Exercise is used as the primary countermeasure to minimize or eliminate skeletal muscle atrophy and bone calcium loss and to prevent physical deconditioning during spaceflight. There are usually two daily exercise sessions performed on a 3-d-on/3-d-off schedule during Russian spaceflights longer than 3 months. Based on exercise heart rates and oxygen uptakes, each exercise session is approximately 75 min, with exercise intensities of 50-75% of maximal oxygen uptake (˙VO2max); the average amount of exercise usually increases in the second half of the flight(15). The importance of regular prescribed physical activity during spaceflight was demonstrated during the 96-d Soyuz 26-Salyut-6 mission, when exercise training was interrupted for the initial 25 d, resulting in diminished exercise endurance (15). Resumption of regular training improved hemodynamic responses, and exercise endurance increased to preflight levels by day 70 of the mission and was maintained thereafter. Despite widespread in-flight utilization of physical exercise, as well as various other procedures alone or in combination, current in-flight exercise has been only partially effective for counteracting deconditioning, improving tolerance to reentry, and accelerating the postflight recovery process (12,15,48).
Other than instances of reduced ability to maintain high levels of physical effort during extravehicular activities (EVA), flight-induced changes in physiological function have not seriously impacted astronaut performance or caused mission termination. However, orthostatic hypotension and presyncopal symptoms in some Shuttle crew members during reentry and impaired neuromuscular function and physical fitness during the immediate postflight egress period have caused serious safety concerns for mission planners(78).
The purpose of this article is 1) to present a physiological basis for the use of exercise as a countermeasure to minimize or eliminate adverse adaptations to spaceflight that may compromise the health, safety, and productivity of space travelers; 2) to outline special considerations for development of exercise countermeasures; 3) to review past and present exercise regimens used during flight and to evaluate the effectiveness and limitations of their application; and 4) to provide new approaches and concepts for the implementation of novel exercise countermeasures for future space travel.
PHYSIOLOGICAL BASIS FOR SPACEFLIGHT EXERCISE COUNTERMEASURES
Aerobic Power (˙VO2max)
Although there are no published spaceflight data on the effects of nonexercise-induced adaptation to microgravity on ˙VO2max, data from ground-based analogs clearly indicate that absence of application of exercise countermeasures results in 10-25% reductions in aerobic power(12,15,23,35,54,79). Reduced stroke volume and cardiac output are the primary factors contributing to lower ˙VO2max following exposure to simulated microgravity, i.e., bed rest (65,79). In addition to reduced cardiac output, there are also reductions in skeletal muscle blood flow(17,29,92) and aerobic pathway enzymes(63), which could limit oxygen delivery and utilization. It is reasonable to propose that regular endurance training using low- to moderate-intensity, long-duration exercise be employed during spaceflight to some degree since it can increase cardiac, hemodynamic, and muscle biochemical capacities to support higher ˙VO2max(7,43) and could guard against reduced functional work capacity.
The first quantitative data on maximal in-flight exercise capacity were obtained during the Skylab flights, when ˙VO2max was measured before and during flight (15,73).˙VO2max levels actually increased over the course of the spaceflight in the Skylab 4 crew. This was undoubtedly because of an intensive daily in-flight exercise program and indicates that a training effect took place. The commander of Skylab 4, who undertook the least extensive exercise training program both before and during flight, showed the smallest decrement compared to the others who exercised more vigorously. This may indicate that intensive exercise may not always be necessary for all individuals during spaceflight. Yet, identification of the minimal amount of exercise needed to maintain preflight fitness levels, or whether it is indeed necessary to maintain preflight fitness throughout long flights, has received little attention to date.
The average ˙VO2 during submaximal exercise at a heart rate of 160 bpm decreased by 21% from preflight levels when measured on the first recovery day in the Apollo 7-11 crews and dropped a similar 17% for Apollo 14-17 astronauts, who also spent 2-4 d on the lunar surface at 0.16 ×g. These exercise decrements after 2 wk of exposure to weightlessness closely match findings obtained following 2- to 4-wk periods of exposure to ground-based analogs of microgravity(12,23). Presence of significant decrements in postflight exercise capacity in Apollo crews who spent time on the lunar surface suggests that exposure to greater intensity, duration, and/or frequency of gravity or exercise will be required to remedy this reduction in physical work capacity.
There have been no documented measures of change in aerobic power in Shuttle crews even though a cycle ergometer and treadmill have been carried aboard all flights. Analysis of such data from astronauts who do and do not exercise during flight should be verified with ground-based controls so that effectiveness of exercise countermeasures on aerobic capacity can be critically assessed.
Data from ground-based analogs of microgravity (bed rest) demonstrate reduction in total circulating plasma and blood volume (i.e., hypovolemia) within 24-48 h of exposure(15,19,23,26); the magnitude of hypovolemia for flights of various durations ranges from 6 to 16%. Although serial measurements of plasma volume during spaceflight have not been reported, pre- and postflight data support the hypothesis that the hypovolemia occurs in the initial days of the mission because the magnitude of reduction in plasma volume during flights of 3-4 d is similar to that of flights as long as 28 d (15,23). In addition, unpublished data on in-flight plasma volume from the SLS-1 mission clearly showed a pronounced hypovolemia (≈approximately 20%) at 36 h into the flight that remained at day 7. Microgravity-induced hypovolemia may have direct operational impact on return from spaceflight since its relative magnitude is highly correlated with decreased ˙VO2max (12,23) and orthostatic compromise (8,15,19,64). Baseline and exercise left ventricular end-diastolic volume, end-systolic volume, stroke volume, and cardiac output are reduced after 30-60 d of flight despite elevations in heart rate (6,15). However, increased baseline and exercise ejection fractions (6) support the concept that reduction in vascular volume is probably a more important contributor to reduced cardiac dynamics during flight than altered myocardial performance.
In addition to hypovolemia, gradual loss of red blood cells approaching 15% and loss of hemoglobin of 10-25% occur regularly during U.S. and Russian flights (15). Reduction of red cell mass results in loss of blood oxygen-carrying capacity, which could also contribute to the limitation of work capacity.
Mechanisms of vascular volume contraction during exposure to microgravity are probably better understood than those restricting its replacement. Fluid-loading techniques used by astronauts just prior to reentry from flight may not be effective in increasing blood volume as the flight duration surpasses 4-7 d. Saline loading failed to increase plasma volume after 7 d of head-down tilt (HDT) (95), and orthostatic heart rates were no different in astronauts who used fluid loading compared to those who did not after 7-8 d of flight (97). The inability to increase plasma volume with fluid ingestion may be related to a resetting of central venous pressure (CVP) to a lower operational point because both plasma volume and CVP are chronically reduced with microgravity exposure(22,28,34,68). Capacity of cardiovascular reflexes to retain volume from acute fluid intake may be limited by its defense of the pressure-volume relationship under hypovolemic conditions.
Endurance-trained athletes have greater plasma volume, total hemoglobin content, and erythrocyte volume compared to sedentary subjects(8,14,16,79). Regular physical activity, especially of moderate intensity and long duration, is effective in expanding plasma volume chronically(14,16,25,33,38), elevating CVP (37,85), and improving orthostatic tolerance(14,21,38). Intensive intermittent exercise training performed during two 30-min periods daily for 5 d·wk-1 during 30 d of exposure to 6° HDT maintained both plasma volume and ˙VO2max at baseline levels compared to 16-18% reductions in control subjects who had no exercise training(53). Prevention of red blood cell loss in Skylab 4 was associated with high levels of in-flight exercise and maintenance of inflight˙VO2max (16,73). Even intense acute exercise is effective for expanding plasma volume within 24 h in ambulatory subjects by an order of magnitude (10-20%) similar to the reduction in flight(50,77) and for completely restoring plasma volume following 16 d of HDT (31). It seems apparent that specific exercise stimuli can be used to replace stimuli normally provided by gravity to maintain elevated CVP and other cardiovascular functions that allow for restoration of vascular volume. Development of effective countermeasures for blood volume maintenance should include in-flight exercise to test these hypotheses.
Metabolic Responses to Work
Adaptation to microgravity is associated with metabolic responses similar to those observed with sedentary lifestyles. After exposure to ground-based analogs of microgravity, greater metabolic acidosis occurs during exercise, with the anaerobic threshold occurring at lower absolute and relative steady-state work rates and greater blood lactates occurring at the same steady-state work rates(23,36,79,98). Oxygen delivery may be compromised by reduced muscle blood flow(29,92) and by lower capillary-to-fiber ratio(17,42,63) in skeletal muscles after microgravity exposure. Limited enzymatic capacity to facilitate oxygen utilization at the cellular level is evidenced by reduction in citrate synthase and β-hydroxyacyl-coenzyme A (CoA) dehydrogenase(17,63) and increased myofibrillar ATPase/SDH ratios in fast-twitch fibers (42) after spaceflight. In addition, substrate availability may also be affected(15,23). These findings suggest that in-flight muscle fatiguability may result from reduced capacity to oxidize pyruvate and greater dependence on anaerobic metabolism.
Data from both U.S. and Russian space crews have demonstrated 20-50% decrements in force development during postflight testing of various leg muscles(15,17,57-60,69,91,94). Reduced force development of skeletal muscle has been associated with 6-8% decrements in the volume of the lower limbs following flights longer than 3 months (91,93). Using HDT as an analog for spaceflight, average reduction in strength for muscles of the lower extremities (0.6%·d-1) is 50% greater than that occurring for handgrip, forearm, and arm strength (0.4%·d-1)(Fig. 1, top panel). These results closely match those from Skylab and more recent Russian missions(12,17,23,53,55). Following 30 d of HDT, average leg volume was reduced by 10%(17,29,30), and the average decrease in angle-specific peak torque across speeds of both concentric and eccentric muscle actions was by 21% for knee extensors and by 10% for knee flexors(17). These changes in leg volume and muscle function induced by exposure to HDT compare favorably with changes in leg volumes and peak torque development in the same muscle groups during spaceflight of similar duration (15,17,23,94).
Reductions in muscle strength during ground-based simulations of microgravity have been successfully ameliorated with exercise training(Fig. 1, bottom panel). It may be important that cycle exercise has been as successful as resistance exercise for maintaining muscle function (52,53,55). To provide effective and efficient exercise equipment and procedures to minimize muscle atrophy and dysfunction during spaceflight, it is essential to understand the nature of skeletal muscle structural and functional changes induced by exposure to microgravity. Results from both spaceflight and ground-based experiments have revealed significant reduction in cross-sectional areas (CSA) of slow-twitch(Type I) and fast-twitch (Type II) muscle fibers of the vastus lateralis(17,42,63). The relative reduction in muscle fiber CSA tended to be greater in fast-twitch (23-36%) compared to slow-twitch fibers (16%). Astronauts from whom biopsies were taken were among 19 crew members in whom the average decline in maximum voluntary concentric force production of ≈15% for the knee extensor group was similar to the≈20% average reduction in muscle fiber CSA (42). The loss of structure and function in both slow- and fast-twitch muscle fibers suggests that a combination of endurance and resistance exercise is probably required to effectively protect the integrity of skeletal muscle during prolonged spaceflight.
The decline in peak force production in skeletal muscle groups of the lower extremeties has been consistently greater than average reduction in CSAs of the total muscle compartment and fiber size. Various muscle ultrastructural abnormalities (63) and alterations in neuromuscular functions (15,17,71) induced by spaceflight or its analogs may also contribute to loss of muscle force generation and should also be considered in the design and development of exercise equipment and prescription for spaceflight. It is also reasonable to suggest that preservation of muscle structure and function during exposure to microgravity would include replacing muscle actions and forces that occur in the normal 1 × g environment. Nearly all muscle actions in microgravity require fiber shortening (concentric actions), whereas fiber lengthening (eccentric actions) is virtually eliminated. Even though eccentric muscle actions are a regular part of our daily ambulatory activities on Earth, it seems that an efficient exercise program during spaceflight would include eccentric in addition to concentric actions by producing greater force development and greater strength gain with minimal added energy cost(89). Ground-based and spaceflight testing of this type of resistance exercise training as a protective measure against in-flight and postflight muscle atrophy and dysfunction should be pursued.
Both increased bone resorption and decreased bone formation have contributed to bone calcium loss following ground-based and spaceflight experiments (4,5,74,76). Because of the 7-12% mineral loss in trabecular bone and throughout the spine after 6-8 months of spaceflight, increased risk of bone fracture must be a concern for flight duration beyond 1 yr.
Although exercise on Earth has improved calcium retention(5,86), extensive use of cycle ergometer and treadmill exercise in flight has not proven effective for eliminating postflight bone density losses. As a result of reduced time of musculoskeletal loading during spaceflight, bone calcium retention may require greater magnitude of loading forces than those generated with cycle or treadmill exercise. Approximately 1 h·d-1 of isokinetic resistance leg exercise reduced calcium excretion in subjects who underwent 30 d of HDT when compared to other subjects who underwent cycle or no exercise(3) (Fig. 2). High priority should be placed on experiments designed to investigate the relationship between musculoskeletal loading and bone density.
Predisposition to hypotension and syncopal symptoms during standing, head-up tilt, or lower-body negative pressure (LBNP) is well documented in subjects exposed to periods of actual or simulated microgravity(8-10,14,18,20,22,26,56,64,67,81,87). Orthostatic hypotension induced by bed rest and spaceflight has been associated with increased venous compliance of the lower extremities(28,29,60), reduced plasma and blood volumes(14,15,26,35,53,54,56), and decreased left ventricular end-diastolic volume with consequent lowering of stroke volume and cardiac output (80). An important operational concern is the postflight presence of orthostatic intolerance with frank syncope in 30-40% of Shuttle crews when not protected by a G-suit and fluid loading (9,10), and 6-9% are unable to immediately egress from the vehicle (78).
Greater reduction of ˙VO2max in fit compared with unfit subjects has been reported during exposure to simulated microgravity(14,23,32,35,79,87). The absence of regular physical activity and consequent deconditioning may contribute to reduced effectiveness of blood pressure control during orthostatic challenges following spaceflight because exposure to ground-based simulations can induce greater physical deconditioning and higher incidence of syncope in athletic subjects compared to their sedentary counterparts(14,87). This observation provides a basis for the contention that exercise may also prove to be effective in protecting against cardiovascular adaptations underlying postflight orthostatic hypotension.
Because reduction of blood volume during spaceflight is related to orthostatic instability, it seems reasonable that exercise regimens designed to promote hypervolemia might prove effective against orthostatic hypotension. However, data from both spaceflight and groundbased experiments do not necessarily support this hypothesis. Despite extensive exercise training which increased ˙VO2max by 8% (15,73), the three astronauts who completed the 84-d Skylab 4 mission experienced 16% plasma volume reduction, increased venous compliance, and orthostatic instability postflight (15,67). Conversely, in a ground-based experiment (56) with intensive cycle exercise training performed for two 30-min periods·d-1 for 5 d·wk-1 during HDT, ˙VO2max and plasma volume were maintained at pre-HDT levels, while groups who performed resistance or no exercise experienced significant reductions in these parameters(Fig. 3A and 3B). However, tolerance times during head-up tilt were significantly reduced in all three groups with no difference between them (Fig. 3C). Because blood volume was maintained in the cycle-trained subjects, physiological mechanisms in addition to hypovolemia must contribute to orthostatic intolerance. These results from spaceflight and ground-based studies suggest that repeated exercise training regimens, designed to defend physical fitness, require more specificity to provide appropriate stimuli to the mechanism(s) that control orthostatic stability.
In addition to hypovolemia, exposure to microgravity causes significant impairment of carotid baroreceptor reflex control of cardiac acceleration during hypotensive challenge(18,22,26,34,47). Acute alterations in plasma volume do not affect carotid-cardiac baroreflex response(18,90). The time course of changes in baroreflex function during ground-based studies does not parallel that of plasma volume(18,19,26), indicating that impaired cardiac baroreflex function that occurs during adaptation to microgravity is independent of hypovolemia. The magnitude of attenuation of carotid-cardiac baroreflex function correlated positively with development of orthostatic hypotension and instability (syncope) immediately upon standing(26,39). These results suggest that microgravity exposure leads to substantial and progressive attenuation of baroreflex heart rate response that contributes to the occurrence of hypotension and syncope during standing immediately upon reambulation at 1 ×g.
The ineffectiveness of extensive cycle and resistance exercise training programs in counteracting orthostatic instability following exposure to microgravity represents a failure to identify and employ specific exercise profiles that reverse or attenuate baroreflex impairment in addition to other factors associated with orthostatic hypotension. Exercise prescriptions used during long-duration spaceflight have been primarily designed with conventional daily dynamic exercise of moderate intensity for durations of 30-120 min·d-1 (15,20). Similar exercise training protocols have been used in ground-based experiments(56). Unfortunately, dynamic and resistance training regimens in 1 × g employing repeated daily exercise have failed to produce chronic changes in the sensitivity of the carotid-cardiac baroreflex (21,84,88). In light of the relationship between impairment of baroreflex responses and orthostatic intolerance induced by exposure to microgravity, it should not be surprising that exercise training regimens that do not alter baroreflex responsiveness in 1 × g fail to provide protection against orthostatic instability following actual or simulated spaceflight.
CONSIDERATIONS FOR EXERCISE PRESCRIPTION DURING SPACEFLIGHT
Early spaceflight mission managers recognized the importance of exercise programs for maintaining astronaut health and fitness during long-duration spaceflight. The goal has been to ensure that each crew member has sufficient power, strength, and endurance to conduct daily tasks and still have sufficient reserve and functional capacity to perform emergency functions. In this respect operational demands for health and safety have taken priority over the quest for scientific knowledge, which in turn has limited the opportunity to conduct well-documented, reproducible, and controlled experiments that provide data to ensure astronaut health and safety. Therefore, it is not surprising that past and present in-flight exercise regimens have not been totally effective in preserving normal physiological function because they have not been based on adequately designed and controlled investigations. These results strongly emphasize the need to place more emphasis on ground-based investigations and to place greater priority on science than operational considerations in the space station program.
Prolonged daily exercise presently used during spaceflight is costly and drains life-support materials, which are expensive to place and maintain in orbit. For instance, the average daily exercise metabolic cost of 725 kcal during Russian missions represents about 25% of the total caloric intake of 3,150 kcal (15). If exercise time and ensuing energy costs could be halved, the savings over a 6-month mission would be enough to supply another crew member with an additional 27 d of food, 23 d of water, and 13 d of oxygen. This issue will become more critical on longer interplanetary missions.
Development of flight exercise countermeasure prescriptions must include consideration of equipment that meets feasible and effective operational requirements. The equipment must be convenient to use so it will promote optimum crew member compliance. Exercise devices should be small and lightweight for easy handling, setup, and storage and should function with minimal external power requirement.
Types of Exercise
Reduction in oxidative enzyme activity and capillary density in skeletal muscles of the lower extremities induced by prolonged exposure to microgravity emphasizes that exercise stimulates aerobic metabolic processes. Although aerobic exercise training has been used for maintaining or increasing aerobic power during spaceflight, it has failed to eliminate postflight orthostatic hypotension. Results from ground-based experiments have clearly shown that graded exercise protocols leading to maximal effort can acutely reverse reductions in ˙VO2max, exercise endurance time, plasma volume, sensitivity of baroreflexes, and fainting episodes following simulated weightlessness (20). If these protocols would be applied every 7-10 d of flight or just prior to reentry, use of more frequent and costly regimes might be avoided, while functional working capacity and orthostatic tolerance could be adequately restored. This challenges the hypothesis that astronauts must be maintained in a1 × g physiological state throughout flight.
Countermeasure prescriptions for spaceflight should include exercise for the upper arms and forearms because physical activity and body mobilization in microgravity depend heavily on use of the upper body. Exercise for the legs should continue to protect function required to reambulate upon return to Earth.
Protection of musculoskeletal integrity may require development of exercise protocols to replace force development stresses on Earth. Eccentric muscle exercise develops force during muscle lengthening, which replaces a stimulus natural in terrestrial gravity but absent in microgravity. There are several operational and functional advantages of eccentric muscle action: it can produce optimum force development independent of limb speed that is more representative of that experienced on Earth, it does not appreciably increase energy cost, and it elicits minimal fatigue compared to concentric actions. The capability of providing eccentric muscle actions during spaceflight will depend on the design and implementation of unique equipment that can generate active resistance without a large external power requirement or use of conventional hydrolic cylinders.
The empirical programs employed during spaceflight to date can change to meet the requirements of optimal physiological fitness only when exercise equipment is employed that is capable of providing more quantifiable data. Present practices, which allow individual crew members to engage in individual exercise training programs both before and after flight, may provide better compliance to in-flight exercise but prolong the opportunity for obtaining predictable information and assessing specific exercise requirements for spaceflight.
The effects of exercise training during spaceflight will vary with the initial fitness state of the crew. Some ground-based investigations(14,23,32,35,79,87) have demonstrated that individuals with high fitness levels have shown greater loss of maximal aerobic capacity than more sedentary individuals. Despite greater absolute and relative reductions in aerobic power, ˙VO2max in exercise-trained individuals after HDT remained higher than initial baseline levels of untrained individuals (32,79). In addition, Saltin et al. (79) indicated that sedentary subjects required only 7-10 d to return to and surpass their basline˙VO2max following 21 d of exposure to bed rest, while highly fit subjects required 4-6 wk to regain their aerobic power. This relationship appears applicable to muscle strength as well(58,59). Thus, an astronaut's preflight fitness level may profoundly influence the prescription for in-flight exercise training and postflight recovery treatment.
Age and Gender
Crew composition is no longer limited to the highly motivated, robust male test pilots who dominated early spaceflights. Nine female astronauts have now flown in the U.S. program (compared to 113 males), and two female (and 98 male) cosmonauts have flown in the Russian program. One astronaut was 56 yr old. A few ground and flight data have shown that older age and gender should not be deterrents to participation in spaceflight (81). However, females have greater loss of orthostatic tolerance, which has been associated with lower baseline end-diastolic volume, stroke volume, cardiac output, and ˙VO2max, and greater body fat content compared to males after 2 wk of bed rest (81). These differences could not be attributed to the menstrual cycle (82). If exercise can provide protection against postflight orthostatic hypotension, these results suggest that females may require more rigorous or unique exercise regimens compared to males.
In addition to operational considerations, the need to minimize time required for in-flight exercise should help motivate crew members to maintain their exercise regimens. Time-intensive flight exercise protocols, which can require as much as 2 h·d-1, attenuate crew members' motivation. In addition to scientific requirements, it will also be important for crew motivation to quantitate the effects of the exercise program on their fitness level as a positive feedback.
Crews have attested to the efficacy of various exercises and to associated improvement in “psychological wellbeing,” i.e., that in-flight exercise hastens postflight recovery despite the absence of systematic assessment of results (58,59). Evaluation of in-flight exercise data has been confounded by varying levels of physical fitness of crew members (athletes vs nonathletes), type and extent of individualized training prior to flight (aerobic vs anaerobic), and nonuniform use of various exercise procedures during a given mission. Although personalized exercise programs may be important for acceptance by crew members, answers to basic physiological questions will be obtained only when preflight, in-flight, and postflight exercise training regimens become standardized.
EXERCISE CURRENTLY USED IN SPACEFLIGHT
Cycle ergometer exercise has been used by astronauts since the early days of spaceflight because of several unique advantages: in addition to its use for maintaining physical conditioning and for measuring aerobic power, it accurately measures work output by systematically controlling pedaling resistance, mechanical efficiency remains essentially constant(15), and it can be used for assessing work capacity and training of the arms. The capacity to perform arm work did not appear to be impaired during the 18-d Soyuz-9 mission: estimated energy expenditure was 160-180 kcal·h-1 (˙VO2 of approximately 0.5-0.6 l·min-1), representing 30-35% of the ˙VO2max for both arms. However, that method of arm flexion testing proved to be highly variable, and a modified arm-crank cycle ergometer could provide a more reliable technique for control of work rate. In addition to testing, a cycle ergometer could be used for training upper-body work capacity. For example, the ˙VO2 during 3-6 h of EVA during various Space Shuttle missions has averaged 0.8 l·min-1, similar to that reported in Apollo and Skylab missions (15). The ˙VO2 required during peak work of short durations (minutes) during nine Shuttle EVA(averaged over six missions) was 1.6 l·min-1, about 80% of the maximal working capacity of the arms and upper-body muscle groups. Similar values for ˙VO2 have been recorded in cosmonauts during short and longerterm EVA (15). It is therefore not surprising that Shuttle astronauts have expressed some degree of fatigue following these long EVAs, which clearly demonstrates the need to design arm exercise training regimens for use during flight to maintain work performance. The cycle ergometer could provide such a function.
Cycle-like devices that could be rotated with either arms or legs were utilized in initial spaceflight programs because of difficulty using treadmills. Cycle ergometers were flown on all Skylab missions and continue to be included aboard all Russian space stations (15). The cycle used aboard Skylab in 1973-1974 (Fig. 4) has served as the prototype for subsequent models. This initial device, although heavy(45 kg) and bulky, had programmable loads ranging from 25 to 300 W at 40-80 pedal rpm that could be controlled by heart rate. Smaller (≈10 kg), more self-contained ergometers are carried aboard present Shuttle vehicles(91). Russian cosmonauts use the VB-3 ergometer, which does not differ significantly from the Skylab device.
A new ergometer, incorporated into the European Space Agency (ESA) Anthrorack research unit, was first flown on the NASA ML-1 mission in 1993(72) and is flown regularly on the Space Shuttle. This ESA ergometer is a constant-load device with work rates that can be set automatically or manually from 0 to 350 W at pedaling rates from 40 to 115 rpm. Subject support is provided when exercising in microgravity by handrails and a restraining body harness. The pedals operate a planetary gear that drives the flywheel, and a motor drives a friction band that provides torque or resistance to the flywheel. The product of flywheel speed and frictional torque dictates the resultant work rate, and a controller maintains a constant work rate independent of actual flywheel or pedal speed. A computer programs variable work rate and speed patterns, providing flexibility for applying step or ramp work rate functions.
A treadmill was first used during the third Skylab mission. Treadmill ergometry is attractive because it can elicit higher ˙VO2max compared to cycle ergometry. However, a major disadvantage is its limitation in obtaining data on mechanical work and efficiency because negligible external work occurs. Therefore, external work can be estimated only during horizontal walking by measuring vertical body motion and by calculating resulting acceleration and deceleration of the limbs, which is impractical during spaceflight. Mechanical efficiency during treadmill exercise in spaceflight is 15-20% lower than on Earth, indicating that the energy cost of locomotion and body stabilization is greater in weightlessness(15). The mechanical systems (bungee cords) used to restrain the subject on the treadmill and provide resistance to movement during exercise in flight contribute significantly to this loss in mechanical efficiency and may explain why treadmill exercise has not been totally effective in maintaining musculoskeletal integrity during long-duration missions. Clearly, new methods for providing mechanical stability during walking and running in weightlessness need to be developed if treadmill exercise is to be effective.
Treadmills have been favored in spaceflight for simulation of ground-based walking and the imposition of gravity-like eccentric loads on the muscles and bones of the lower extremities (91). Passive and/or motor-driven treadmills have been developed for use in space. If a moving track (belt) is provided and forces are properly distributed over the body, a close approximation to walking can be achieved during weightlessness.Figure 5 shows the in-flight device developed by the Russians, in which maximal restraining forces have been limited to 50 kg (0.7× g·70 kg-1 man). This partial loading represents a compromise as a result of discomfort (pressure, chafing) caused by the restraining harness. To tolerate long periods of simulated walking and running, cosmonauts wear the TNK V-1 load suit with boots(Fig. 5).
A human-powered treadmill consisting of an aluminium-Teflon walking surface attached to an iso-grid floor was used 10 min·d-1 during the 84-d Skylab 4 mission (91). Subjects moved the surface against resistance with their foot motions. Four bungee cords attached to a shoulder and waist harness provided an equivalent weight of 80 kg for this treadmill. By angling the bungees, subjects were able to simulate climbing a slippery hill under 1.1 × g. Three crew members who each flew on Skylab 2 (28 d) and Skylab 3 (59 d) missions used only the cycle ergometer for exercise. Reduction in postflight muscle strength was markedly less in Skylab 4 astronauts, who combined treadmill exercise with cycling(91,94). But additional exercise on the first and second missions may have produced comparable results. This perceived positive experience with in-flight treadmill exercise led to the design of the collapsible subject-driven treadmill now used on Space Shuttles(91). The new treadmill is 109 cm long, 34 cm wide, 71 cm high, and weighs 17 kg. Subjects can generate low-friction belt speeds of 50-265 m·min-1. Rubber bungee cords attached to a hip-shoulder harness provide a “weight” equivalent force of 0.5-2.0 ×g.
Although the cycle ergometer is an excellent device for testing and maintaining cardiorespiratory endurance, current models cannot generate forces and eccentric muscle actions in the legs equivalent to those occurring during ambulation in terrestrial gravity. Therefore, sole use of a cycle ergometer may result in marked gravity underloading of leg muscles and may not adequately defend against potential losses in strength, muscle mass, and neuromuscular function. The ability to impose gravity loading approximating that during Earth-based ambulation makes a treadmill device the choice for exercise training during long-term spaceflight. Hopefully its use might also minimize calcium loss from bone during spaceflight.
Strength Training Devices
The efficacy of strength training procedures in flight has been limited because quantification of forces has not been controlled or measured.
Devices have ranged from simple exer-gym “chest expanders”(Fig. 6) to “rope and pulley” machines(Fig. 7). These strength training devices have been principally for upper-body muscles because cycle ergometers or treadmills do not adequately exercise those muscle groups. However, the necessity for significant upper-body exercise is questionable because findings from ground-based studies have demonstrated little degradation of upper-body muscle strength (23,55). On the other hand, resistive exercise for leg muscles may prove important because significant atrophy and loss of function by as much as 50% of baseline values have been reported following spaceflight.
The rope and capstan device (Fig. 6A) is representative of the entire class of available resistance devices for loading muscles of the upper extremities. Applied force is proportional to the speed at which the rope or handle is pulled through its resistance container. These devices have the advantages of being small and lightweight and of developing high resistance. However, they become unpleasant to use at levels required to induce significant physiological change. Although included on all Apollo and Skylab missions, they were not frequently utilized. When used at low resistances and high pulling rates, these devices induce rapid muscle fatigue, which results in dynamic endurance rather than resistive muscular strength training effects. The simple spring-resistance Type B device(Fig. 6) induces greater force the further the handles are pulled apart due to elastic- or spring-resistive elements. Exercise on the Type B device seems more acceptable to crew members than the purely resistance type of device, even though this is an unnatural way of muscle loading.
Isokinetic devices (Fig. 7) were employed as a result of limited acceptance of the purely resistance devices used on early spaceflight missions and because of the limited number of muscles they affected. These MK-1 exercisers were first used aboard Skylab 3 and 4. A rope and capstan mechanism allowed a wide range of motion and was controlled by a constant-rate governor with a variable set-point. This lightweight device exercised upper-extremity and back muscles with higher mechanical loading on bones than could be provided by a cycle ergometer or treadmill(66). Similar expanders are also used by cosmonauts. Although use of these devices approximated weightlifting, there have been limited data on their effectiveness.
Application of electrical stimulation to the large skeletal muscle groups of the lower extremities is another method for counteracting muscle atrophy during spaceflight because astronauts can be stimulated while conducting daily tasks (41). However, electrical stimulation can result in painful tetanic contractions if electrical impulses become too frequent. Results from animal studies demonstrated prevention of muscle fiber atrophy by nerve stimulation (71), and, based on extensive ground-based investigations on human subjects(11,41,49,58,99), the Russian space program has included muscle electrical stimulation. While cosmonauts have applied electrical stimulation to leg and back muscles during long-term space missions, the U.S. space program has yet to adopt this methodology. Current data support consideration for the use of this technique as an additional “exercise” countermeasure against in-flight muscle atrophy.
Russian crews have also worn a special elasticized garment, the TNK V-1 load, or “penguin,” suit, during their entire 8- to 12-h in-flight workday, as well as during treadmill exercise (15). Rubber bands woven into the fabric, extending from the shoulders to the waist and from the waist to the lower extremities, produce tension on antigravity muscles. Although a quantitative assessment of potential benefit has not been made, daily long-term use of the penguin suit may provide protection against musculoskeletal atrophy because gravitational loads equivalent to 70% of body mass on Earth can be provided.
NEW APPROACHES TO EXERCISE COUNTERMEASURES FOR SPACEFLIGHT
Moderate effectiveness of current in-flight exercise protocols has led to a number of additional proposals designed to apply missing gravitational stimuli during exercise during a mission. Although none has been used during flight, their use in combination with presently employed exercise regimes may help alleviate spaceflight deconditioning.
A mechanically coupled, counter-rotating system using two bicycles(Fig. 8) may provide effective endurance exercise training in microgravity under a gravitational load(1,2). Moving on the inner wall of a cylindrical surface (spacecraft module), two subjects can develop sufficient speed to generate centrifugal acceleration at their feet equivalent to +1 ×g. The wheels of the bicycles run on two parallel rack-rails, which provide the friction necessary for movement. The bikes move at the same speed but in opposite directions to minimize yaw and disturbance to the spacecraft. To ensure that the subjects follow a matched sinusoidal function, an adjustable computer-controllable mass is mounted to an axle that moves in concert with the subjects. Use of a short radius (<3 m) results in a significant head-to-foot gravity gradient that ceases to be problem with radii>4 m.
Another approach to a countermeasure designed to simultaneously apply cycle exercise with gravity acceleration is a short-arm, dual-couch, human-powered centrifuge fabricated at NASA's Ames Research Center (Fig. 9). The device has three cycle stations, two on-board and one off-board the centrifuge (1.8-m radius). The centrifuge can be driven by any one or a combination of these chain-powered cycle stations, providing gravitational load to the riders of the on-board cycles. Instruments for monitoring physiological (e.g., heart rate) and centrifuge (e.g., rpm, g level) parameters are integrated into the system to allow feedback regarding the physiological and gravitational stimuli provided by the exercise countermeasure.
Development of motion sickness due to the Coriollis effect(51) from head motions of subjects on a short-armed device represents a potential limiting factor with the twin-bicycle technique. Preliminary tests with a bicycle on the centrifuge have failed to demonstrate motion sickness problems while pedaling up to +3 × g when the rotational radius was 2.5-3 m (2). Previous findings also indicate that such Coriollis-induced symptoms may be actually ameliorated rather than aggravated with spaceflight (70). Advantages of this approach to impose a gravity load during exercise strongly recommend its consideration for further development and in-flight testing.
Dynamic Resistance Exercise Devices
A resistance device that consists of a modular rope, instrumented pulley, and harness (83) and operates on the principle of“dynamic tensioning,” where one limb is pushed directly against the other, has been recommended for use in microgravity. During leg exercise, for example, extension of one leg transmits force through a rope and pulley so that the flexor muscles of the other leg develop tension eccentrically by resisting the force. Advantages of this device include full control over the range of motion, speed, and magnitude (900 Nm per leg) of forces generated by the subject; maximum force developed is limited only by strength of the involved muscle groups (83). This device can induce quantifiable, repeated, eccentric, and concentric muscle actions. Due to its simplicity of design, low cost, light weight, power-free requirements, and ease of operation, it should find ready in-flight application.
Use of isokinetic exercise devices that function at a preselected constant velocity against a resistance that automatically adapts to applied force has been suggested for spaceflight (53,83). Essentially all instruments (including rowing machines or pneumatic resistance devices) are too bulky for use in space. A disadvantage of isokinetic exercise is that it may not mimic the natural neuromuscular activity in terrestrial gravity, where muscular actions require movements at varying speeds. However, when a space-efficient isokinetic device becomes available, it should accurately assess muscle function by providing in vivo force-velocity relationships as well as function as a diagnostic tool for physical fitness testing over the course of a space mission. This could provide important information concerning countermeasure effectiveness and allow for continuous updating and optimization of the treatments being used.
Maximal Exercise Effects
Since long-term submaximal exercise training has proven ineffective in counteracting orthostatic instability following exposure to microgravity, new protocols using acute, intense bouts of exercise have been tested in ground-based experiments. With a graded work-rate protocol, ambulatory, bed-rested, and paraplegic subjects who exercised to volitional exhaustion in 15-20 min demonstrated significant increases in baroreflex function(18,20,24,27,44-46), plasma volume (31,50), and elimination of orthostatic hypotension (44,45,46) throughout 24 h of postexercise recovery. In addition, the reduced insulin sensitivity induced by bed rest or physical deconditioning(40,62) can be reversed by one bout of maximal exercise following 10 d of detraining (62). One bout of maximal cycle exercise at the end of 10 d of HDT restored ˙VO2max, heart rate, blood pressures, rate-pressure product, oxygen pulse, endurance time, and orthostatic stability during treadmill exercise to pre-HDT levels within 2 h of ambulation (13). Acute maximal exercise has also reversed fainting episodes following acute exposure to simulated microgravity (92). These observations indicate that application of acute maximal exercise may prove an effective countermeasure against various cardiovascular and metabolic functions that are normally compromised in microgravity. An exercise prescription that uses less-frequent and more-intense exercise loads would be cost-effective by minimizing in-flight work time, food, water, and oxygen. These factors will become paramount during extended-duration spaceflight, in which life-support systems will be at a premium.
Exercise and Lower Body Negative Pressure
With the demonstration that LBNP of 40-50 mm Hg results in body fluid and hemodynamic responses equivalent to those induced by head-up tilt at +1× g (21), the use of exercise simultaneously with LBNP has been proposed (Fig. 10). This concept assumes that LBNP applied during exercise in microgravity can generate higher musculoskeletal forces by inducing artificial gravity without rotation of the spacecraft and shorten flight exercise times(61,75,96). During application of negative pressure, an axial force is developed by higher external pressure acting on the CSA of the body at the waist seal. The developed ground reaction force is a function of inertial forces caused by accelerations of the body's center of mass while walking, jogging, or running and the differential negative pressure. Preliminary ground-based studies have shown this approach to have promising physical and physiological effects in addition to several attractive features such as low implementation costs and easy use in spaceflight(61,75,96).
SUMMARY AND RECOMMENDATIONS
Use of exercise as an effective countermeasure will depend on a thorough understanding of operant mechanisms of physiological adaptation to microgravity and how they are affected by the stimulus of specific intensities, durations, frequencies, and modes of exercise that have yet to be clearly defined. Present practices appear to be excessively costly in terms of both time and money. Better exercise protocols are available and need to be tested on the ground and during spaceflight. Data from both ground-based and in-flight investigations have suggested several findings and strategies that can be used for development of exercise countermeasures for training before, during, and after spaceflight:
1. It is clear that current exercise countermeasures applied during exposure to microgravity have not been completely successful in maintaining or restoring impaired cardiovascular and musculoskeletal functions.
2. HDT can be used effectively as a model to investigate alterations in physiological functions induced from exposure of crews to microgravity. This model provides a less expensive and more controlled laboratory environment to conduct experiments for later in-flight testing.
3. Musculoskeletal structure and function are reduced in microgravity in spite of heavy in-flight cycle and treadmill exercise (the latter with some degree of gravity loading), electrical muscle stimulation, and wearing of a load suit. Use of more-intense resistance exercise, including eccentric contractions, must be given greater consideration.
4. Performance of maximal exercise within 24 h of reambulation has been successful in reversing hypovolemia and orthostatic instability in ground-based experiments and could easily be employed during space missions by using currently available exercise ergometers.
5. In addition to exercise alone, some degree of gravity loading with acceleration or LBNP may be required to provide optimum effects.
Exercise countermeasure development needs more attention. Future in-flight exercise training programs will probably require a mix of dynamic and resistance modes to maintain both anatomical structure and physiological function. Development of training protocols for the arms and upper body has received far less emphasis than protocols for the lower extremities. Provision of operationally appropriate equipment is mandatory.
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MICROGRAVITY; ORTHOSTATIC TOLERANCE; CARDIOVASCULAR FUNCTION; MUSCLE FUNCTION; BONE
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