Even before humans began to venture into space, serious and reasonable concerns were expressed about exposure of humans to the microgravity of space due to the potential systemic effects on terrestrially evolved life-forms adapted to Earth gravity. Unloading of skeletal muscle, both on Earth via bed-rest experiments and during spaceflight, result in remodeling of muscle (atrophic response). As a result, decrements occur in skeletal-muscle strength, fatigue resistance, motor performance, and connective-tissue integrity. In addition, weightlessness causes cardiopulmonary and vascular changes, including a significant decrease in red blood cell mass, that affect skeletal muscle function. Normal adaptive response to the microgravity environment may become a liability, resulting in increased risk of an inability or decreased efficiency in crewmember performance of physically demanding tasks during extravehicular activity (EVA) or upon return to Earth. In the US human space-program, the only in-flight countermeasure to skeletal muscle functional deficits that has been utilized thus far is physical exercise. In-flight exercise hardware and protocols have varied from mission to mission, somewhat dependent on mission duration and the volume of the spacecraft available. Collective knowledge gained from these missions has aided in the evolution of exercise hardware and protocols designed to minimize muscle atrophy and the concomitant deficits in skeletal muscle function. Russian scientists have utilized a variety of exercise hardware and in-flight exercise protocols during long-duration spaceflight (up to and beyond one year) aboard the Mir space station. On the International Space Station (ISS), a combination of resistive and aerobic exercise has been used. Outcomes have been acceptable according to current expectations for crewmember performance on return to Earth. However, for missions to the Moon, establishment of a lunar base, and interplanetary travel to Mars, the functional requirements for human performance during each specific phase of these missions have not been sufficiently defined to determine whether currently developed countermeasures are adequate to meet physical performance requirements. Research access to human crewmembers during space flight is limited. Earth-bound physiologic models have been developed and findings reviewed. Models include horizontal or head-down bed rest, dry immersion bed rest, limb immobilization, and unilateral lower-limb suspension. While none of these ground-based analogs provides a perfect simulation of human microgravity exposure during spaceflight, each is useful for study of particular aspects of muscle unloading as well as for investigation of sensorimotor alterations. Development, evaluation and validation of new countermeasures to the effects of skeletal muscle unloading will likely employ variations of these same basic ground-based models. Prospective countermeasures may include pharmacologic and/or dietary interventions, innovative exercise hardware providing improved loading modalities, locomotor training devices, passive exercise devices, and artificial gravity (either as an integral component of the spacecraft or in a discrete device contained within it). With respect to the latter, the hemodynamic and metabolic responses to increased loading provided by a human-powered centrifuge have been described.
Historical overview
U.S. human spaceflight programs
Mercury and Gemini Prior to launch of the first American astronaut, suborbital flights of non-human primates (chimpanzees) demonstrated that launch and entry, as well as short-duration microgravity exposure, were all survivable events. The initial biomedical problem faced by Project Mercury (which ran from 1959 – 1963) was establishment of selection criteria for the first group of astronauts. Medical requirements for the Mercury astronauts were formulated by the NASA Life Sciences Committee, an advisory group of distinguished physicians and life scientists. Final selection criteria included results of medical testing as well as the candidates' technical expertise and experience. Aeromedical personnel and facilities of the Department of Defense were summoned to provide the stress and psychological testing of astronaut candidates. The screening and testing procedures defined for the selection of Mercury astronauts served as the basis for subsequent selection of Gemini and Apollo astronauts when those programs were initiated. While the Mercury flights were largely demonstration flights, the longest Mercury mission being only about 34 hours, Project Mercury clearly demonstrated that humans could tolerate the spaceflight environment without major acute physiological effects and some useful biomedical information was obtained, which included the following:
Pilot performance capability as unaltered by spaceflight All measured physiological functions remained within acceptable normal limits No signs of abnormal sensory or psychological responses were observed The radiation dose received was considered insignificant from a medical perspective Immediately after landing, an orthostatic rise in heart rate and drop in systemic blood pressure were noted, which persisted for 7 to 19 hours post landing Because of the short mission durations of Project Mercury, there was little concern about loss of musculoskeletal function; hence no exercise hardware or protocols were developed for use during flight. However, the selection criteria ensured that astronauts were in excellent physical condition before flight. Biomedical information acquired during the Mercury flights provided a positive basis to proceed with the next step, the Gemini Program, which took place during the 20 months from March 1965 to November 1966. The major stated objective of the Gemini Program was to achieve a high level of operational confidence with human spaceflight. To prepare for a lunar landing mission, three major goals had to be realized. These were:
to accomplish rendezvous and docking of two space vehicles to perform extravehicular activities and to validate human life support systems and astronaut performance capabilities under such conditions (germane to this topic) to develop a better understanding of how humans tolerate extended periods of weightless flight exposure Thus, Project Gemini provided a much better opportunity to study the effects of the microgravity of spaceflight on humans. In the 14-day Gemini 7 flight, salient observations were undertaken to more carefully examine the physiological and psychological responses of astronauts as a result of exposure to spaceflight and the associated microgravity environment. The Gemini Program resulted in about 2000 man-hours of weightless exposure of U.S. astronauts. Additional observations included the presence of postflight orthostatic intolerance that was still present for up to 50 hours after landing in soe crewmembers, a decrease in red cell mass of 5 – 20% from preflight levels, and radiographic indications of bone demineralization in the calcaneus. No significant decrements in performance of mission objectives were noted and no specific measurements of muscle strength or endurance were obtained that compared preflight, in-flight and postflight levels.
Apollo The major objective of the Apollo Program was the landing of astronauts on the lunar surface and their subsequent safe return to Earth. The Apollo (1968–1973) biomedical results were collected from 11 crewed missions that were completed within the five-year span of the Apollo Program, from pre-lunar flights (missions 7 through 10); the first lunar landing (mission 11), and five subsequent lunar exploratory flights (mission 12 through 17). Apollo 13 did not complete its intended lunar landing mission because of a pressure vessel explosion in the Service Module. Instead, it returned safely to Earth after attaining a partial lunar orbit. Essential to the successful completion of the Apollo Program was the requirement for some crew members to undertake long and strenuous periods of extravehicular activity (EVA) on the lunar surface. There was concern about the capability of crew members to accomplish the lunar surface excursions planned for some of the Apollo missions. Although reduced lunar gravity was expected to make some tasks less strenuous, reduced suit mobility coupled with a complex and ambitious timeline led to the prediction that metabolic activity would exceed resulting levels for extended periods. Since the nature and magnitude of physiological dysfunction resulting from microgravity exposure had not yet been established (and is still not concisely defined), suitable physiological testing was completed within the constraints of the Apollo Program to determine if crewmember physiological responses to exercise were altered as a consequence of spaceflight. Initial planning for the Apollo Program included provisions for in-flight measurements of salient parameters of concern including physiological responses to exercise. However, the fire in the Apollo 204 spacecraft (also known as Apollo 1), fatal to astronauts Grissom, White, and Chaffee, resulted in NASA management initiating changes in the program that eliminated such prospects. This, investigators were left with only the possibility to conduct pre-flight and post-flight exercise response studies and to assume that these findings reflected alterations of cardiopulmonary and skeletal muscle function secondary to microgravity exposure. It was realized early on that within the context and constraints imposed by the realities of the Apollo missions, the inability to control certain experiment variables would present challenges to many biomedical investigations. Firstly, re-adaption to Earth gravity procedures introduced additional challenges to a well-controlled experiment design since Apollo crew members spent variable amounts of time in an uncomfortably warm spacecraft bobbing in the ocean and additionally, orbital mechanics constraints on re-entry times imposed crew recovery times that prevented the possibility of conducting pre- and post-flight testing within a similar circadian schedule. The effect of these uncontrollable conditions and that of other physical and psychological stresses could not be separated from responses attributable to microgravity exposure alone. Thus, data relating to the physiological responses to exercise stress in Apollo astronauts must be interpreted within this overall context. No standardized in-flight exercise program was planned for any of the Apollo flights; however, an exercise device (Figure 6-1) was provided on some missions. Crewmembers, when situated in the Command Module (CM), typically used the exerciser several time per day for periods of 15–20 minutes. The pre- and post-flight testing consisted of graded exercise tests conducted on a bicycle ergometer. Heart rate was used for determining stress levels, and the same heart rate levels were used for pre- and postflight testing.
Although the exact duration of each stress level was adjusted slightly (1–2 minutes) for the later Apollo missions to obtain additional measurements, the graded stress protocol included exercise levels of 120, 140 and 160 beats per minute, corresponding to the light, medium, and heavy work respectively for each individual. For the Apollo 9 and 10 missions, a stress level of 180 beats per minute was added. The entire test protocol was conducted three times within a 30-day period before lift-off. Postflight tests were conducted on recovery (landing) day and once more at 24 to 36 hours after recovery. During each test, workload, heart rate, blood pressure, and respiratory gas exchange (oxygen consumption, carbon dioxide production, and minute volume) measurements were made. For Apollo 15 to 17 missions, cardiac output measurements were obtained by the single-breath technique. Arteriovenous oxygen differences were calculated from the measured oxygen consumption and cardiac output data. The data collected were voluminous and are summarized in tabular form by Rummel et al. Dietlein has provided a concise synopsis of the findings. In brief, reduced work capacity and oxygen consumption of significant degree was noted in 67% (18 of 27) of the Apollo crewmembers tested on recovery. This decrement was transient, and 85% of those tested (23 of 27) returned to preflight baseline levels within 24–36 hours. A significant decrement in cardiac stroke volume was associated with diminished exercise tolerance. It was not clear whether the exercise decrement had its onset during flight. If it did, the Apollo data did not reveal the precise in-flight time course because of lack of in-flight measurement capabilities. The astronauts' performance on the lunar surface provided no reason to believe that any serious exercise tolerance decrement occurred during flight, except that related to lack of regular exercise and muscle disuse atrophy. The studies completed during Apollo, although less than optimal, left no doubt that a decrement in exercise tolerance occurred in the period immediately after landing, although it is believed that such decrements were not present during surface EVA. It seems likely that multiple factors are responsible for the observed decrements. Lack of sufficient exercise and development of muscle disuse atrophy probably contributed. Catabolic tissue processes may have been accentuated by increased cortisol secretion as a consequence of mission stress and individual crew member reaction to such stress. Additional factors associated with the return to Earth's gravity may also be implicated. This, the observed diminished stroke volume (cardiac output) is certainly contributory and, in turn, is a reflection of diminished venous return and contracted effective circulating blood volume induced by spaceflight factors. Skeletal muscle atrophy is mentioned with respect to its possible contribution to exercise intolerance, and in some of the later Apollo flights lower limb girth measurements were completed (data not published) that provided the first evidence for loss of muscle mass in the legs.
Skylab The Skylab program (May 1973 – November 1974) was from the onset, intended to provide a life sciences laboratory in space. A significant number of experiments were conducted to provide physiologic data from humans exposed to long-duration stays in a microgravity environment. A 56-day ground-based simulation of many of the Skylab experiments, conducted in an environmentally controlled, enclosed chamber, was termed the Skylab Medical Experiments Altitude Test and represented the first mission. The three subsequent orbital missions were termed Skylab 2, 3 and 4. These three long-duration mission were 28, 56 and 84 days in duration, respectively. Collectively, the Skylab missions achieved a milestone in providing a vast array of human spaceflight biomedical information during missions of longer duration than any previous mission. With respect to the current issue of loss of muscle mass and function, two key studies were performed during the course of the three Skylab orbital missions. First, leg and arm volumes were calculated by measuring the girth (circumference) of contiguous 3-centimeter arm and leg segments, treating all the segments as a short tapered cylinder, and then summing the segment volumes to obtain the volume of each extremity. The second study included the first muscle strength measurements by means of a dynamometer. In addition to measurements relating directly to skeletal muscle strength and mass, indirect measurements were made that demonstrated that all Skylab crewmembers had a negative nitrogen balance indicative of skeletal muscle attrition. This was also observed 10 years later in short-duration Space Shuttle crewmembers.
Upper and lower limb volumes obtained on the three crewmembers of Skylab 4 are shown in figure 6-2. Fluid shifts contributed the largest changes to lower limb volumes, but loss of leg tissue mass is clearly evident, particularly in the Commander. As shown in the graphs, significant loss of leg volume occurs within the first few days of microgravity exposure while changes in the upper limbs are less remarkable. Upon return to Earth, much of the loss of leg volume is corrected and there is often a short over-correction or overshoot. Once this fluid shift resolves, the true loss of muscle mass remaining in the legs is revealed that more slowly returns to the baseline or preflight level (see figure 6-2, leg during recovery on right side of graph for all three crewmembers). In the Skylab 4 Commander, the loss in leg volume appears to be nearly 300 cc. (figure 6-2, topmost graph). Because the complement of exercise equipment for this mission was the largest (consisting of a cycle ergometer, passive treadmill, and the "Mini gym", modified commercial devices that provided the capability for low-load resistive exercises) losses in muscle mass and strength were less than in the previous two missions of shorter duration. During the Skylab program, exercises and exercise devices were added incrementally and the testing expanded with each mission. This produced a different exercise environment for each flight so that in reality, there were three separate but related orbital experiments, each with N=3. The results from each mission significantly affected the next. Preflight and postflight evaluation of muscle strength was performed on the right arm and leg of each crewmember for all three Skylab orbital missions by means of a Cybex isokinetic dynamometer. The protocol completed on each crewmember included a thorough warm-up, and 10 maximum-effort full flexions and extensions of the arm at the elbow and of the hip and knee at an angular rate of 45° per second. The isokinetic leg strength from all three missions, as well as body weights and leg volumes, are presented in Figure 6-3.
On Skylab 2, only the bicycle ergometer was available for the in-flight exercise, with testing performed 18 days before launch and 5 days after landing. While it was realized that these times were too temporally remote from the flight, this was the best that could be achieved due to schedule constraints. By the time day 5 muscle testing was completed, some recovery in function had likely occurred; however, a marked decrement still remained. The decrement in leg extensor strength was nearly 25%; the arms suffered less but also exhibited marked losses (data not shown). The Commander's arm extensors showed no loss, since he used these muscles in hand-pedaling the bicycle, being the only Skylab crewmember to adopt this mode of arm exercise. This illustrated a fundamental point in muscle conditioning: to maintain the strength of a muscle, it must be stressed to or near the level at which it will have to function. Leg extensor muscles, important in standing and providing propulsive forces during walking, are capable of generating forces of hundreds of pounds, while the arm extensor forces are measured in tens of pounds. Forces developed in pedaling a bicycle ergometer are typically tens of pounds and are thus incapable of maintaining leg strength. The bicycle ergometer proved to be an excellent machine for aerobic exercise and cardiovascular conditioning, but it was not capable of developing either the type or level of forces needed to maintain strength for walking under 1G. Immediately after Skylab 2, work was started on devices to provide adequate exercise to arms, trunk, and legs. A commercial device, termed "Mini Gym", was modified extensively and designated "MK-I". Only exercises that primarily benefited arms and trunk were achievable with this device. While forces transmitted to the legs were greater than those from the cycle ergometer, they were still limited to an inadequate level, since this level could not exceed the maximum strength of the arms, which represents a fraction of leg strength. A second device, designated "MK-II", consisted of a pair of handles between which up to five extension springs could be attached, allowing development of maximum forces of 25 pounds per foot. These two devices were flown on Skylab 3, and in-flight nutrition support and exercise time and food were increased. The crew performed many repetitions per day of their favorite maneuvers on the MK-I and to a lesser extent, on the MK-II. Also, the average amount of work done on the bicycle ergometer was more than doubled on Skylab 3, with all crewmembers participating actively. It was perceived by Skylb life scientists that a device that allowed walking and running under forces equivalent to Earth gravity would provide more strenuous exercise. Immediately after completion of Skylab 2, work was begun on a treadmill for Skylab 4. As mission preparation progressed, the launch weight of Skylab 4 escalated so much that the final design of the treadmill was constrained by weight limitations. The final weight for the device was a mere 3.5 pounds. This passive device (figure 6-4) consisted of a Teflon-coated aluminum walking surface attached to the Skylab iso-grid floor. Four rubber bungee cords provided an equivalent weight of about 80 kilograms (175 lbs) and were attached to a shoulder and waist harness worn by crewmembers during use. By angling the bungee cords to that the user was pulled slightly forward, an equivalent to a slippery hill was created. High loads were placed on some leg muscles, especially the calf, and fatigue was so rapid that the device could not be used for significant aerobic work because of the bungee/harness design. It was absolutely necessary to wear socks and no shoes to provide a low-friction interface to the Teflon surface.
On Skylab 4, the crew used the bicycle ergometer at essentially the same rate as on Skylab 3, as well as the MK-I and MK-II Mini Gym exercisers. In addition, they typically performed 10 minutes per day of walking, jumping and jogging on the treadmill. Food intake had again been increased. Upon their return to Earth and even before muscle testing, it was apparent that the Skylab 4 crewmembers were in very good physical condition. They were able to stand and walk for long periods without apparent difficulty on the day after landing (R+1), in contrast to the crewmembers from the earlier two missions. Results of strength testing confirmed a surprisingly small loss in leg strength even after nearly 3 months of microgravity exposure (figure 6-3). In fact, knee extensor strength increased over the pre-flight level (figure 6-13).
Space Shuttle A variety of investigations related to skeletal muscle function have been completed during the course of the Space Shuttle Program (1981–2011). The most comprehensive of these was a suite of investigations accomplished during the Extended Duration Orbiter Medical Project (EDOMP), which was carried out during 1989 – 1995 with missions of up to 16 days. Studies most relevant to the risk on which this report focuses include the following Detailed Scientific Objectives (DSO):
DSO 475 – Direct assessment of muscle atrophy and biochemistry before and after spaceflight DSO 606 – Evaluating concentric and eccentric skeletal muscle contractions after spaceflight DSO 617 – Evaluating functional muscle performance
The collective specific aim of DSO 477 and DSO 617 was to evaluate functional changes in concentric and eccentric strength (peak torque) and endurance (fatigue index) of the trunk, arms, and legs of crewmembers before and after flight. LIDO® dynamometer located at the Johnson Space Center and at both the prime and contingency landing sites were used to evaluate concentric and eccentric contractions before and after flight. Test subjects in this study exercised during flight for various durations, intensities and numbers of days on the original Shuttle treadmill (figure 6-5) (as opposed to the EDO treadmill, which flew on later Shuttle missions and was the bases for the ISS treadmill) as part of separate in-flight investigations. Exercise protocols included continuous and interval training, with prescriptions varying from 60% to 85% of preflight maximal oxygen uptake as estimated from heart rate (HR) Some subjects had difficulty in achieving or maintaining their target HR during flight. The brake (figure 6-5). A harness and bungee/tether system was used to simulate body weight by providing forces equivalent to an approximate 1-G body mass. Subjects on this non-motorized treadmill were required to walk and run at a positive percentage grade to overcome mechanical friction. Study participants were familiarized with the LIDO® test protocol and procedures about 30 days before launch (L-30), after which six test sessions were conducted. Three sessions were completed before launch (L-21, L-14 and L-8 days) and three after landing (R+0, R+2 and R+7 to R+10 days). The muscle groups tested are shown in table 6-1. Torque and work data were extracted from force-position curves. Peak-torque, total work, and fatigue index measured in the three preflight test sessions were compared; when no differences were found between sessions, values from the three preflight sessions were averaged and this average was used to compare preflight values with those on landing day and during the postflight period. Skeletal-muscle strength was defined as the peak torque generated throughout a range of motion from three consecutive voluntary contractions for flexion and extension. Eccentric contractions are actions of the muscle in which force is generated while the muscle is lengthening, as opposed to the concentric actions in which the muscle is shortening (contracting) while generating force. Skeletal-muscle endurance was defined as the total work generated during 25 repetitions of concentric knee exercise, as determined from the area under the torque curve for a complete exercise set. Work also was compared between the first 8 and last 8 repetitions. Endurance parameters were measured during concentric knee flexion and extension activity only. On R+0, significant decreases in concentric and eccentric strength were shown in the back and abdomen when compared to the preflight means (table 6-1).
Concentric back extension and eccentric dorsiflexion remained significantly less than preflight values on R+7. Recovery (an increase in peak torque from R+0 to R+7) was demonstrated for the eccentric abdomen and the concentric and eccentric back extensors. However, the data depicted in table 6-1 may be somewhat misleading because in some cases there were tremendous differences in strength between crewmembers who exercised during flight versus those who did not. For example, some crewmembers who exercised during flight actually gained in isokinetically measured strength in the ankle extensor/flexor muscles (anterior versus posterior calf muscles, that is m. tibialis anterior versus the gastrocnemius/soleus complex) compared to crewmembers who did not exercise and who actually showed a decrease in isokinetically measured strength in these muscles (figure 6-6).
With respect to endurance, a majority of the decrease in the total quadriceps work occurred on R+0. This likely reflects significant loss in the first third of the exercise bout (−11%). The declines in peak torque at the faster endurance test velocities are consistent with changes seen at the slower angular velocity used during the strength tests. Torque for the quadriceps at 75° per second was 15% less than preflight values but for the hamstrings was 12% less than the preflight mean at 60° per second. Endurance data showed little difference between preflight and R+7 tests, suggesting that crewmembers had returned to baseline by 1 week after landing. Additionally, subjects who did exercise during flight compared to those who did not had significantly greater (p < 0.05) losses within 5 hours of landing in concentric strength of the back, concentric and eccentric strength of the quadriceps (30° per second), and eccentric strength of the hamstrings, relative to the respective preflight values (data not shown here). According to Greenisen et al., non-exercisers also had significantly less concentric strength of the quadriceps at 75° per second and lower total work extension, work first-third flexion, and work last-third extension, immediately after landing, than before flight. The conclusions reached by the investigators were that the data indicate that muscles are less able to maintain endurance and resist fatigue after spaceflight, and that exercise may avert decrements in these aspects of endurance. Conversely, crewmembers who exercised during flight had greater losses in trunk muscles strength as measured at landing than did the non-exercising group (figure 6-7). However, preflight strength in trunk flexion and extension was substantially greater in the exercising group than in the non-exercising group. Apparently treadmill exercise did not prevent decrements in trunk strength after 9 to 11 days of spaceflight, and the investigators proffered the explanation that preservation of muscle function may be limited only to those muscles that are effectively used as part of the exercise regimen.
The specific aim of DSO 475, "Direct Assessment of Muscle Atrophy Before and After Short Spaceflight" was to define the morphologic and biochemical effects of spaceflight on skeletal fibers. To obtain myofiber biomechanical and morphological data from Space Shuttle crewmembers, biopsies were conducted once before flight (L – > 21 days) and again on landing day (R+0). The subjects were eight crewmembers, three from a 5-day mission and five from an 11-day mission. Biopsies of the mid-portion of the m. vastus lateralis were obtained by means of a 6-mm biopsy needle with suction assist. A one-tailed paired t-test was used to identify significant differences (p < 0.05) between the mean values of fiber cross-sectional area (CSA), fiber distribution, and number of capillaries of all crewmembers before flight and the mean values for all crewmembers after flight. According to this report, CSA of slow-twitch (Type I) fibers in postflight biopsies was 15% less than in preflight biopsies; the CSA of fast-twitch (Type II) fibers was 22% less after flight than before (figure 6-8). Mean values did not reflect the considerable variation seen in the biopsies from the eight astronauts who participated. At least some of this variation likely resulted from differences in the types and quantities of preflight and in-flight countermeasures (exercise or lower body negative pressure) used by the different crewmembers. The relative proportions of Type I and Type II fibers were different before and after the 11 day mission: the fiber distribution also seemed to follow the same trend after the 5 day mission (more Type II and fewer Type I fibers after than before), but the sample size was too small to reach statistical significance. The number of capillaries per fiber was significantly reduced after 11 days of spaceflight.
However, since the mean fiber size was also reduced, the number of capillaries per unit of CSA of skeletal muscle tissue remained the same. Atrophy of both major myofiber types, with atrophy of Type II > Type I, is somewhat different from the more selective Type I myofiber atrophy observed in unloaded Sprague-Dawley and Wistar rat muscle representing an uncommon case in which difference exist between responses of human and murine skeletal muscle. The purpose of DSO 606, "Quantifying Skeletal Muscle SIze by Magnetic Resonance Imaging (MRI)", was to non-invasively quantify changes in size, water, and lipid composition in antigravity (leg) muscles after spaceflight. This experiment was the first attempt to measure limb volumes before and after flight since the less sophisticated methods of measuring limb girths during Apollo and SKylab programs were used. The subjects included a total of eight Space Shuttle crewmembers, five from a 7-day flight and three from a 9-day flight. All subjects completed one preflight and two postflight tests on either L-30 or L-16 and on R+2 and R+7. Testing involved obtaining an MRI scan of the leg (soleus and gastrocnemius) at The University of Texas – Houston Health Science Center, Hermann Hospital. Multi-slice axial images of the leg were obtained to identify and locate various muscle groups. Changes in water and lipid content were measured, in addition to CSA, to distinguish changes in fluid versus tissue volumes. Multiple slices were measured by computerized planimetry. CSA and volume of the total leg compartment, soleus, and gastrocnemius were evaluated to assess the degree of skeletal muscle atrophy. The volumes of all 3 compartments were significantly smaller (p < 0.05) after both the 7 and 9 day Shuttle flights than they were before flight. Volume decreased by 5.8% in the soleus, 4.0% in the gastrocnemius, and 4.3% in the total compartment. These losses were stated to represent the true level of skeletal muscle tissue atrophy and not changes associated with fluid shifts. No recovery was apparent by 7 days after landing (data not shown). This finding indicates that the losses were not due to fluid shifts, but the delay in recovery after these rather short flights is contrary to what was observed and documented during the Skylab program of flights much longer in duration, albeit by less sophisticated methods during Skylab. The Space Shuttle Program and, in particular, EDOMP has provided a great deal of knowledge about the effects of spaceflight on human physiology and specifically on alterations in skeletal muscle mass, strength, and function. Once again, losses of skeletal muscle mass, strength, and endurance were documented, in some cases in spite of exercise countermeasures. But some findings were encouraging, particularly indications that in-flight exercise does have a positive effect in countering losses in muscle strength at least in the legs (see table 6-1 and figure 6-6), as predicted from the results of the 84-day Skylab 4 mission when multiple modesof exercise were used including a unique "treadmill" device (see figure 6-4). This unusual treadmill provided loads of sufficient magnitude to the legs in a fashion approaching resistance exercise. However, the data provided by MRI volume studies indicate that not all crewmembers, despite utilization of various exercise countermeasures, escape the loss in muscle mass that has been documented during most of the history of U.S. human spaceflight since Project Mercury. This, additional research is needed to continue the development of countermeasures and equipment that will eventually provide a successful solution for all human space travelers.
Shuttle-Mir and NASA-Mir During the seven NASA-Mir flights, seven U.S. astronauts trained and flew jointly with 12 Russian cosmonauts over a total period of 977 days (the average stay was 140 days) of spaceflight, which occurred during the period from March 1995 to June 1998. The major contribution of the joint U.S./Russian effort on the Mir space station relevant to the current risk topic was the first use of MRI to investigate volume changes in the skeletal muscles of astronauts and cosmonauts exposed to long-duration spaceflight. This began with the first joint mission, Mir-18, and continued until the final Mir-25 mission. The data indicated that loss of muscle volume, particularly in the legs and back, was greater than with short-duration spaceflight but not as great as the data from short-duration flight might have predicted. A comparison between volume losses in the selected muscle groups in short-duration spaceflight on the Space Shuttle, long-duration (119 d) bed rest, and a (115 d) Shuttle-Mir mission demonstrates the relative time course of the losses (figure 6-9).
There is good correlation between long-duration bed rest and spaceflight of similar duration except that losses in the back muscles are much less with bed rest. This likely reflects use of these muscles during bed rest to adjust body position and to reduce the potential for vascular compression and tissue injury. During spaceflight the back muscles are apparently less used because they do not have to support the upright body against Earth gravity and are not used with great force to make positional adjustments of the body as they are during the recumbency of bed rest.
International Space Station (ISS) The International Space Station's (ISS) first crew (Expedition 1) arrived in October 2000; since then there have been 15 additional Increments. The data presented here were collected during the first 11 of the ISS Expeditions. The complexities and shortcomings of collecting scientific data from a laboratory orbiting more than 300 miles above the Earth and completing 18 orbits per day at a speed of more than 17,000 mph with discontinuous voice and data communications, combined with the constraints and limitations of up mass, crew time, and on-board logistics, cannot be overstated.
Another problem was exercise hardware that was built and launched but failed to meet science requirements. (The Resistive Exercise Device [RED] science requirement was to provide a load of up to an equivalent of 600 lbs., but the Interim Resistive Exercise Device (iRED) provides only half of that amount. Ground-based studies have shown that it does produce a positive training effect similar to equivalent free weights when used in a high-intensity program, but it will likely not provide sufficient load in a zero-gravity environment to prevent loss of muscle and bone tissue, as determined from parabolic flight studies.) Other problems were failure at one time or another of each piece of onboard exercise hardware with reduced utilization at other times, and other limitations imposed because transmission of forces to the space frame have confounded inflight exercise sessions. In fact, during the first eleven ISS Expeditions, only for 2 short periods during Expeditions 3 and 4 were all three U.S. onboard exercise devices (Cycle Egometer with Vibration Isolation System [CEVIS], Treadmill with Vibration Isolation System, and iRED) capable of being used under nominal conditions (Figure 6-10). The almost continuously suboptimal availability of exercise equipment likely has reduced maintenance of crew physical fitness.
Despite these shortcomings, lean tissue mass data collected by means of dual-energy x-ray absorptiometry (DEXA) before and after flight compares favorably with data from NASAMir, and the total body and leg losses are in fact less than seen during NASA-Mir or during three separate bed rest studies of similar durations in the range of 20–170 d (Figure 6-11). However, the news is not entirely good since knee extensor and knee flexor strength losses in long-duration crewmembers after flights aboard Mir and ISS were ~23% and ~25%, respectively (Figure 6-12), indicating that strength losses in the quadriceps and hamstring muscle groups were significant and similar for NASA-Mir and early ISS missions, despite apparent slightly increased preservation of muscle mass (lean tissue) in the legs of ISS crewmembers compared to crewmembers on NASA-Mir missions (also Figure 6-11). These near equivalent losses occurred in spite of iRED being present on the ISS. Unfortunately, MRI data collected by Fitts and colleagues to assess skeletal muscle volumes in ISS crewmembers are not yet available to allow comparison with those from NASA-Mir. With respect to endurance, the following comparison (Figure 6-13) shows a trend for improved maintenance of muscle endurance on ISS with respect to NASA-Mir although the loss of endurance on ISS was greater than that documented during short-duration Space Shuttle missions (for ISS, n = 2).
ISS crewmembers, under