimens and hardware will be required, not only for countering muscle atrophy but for the reasons stated by apollo astronauts above as well. how much exercise is needed and the proper way to implement it are certainly knowledge gaps that will require innovative research to fill. part of this research will unquestionably help to define the level of risks to which crews will be exposed but will also be helpful in properly mitigating those risks. mars transit without doubt, transport between the earth and mars as well as the return trip represent the greatest risks to humans encountered in the history of human spaceflight. notwithstanding the risks of radiation exposure, deterioration of the musculoskeletal system must be prevented or a mission to mars (and back) will not be successful. highly refined exercise protocols and robust exercise equipment and methods to monitor functional capacity are mandatory for mitigation of the risks inherent in long-duration exposure of humans to microgravity. a huge challenge will be to provide the above within the current design of the crew exploration vehicle (cev), which provides trivial space for equipment and crew. the cramped confines will afford little room for stretching or exercise. modest or no power for equipment and a human life support system whose design may be marginal to support a full complement of exercise by efficiently dealing with the heat, water vapor, and carbon dioxide that are byproducts of human exercise are additional challenges that must be overcome. mars outpost knowledge gained during lunar outpost missions will be highly relevant to successful establishment of a martian outpost. if the challenges posed by the long transit to mars and the extended period of microgravity exposure can be met, the outpost phase should represent a much lower risk by comparison, since lunar outpost experience will have allowed significant opportunity to develop risk-mitigation strategies for this phase. the gravitational environments are similar; in fact, the martian gravity field, being greater than that of the moon, will provide a less formidable setting. however, capability to provide sufficient exercise capacity during the martian outpost phase is essential in preparing the crew for a long-duration exposure to microgravity on the transit back to earth. this probably represents the greatest challenge with respect to maintaining a safe level of skeletal muscle performance for exploration-class missions. current gaps despite four decades of effort, success in prevention of spaceflight muscle atrophy and skeletal muscle functional deficits has not yet been achieved in every case although progress has been made. gaps in our knowledge have prevented us from implementing a countermeasures program that will fully mitigate the risks of losing muscle mass, function, and endurance during exposure to the microgravity of spaceflight, particularly during long-duration missions. there are also gaps in our knowledge about working and living in partial-g environments and the effect that wearing an eva suit has on human performance in such an environment. in microgravity the major knowledge gaps that must be addressed by future research to mitigate this risk of loss of skeletal muscle mass, function, and endurance include the following: for humans living in a microgravity environment, the optimal exercise regimen, including the modes, intensity, and volume needed to minimize or fully mitigate risk, is not known. an appropriate exercise prescription must be developed and validated during spaceflight. the types and functional requirements of exercise hardware and the most comfortable human-to-hardware interfaces needed to minimize or fully mitigate risks is not known. such hardware is likely to be mission-specific and should be validated in the appropriate environment. the effect on maintenance of skeletal muscle strength by in-flight use of the currently developed advanced resistive exercise device (ared) is not known. because of the inherent shortcomings in the interim device (ired) (maximum achievable load ~300 lbs), we have not provided an optimal resistance exercise opportunity for flight crews. an in-flight study utilizing ared is essential in determining the efficacy of a program of combined aerobic and resistive exercise during long-duration microgravity exposure. the expected composite of mission-specific critical mission tasks and their physiologic costs to crewmembers during surface eva operations is not well defined. this is essential to determining human functional requirements and attendant risks. the level of skeletal muscle loading and aerobic exercise provided by surface eva on the moon must be determined either through modeling or by lunar analog studies and then validated. for humans living in partial g environments, the optimal exercise regimens, including the modes, intensity, and volume needed to minimize risk, are not known. appropriate exercise prescriptions must be developed and validated for partial g environments. eva suits are known to reduce the effective maximum forces that can be generated by crewmembers for task completion so that a portion of the crewmember's work