April 2016, No.7

Special Edition on Long Duration Spaceflight

STEM TODAY April 2016, No.7

STEM TODAY April 2016 , No. 7

CONTENTS Muscle physiology Skeletal Muscle Structure and Function Human Spaceflight Evidence Muscle Volume, Strength, Endurance, and Exercise Loads During 6Month Missions in Space Human skeletal muscle after 6 months aboard the International Space Station MicrogravityInduced Fiber Type Shift in Human Skeletal Muscle Effects of Sex and Gender on Adaptation to Space: Musculoskeletal Health Effects of prolonged space flight on human skeletal muscle enzyme and substrate profiles Simulated Microgravity Contributes to Autophagy Induction by Regulating AMP Activated Protein Kinase

Editorial Editor: Mr. Abhishek Kumar Sinha Advisor: Mr. Martin Cabaniss

STEM Today, April 2016, No.7

Cover Page Daybreak at Gale Crater This computer-generated images depicts part of Mars at the boundary between darkness and daylight, with an area including Gale Crater, beginning to catch morning light. Northward is to the left. Gale is the crater with a mound inside it near the center of the image. NASA selected Gale Crater as the landing site for Curiosity, the Mars Science Laboratory. The mission’s rover will be placed on the ground in a northern portion of Gale crater in August 2012. Gale Crater is 96 miles (154 kilometers) in diameter and holds a layered mountain rising about 3 miles (5 kilometers) above the crater floor. The intended landing site is at 4.5 degrees south latitude, 137.4 degrees east longitude. This view was created using three-dimensional information from the Mars Orbiter Laser Altimeter, which flew on NASA’s Mars Global Surveyor orbiter. The vertical dimension is not exaggerated. Color information is based on general Mars color characteristics Image Credit: NASA/JPL-Caltech Background Stormy Seas in Sagittarius Some of the most breathtaking views in the Universe are created by nebulae - hot, glowing clouds of gas. This new NASA/ESA Hubble Space Telescope image shows the center of the Lagoon Nebula, an object with a deceptively tranquil name, in the constellation of Sagittarius. The region is filled with intense winds from hot stars, churning funnels of gas, and energetic star formation, all embedded within an intricate haze of gas and pitch-dark dust. Image Credit: NASA, ESA, J. Trauger (Jet Propulson Laboratory) Back Cover New Gravity Map Gives Best View Yet Inside Mars A map of Martian gravity looking down on the North Pole (center). White and red are areas of higher gravity; blue indicates areas of lower gravity. The map was derived using Doppler and range tracking data collected by NASA’s Deep Space Network from three NASA spacecraft in orbit around Mars: Mars Global Surveyor (MGS), Mars Odyssey (ODY), and the Mars Reconnaissance Orbiter (MRO). Like all planets, Mars is lumpy, which causes the gravitational pull felt by spacecraft in orbit around it to change. For example, the pull will be a bit stronger over a mountain, and slightly weaker over a canyon. Slight differences in Mars’ gravity changed the trajectory of the NASA spacecraft orbiting the planet, which altered the signal being sent from the spacecraft to the Deep Space Network. These small fluctuations in the orbital data were used to build a map of the Martian gravity field. Image Credit: MIT/UMBC-CRESST/GSFC

STEM Today , April 2016

Special Edition on Long Duration Spaceflight Muscular System

After a few days of exposure to microgravity, muscle atrophy begins and the urinary excretion of nitrogen compounds increases. This atrophy is characterized by structural and functional alterations in the muscle tissue. There is a decrease in muscle fiber size, with no apparent change in fiber number. Atrophy is considerably greater for postural muscles, i.e., those muscles that support activities such as walking, lifting objects, and standing on Earth, as compared to the non-postural muscles, which undergo only marginal changes. Astronauts lose 10-20% of their muscle mass on short missions. On long-duration flights, the muscle mass loss might rise to 50% in the absence of countermeasures. The visible reduction in the leg circumference has been used as an indicator of muscle atrophy. However, this reduction is also influenced by the shift of fluids from the lower to the upper body in microgravity. The muscle loss is presumably caused by changes in the muscle metabolism, i.e., the process of building and breaking down muscle proteins. Experiments performed during long-duration missions on board Mir have revealed a decrease of about 15% in the rate of protein synthesis in humans.


In addition to pure muscle loss, the fibers involved in muscle contractions change their contractile properties and are weakened. Significant decreases in strength of the trunk, knee and shoulder muscles have been found in as few as 6 days in microgravity. Extensor muscles are more affected than flexor muscles. Animal studies also revealed that muscle fiber regeneration is less successful in space. The associated continued excretion of nitrogen may also have deleterious hormonal and nutritional effects. Spaceflight also results in increased susceptibility of skeletal muscle to contraction damage, which occurs in muscular atrophies on Earth-bound patients. These effects may compromise the ability of astronauts to perform some of their activities in orbit. Likewise, they may not be able to withstand the stress of 1-g upon return to Earth. In fact, the muscle weakness, fatigue, faulty coordination, and delayed-onset muscle soreness that astronauts experience after spaceflight mimics the changes seen in bed rest patients and the elderly. Finally, it is important to bear in mind that muscle atrophy caused by weightlessness also participates in the postural instability and locomotion difficulties seen after spaceflight. Muscle physiology There are several types of muscle tissue in the human body. The muscles that are the most affected by spaceflight are the skeletal muscles, which are those directly attached to the skeleton. Skeletal muscles are the largest tissues in the body, accounting for 40-45% of the total body weight. These muscles are attached to the bones by tendons. Their contraction allows for the movement of joints in everyday activities, like walking, lifting objects and standing. The anti-gravity muscles, also known as postural muscles, owe their importance and strength to the presence of gravity. Skeletal Muscle Structure and Function Functional Anatomy of Normal Human Skeletal Muscle The human skeletal muscle system comprises about 220 specific muscles with various sizes, shapes, locations, and functions in the body. Some are relatively small (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 assistance. Muscle fiber crosssectional area (CSA), fiber distribution, and number of capillaries were determined for all crewmembers before and after flight.

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The CSAs of slow-twitch (Type I) fibers in post-flight biopsies were 17% and 11% lower than those in preflight biopsies for 11- and 5-day flyers, respectively. Similarly, the CSAs of fast-twitch (Type II) fibers were 21and 24% lower in post-flight compared with pre-flight biopsies for 11- and 5-day flyers. Due to the extremely small sample sizes, these numbers do not reflect significant differences but nevertheless provide evidence that space flight-induced muscle atrophy occurs at the cellular level. Interestingly, when samples were further analyzed for changes in Type II sub-types, significant CSA reductions were detected in Type IIA (-23%) and Type IIB (-36%) fibers from crewmembers involved in the 11-day mission. The relative proportions of the Type I and Type II fibers were different before and after the 11-day mission; the fiber distribution followed the same trend after the 5-day mission (increased Type II and decreased Type I fibers compared with pre-flight), but the sample size was too small to reach statistical significance. This shift is consistent with the observed reduction in the number of individual muscle fibers that expressed the Type I myosin heavy chain protein. While no specific enzymatic activities involved in energy metabolism were found to be significantly different in muscle biopsy samples from returning crewmembers, the glycolytic/oxidative enzyme ratio of α- glycerophosphate dehydrogenase/succinate dehydrogenase activity was found to be increased , suggesting a shift resulting in decreased oxidative and increased glycolytic capacity in muscle fibers. The implication of such a shift is the potential of reduced fatigue resistance of the muscle during work. The number of capillaries per fiber was significantly reduced after 11 days of spaceflight. However, because 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 differences 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 the Apollo and Skylab programs were used. The subjects included four Space Shuttle crewmembers from an 8-day mission. All subjects completed three pre-flight tests and two post-flight tests at R+1 and R+15/16. Testing involved obtaining a 1.5-Tesla MRI scan of the lower body. Multi-slice axial images of the leg were obtained to identify and locate various muscle groups. Muscle volumes for the calf, thigh, and lumbar regions were measured to assess the degree of skeletal muscle atrophy. Significant reductions were observed in the anterior calf muscles (-3.9%), the gastrocnemius/soleus muscles (-6.3%), hamstrings (-8.0%), and intrinsic back muscles (-10.3%). After two weeks of recovery, some residual atrophy still persisted. These whole muscle measures along with the cellular measurements clearly established that muscle atrophy begins rapidly in the unloaded environment of space and accounts, at least in part, for the observed losses in muscle strength. The EDOMP provided significant knowledge on 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, despite the use of exercise countermeasures in some cases. However, 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 1 and Figure 6), as predicted from the results of the 84-day Skylab 4 mission when multiple modes of exercise were used, including a unique "treadmill" device (see Figure 4). This unusual treadmill provided loads of sufficient magnitude to the legs in a manner 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. In addition to the EDOMP, the Life and Microgravity Spacelab (LMS) experiments represent another hallmark Space Shuttle Program initiative to better understand the physiological adaptations to spaceflight. LMS was conducted aboard STS-78 and involved four crewmember subjects who participated in each of the following muscle physiology studies during their 17-day mission. Studies of muscle function and physiology Muscle atrophy was assessed during LMS by MRI using procedures similar to those used for STS-47 . Post-flight muscle volumes were significantly reduced (7-12%) in back muscles, quadriceps, gastrocnemius, soleus, and gluteal muscles on landing day . By R+10, all changes in muscle volume had reverted to preflight levels. The observed reductions in gastrocnemius, soleus, and quadriceps muscles following the 17-day LMS mission were on average larger than those reported for the 8-day STS-47. The MRI results not only directly confirm that muscle atrophy is an early consequence of space flight, but they also suggest that muscle atrophy continues during longer exposures to microgravity. Whole muscle strength was measured in knee extensors and plantar flexors during LMS. The production of force by knee extensors was determined under isoinertial and isometric conditions. Pre-flight and post-flight measurements were obtained with an instrumented leg press device that uses inertial flywheels as the resistance mode. The device could also be locked in place at a 90-degree knee

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angle for the measurement of maximal isometric force. Consistent with the reported reduction in quadriceps CSA, knee extensor (leg press) strength was reduced post-flight (R+1). Maximal isometric force was reduced by 10.2%, whereas concentric and eccentric strength were reduced by 8.7% and 11.5%, respectively. In separate experiments involving the same astronaut subjects, calf muscle performance was assessed before, during, and after STS-78 with a torquevelocity dynamometer (TVD) . The TVD was a mission-specific piece of hardware that measured ankle plantar flexion and dorsiflexion strength under isometric or isokinetic (fixed angular velocity) conditions. Angle-specific tests for isometric strength (80, 90, 100 degrees), isokinetic strength at speeds from 30-360 degrees/seconds, and isokinetic endurance were performed before, during and postflight. In-flight tests were conducted on flight day (FD)2/3, FD8/9, and FD12/13. Postflight tests were performed on R+2 and R+8. Muscle strength values were reported to be ≈50% lower at the first two in-flight time points, but the charges were attributed to issues with the system that secured the TVD in place. The TVD was reported to be "lifting and floating" during testing. The issue was resolved prior to FD12/13 testing, at which time no differences in torque generation compared with pre-flight values were observed. Likewise, post-flight values were not significantly different than pre-flight values.

The authors of the investigation have suggested that the lack of change during 17 days of space flight may have been due to the nature in which the testing was conducted; that is, the in-flight testing may have served as an unexpected, yet effective, exercise countermeasure to protect the calf muscle from strength loss. The three inflight calf muscle test sessions during STS-78 involved making ≈ 525 calf muscle contractions on the TVD , half of which were made at 80% to 100% of each individual’s maximal values. In contrast, the same LMS crew displayed significant deficits in both size and strength of the quadriceps , a muscle group that was not tested during flight. This result suggests that high-intensity muscle contractions, which are performed less than daily, may protect muscle strength during missions of up to 17 days. Loss of skeletal muscle strength is a consequence not only of reduced muscle size, but also of decreased neural drive and myocellular damage. Studies were performed on the calf muscles (contralateral leg to that used in studies described above) before flight, during flight (four time points), and after flight to separate the causal effects of muscle atrophy from reduced neuromuscular recruitment to address this question. Surface electrodes were placed over the subjects’ gastrocnemius and soleus, and a percutaneous electrical muscle stimulator (PEMS) unit was used to directly cause forced whole-muscle contractions independent of any voluntary input provided by the crew member. No measureable losses in electrically evoked calf muscle performance were observed. However, post-flight (R+8) reductions in force production were observed. Given the lack of change during late in-flight testing (FD16), it was suggested that alterations are likely due to muscle damage due to gravitational reloading of the muscles during normal ambulation. This notion was supported by MRI analyses. MRI transverse relaxation time (T2) of skeletal muscle is an indicator of increased tissue fluid volume and can be a marker of myocellular damage (inflammation/edema). In these crewmembers, T2 values were elevated at R+2 and stayed elevated at R+10. Studies of muscle morphology and cellular function. Muscle biopsy samples were obtained from the 4 LMS crew members who par-

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Special Edition on Long Duration Spaceflight ticipated in the whole-muscle size and function testing . Biopsies were obtained from the gastrocnemius and soleus muscles before flight and again within three hours of landing. Functional analyses of single muscle fibers provide the most direct evidence of space flight-induced changes in the function of the muscle mechanics without the influence of factors such as changes in neuromuscular recruitment patterns or differences in volitional effort. Using calcium-activated individual muscles, any observed alterations in mechanics can be attributed to alterations in the myofiber itself. Individual muscle fibers from the LMS crew were isolated and mounted between a force transducer and a servomotor for analyses. Space flight produced a small decrease (-6%) in type I single-fiber peak calciumactivated force production (Po ) in samples from the gastrocnemius.


However, no difference was observed when these measurements were corrected for muscle fiber CSAs. No mean differences were found in Po or fiber CSA for fibers that either expressed type IIa myosin heavy chain (MHC) or co-expressed both type IIa and IIx MHC. While mean differences in fiber mechanics were not observed in subjects as a group, significant changes occurred within individual subjects when subject-by flight analyses were conducted (each subject had a cohort of fibers that were analyzed). In one subject, Po and CSA in Type IIa fibers were reduced by 19% and 12%, respectively. In another subject, Po was reduced by 23% in Type I fibers and 15% in Type IIa fibers, with reductions in fiber CSA of 7% for type I and 12% for type IIa. The investigators point out that the variability in space flight response seems to result, at least in part, from initial fiber size. Fibers with the greatest reduction in size and Po tended to come from the crew members who had larger pre-flight fibers. In the soleus muscle, a calf muscle adjacent to the gastrocnemius but one that is more slow and oxidative in nature, 91% of muscle fibers expressed only type I MHC before flight . After space flight, the number of Type I fibers decreased to 79%. Space flight also resulted in a 21% decrease in mean Po. This decline in Ca2 -activated peak force was paralleled by a 15% decrease in fiber CSA, which indicates that muscle atrophy accounted for most of the loss of function, although a 4% residual loss of Po remained when Po was normalized by individual fiber CSA. Skeletal muscle power is generally viewed as a functional measure of muscle performance because, like most physical tasks that require high levels of exertion, peak values actually occur at submaximal loads. The power of single fibers was measured in a manner similar to the Po measurements; however, instead of the measures being isometric, they were obtained with isotonic load clamps. No significant main effect of space flight was found on muscle power for single fibers from either the gastrocnemius or the soleus muscles. Despite some variability among crew members in the effect of space flight on Po in various muscle fiber types, the overall trend showed that increases in maximal shortening velocity (Vo ), which are attributed to decreased thin filament density based on observations from electron microscopy , compensate for the loss of Po to maintain muscle power at the cellular level. Skeletal muscle is a highly metabolic tissue. As is true for muscle size, the intensity and volume of physical activity are also major determinants of the readily adaptable bioenergetic capacity and composition of the muscle. Portions of the biopsy specimens from the gastrocnemius and soleus were used to perform biochemical analyses of oxidative and glycolytic enzymes. Despite some evidence of a metabolic shift toward glycolysisderived energy sources in biopsy samples after the 11-day STS-32 mission , no differences were detected in citrate synthase, phosporylase, or β-hydroxyacyl-CoA dehydrogenase in samples after the 17- day LMS mission . Accordingly, no post-flight changes were observed in muscle glycogen content. Therefore, while space-flight appears to promote a slow-to-fast shift in MHC, there does not appear to be a similar systemic metabolic shift. Shuttle-Mir and NASA-Mir Programs 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 that in short-duration spaceflight but not as great as the data from short-duration flight may 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 8).

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There is a good correlation between long-duration bed rest and spaceflight of similar duration except that losses in the back muscles are much lower with bed rest. This result likely reflects the 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’s 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 first ISS crew (Expedition 1) arrived in October 2000; since then, there have been 40 Increments. Two major research study complements addressing the Risk of Impaired Performance Due to Reduced Muscle Mass, Strength, and Endurance were conducted during the early phase of ISS exercise countermeasures evaluation. During these complements, subjects had access to the CEVIS cycle ergometer, the TVIS treadmill, and, importantly, the interim Resistive Exercise Device (iRED). iRED was an elastomer-based piece of resistance exercise hardware. This device was limited to a 300-pound maximum load. By comparison, the currently available ARED has a 600 pound load capacity. One investigation during the "iRED era" involved four ISS astronauts with mission durations of 161-194 days , and the other studied 10 astronauts and cosmonauts whose mission durations spanned a very similar 161-192 days in space . Each of these studies investigated changes in muscle size and strength, with one focusing on a larger array of muscle groups and the other performing a diverse set of whole muscle, cellular, and biochemical measures on the postural muscles of the calf.

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Special Edition on Long Duration Spaceflight Initial post-landing MRI data for both studies were obtained on a relatively similar timeline (5 ± 1 and 4 ± 1 days). Calf muscles were found to undergo greater decrements than thigh muscles (10-18% and 4-7% loss, respectively). Both studies reported the greatest loss in the soleus muscle (%) with less, but substantial, decrements in the gastrocnemius . Approximately half of the loss of muscle mass still existed up to two weeks following return to Earth .

Although these MRI results highlighted a clear need for improved countermeasures hardware and/or strategies, they also demonstrate an incremental improvement in the countermeasures targeted to mitigating muscle loss compared with the more dramatic reductions observed during Shuttle-Mir missions . Muscle strength measurements in ISS crew members were not measured until approximately one week following landing. Nonetheless, strength losses accompanied muscle atrophy in both upper and lower leg muscles. Isokinetic strength measures in thigh knee extensor muscles revealed a 10% loss , whereas calf muscle strength was reduced by 24% , again demonstrating that the calf muscles are most susceptible to spaceflight induced decrements.


The drop in torque production of the calf muscles was observable across the entire range of speeds used from 0-300 degrees/second . This reduction in calf muscle performance, initially measured one week post-landing, persisted until at least two weeks after return despite a partial restoration in muscle volume . Taken together, the results suggest that impairments in muscle strength are likely perturbed by muscle damage and/or soreness derived from gravitational reloading of the muscles. Various structural and functional analyses were performed on muscle biopsy samples from the gastrocnemius and soleus muscles from nine ISS crewmembers. Mirroring what was observed at the whole-muscle level, individual muscle fiber analyses also revealed muscle atrophy at the cellular level . Cross sectional areas were determined in individual muscle fibers that were set at a standardized sarcomere length. The number of slow type I muscle fibers was reduced by 24% and 33% in the gastrocnemius and soleus muscles, respectively. The number of fast type II fibers (of all subtypes, excluding hybrids) was also reduced in the soleus muscle (29%) but was unchanged in the gastrocnemius. Measures of muscle fiber mechanics clearly demonstrated decrements of function at the cellular level . Peak calcium activated force, maximal shortening velocity, and peak power were all markedly reduced in postflight samples taken from gastrocnemius and soleus muscles, with the most dramatic change being a 45% loss of power production in type I soleus muscles. This is in stark contrast to responses to short-term Space Shuttle flights, where increases in maximal shortening velocity were able to compensate for reduced force production to maintain peak power levels. Power was also reduced in type II fibers, with reductions to maximal shortening velocity and peak force being contributing factors for fibers from gastrocnemius and soleus muscles, respectively. In both gastrocnemius and soleus muscles, a clear shift in the contractile machinery was observed with a slowerto- faster phenotype reported . This can be observed from MHC protein expression in the individual fibers that were analyzed for contractile properties. Both gastrocnemius and soleus muscles exhibited reductions in the amount of fibers expressing type I MHC. This corresponded to increases in the percentages of type IIa fibers and type I/IIa hybrid fibers from gastrocnemius muscle. A similar pattern occurred in the soleus muscle, although increases were primarily observed in the various hybrid fibers distributed in a manner such that significant changes were only detected in hybrid fibers grouped together. Although limitations in the availability and accuracy of iRED loading data prevented investigators from making meaningful analyses of the relationships between resistance training loads and muscle adaptions during these ISS missions, a number of observations were made regarding treadmill running and alterations in the calf muscles . Treadmill use ranged from less than 50 minutes a week to greater than 300 minutes per week. Subjects who ran on the treadmill the most preserved muscle better than those who ran less. When total aerobic exercise (TVIS treadmill + CEVIS bicycle ergometer) was compared with changes in muscle volume, this correlation was lost. Data demonstrating that foot forces are much higher during treadmill running versus cycling aboard ISS support the argument that higher forces are vital to protecting against muscle atrophy during spaceflight.

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Special Edition on Long Duration Spaceflight Results for treadmill use were not restricted to in vivo whole-muscle observations. Subjects who used the TVIS treadmill more than 200 minutes per week generally fared better than those who ran less than 100 minutes per week in terms of single fiber CSA, peak force, and power .

In addition to muscle mass and the function of the cellular contractile proteins, changes to the molecular mechanisms that control energy metabolism also have the potential to negatively affect human performance following exposure to long-duration space flight. Activities of a battery of oxidative and glycolytic enzymes were therefore measured in crewmembers before and after ISS missions . Overall, the observed spaceflight effects on metabolic enzymes in skeletal muscle were minimal. No changes in activities of citrate synthase, β-hydroxyacyl- CoA, lactate dehydrogenase, or phosphofructokinase were observed in calf muscles following 6 months aboard ISS.


Rather, spaceflight and exercise countermeasures play a more limited role in select adaptions to metabolic enzymes in calf skeletal muscles. For example, the mitochondrial enzyme cytochrome oxidase was reduced in spaceflight by 35% in type I soleus muscle in all crewmembers studied. However, this result was entirely accounted for by the crewmembers in the low treadmill use group (less than 100 minutes/week), in which a 59% reduction occurred. Activity levels in the high treadmill use group were unchanged. In short, metabolic adaptations in skeletal muscle appear to be less sensitive to unloading compared with structural and functional changes related to morphology and contractility. Furthermore, countermeasure strategies that are insufficient to fully protect muscle from unloading-induced atrophy appear to be more effective in protecting against changes to the metabolic phenotype of the muscle.

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These two major studies point to the need for high load intensity if prevention of muscle mass and strength is to be accomplished. In these early years, both hardware capabilities and reliability certainly contributed to this condition not being met. The iRED science requirement was to provide a load of up to an equivalent of 600 lb (273 kg); however, as mentioned above, the delivered hardware product provided only approximately half of that amount. Ground-based studies have shown that it does produce a positive training effect similar to that of equivalent free weights when used in a highintensity program , but it will likely not provide sufficient loading in a zero-gravity environment to prevent loss of muscle and bone tissue, as determined from parabolic flight studies . For whole-body resistance exercises, such as squats, one’s own body weight contributes a significant amount of load in a 1-G environment.

In the weightlessness of space, this contribution is lost. For this reason, load capacities for resistance exercise devices for use in space must be able to replace the body loads that are lost in the microgravity environment on top of the normal loads that one would use on the ground. Other problems in meeting load requirements were related to failures of the onboard exercise hardware with reduced utilization at other times, as well as use restrictions imposed due to transmission of forces into the structure of the space station itself. In fact, during the first eleven ISS Expeditions, there were only two short periods during Expeditions 3 and 4 when all three U.S. onboard exercise devices (CEVIS, TVIS, and iRED) were capable of being used under nominal conditions (Figure 9). The almost continuously suboptimal availability of exercise equipment likely has had a negative impact on maintenance of crew physical fitness during this time. Since the time depicted in Figure 9, both the reliability and capability of the ISS exercise countermeasures hardware have continued to mature. The second-generation treadmill (T2) and the Advanced Resistive Exercise Device (ARED, Figure 10) were delivered to ISS in 2009 and 2010, respectively. The T2 allows for motor-driven running speeds up to 15 mph in addition to being able to be used in a passive resistance mode (the user rather than a motor drives the belt against resistance). ARED provides adjustable loads of 600 pounds provided by vacuum canisters that provide a constant force and inertial flywheels that simulate the inertial loads that would be experienced using free weights in 1-G. ARED allows for most multi-joint bar-based resistance exercises to be performed, including the squat, deadlift, heel raise, and bench press. Additionally, ARED can support cable pull exercises with loads up to 150 pounds. ARED was delivered to the ISS with expectations of improving muscle outcome measures due to the additional load capacity and the changes in exercise prescription that this improvement affords. During ≈6-month ISS missions, iRED crewmembers lost 0.42±0.39 kg of total body lean mass while ARED users gained 0.77±0.30 kg, as determined by whole-body DXA scans (Table 2); a limitation of these measurements is that the mean post-landing time required to obtain these measurements was 13± 2 d and 8±1 d for iRED and ARED crewmembers, respectively. Regardless, a clear trend exists for the improved protection of muscle mass in more recent missions; this is likely due to a combination of enhanced resistance exercise loading (ARED) and improved caloric intake during flight , two key factors in skeletal muscle outcomes during unloading. ARED users lost more fat mass during flight, but because of an increase in muscle mass, ARED crewmembers had a smaller net decrease in total body mass compared with iRED crewmembers. All United States Operating Systems (USOS; NASA, Japan Aerospace Exploration Agency, European Space Agency, and Canadian Space Agency) crewmembers undergo specific medical requirement testing before and after their ISS missions. Part of this testing includes isokinetic muscle strength and endurance testing of the legs and trunk muscles. Post-flight testing occurs 5- 7 days after landing. In Figure 11, we present results as the

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percent change from pre-flight for isokinetic strength and endurance testing for crewmembers.

Results are divided into two groups: those who used iRED and those who used ARED during their flight. Isokinetic strength around the knee joint was measured at 60◦ /s. For iRED crewmembers, the mean decrements in knee extensor and knee flexor strength were -13.7% and -19.5%, respectively. ARED users still exhibited losses of knee extensor and flexor strength, but the values were improved at -6.9 and -11.1, respectively. Muscle endurance was measured in knee extensor and flexor muscles based on total work production during a 20 repetition effort at 180◦ /s. The average loss for iRED crewmembers was -10.7% in knee extensors and -8.9% in knee flexors. Mean values for loss of endurance in ARED users were lower than that in iRED users (-7.5% for both knee extension and flexion), but any improvements were more subtle than those for strength. Studies in bed rest analogs for long-duration spaceflight deconditioning have typically shown that calf muscle mass and strength are more difficult to protect than quadriceps muscle mass and strength . Here, authors show that calf muscle strength in ISS crewmembers using iRED was reduced 14.2% compared with pre-flight values. While this value is on par with knee extensor results (-13.7%), the improvement in ARED users’ calf muscle strength loss is more modest (-11.6% versus preflight) than that of ARED users for the knee extensors. Trunk extensor strength losses equaled -7.4% and -5.5% for iRED and ARED, respectively.

The current permissible outcome limit for muscle strength in returning crewmembers is at or above 80% of baseline values (NASA Space Flight Human System Standard Volume 1: Crew Health; NASA-STD-3001). Although ARED-era crewmembers have fared better than iRED-era crewmembers, on average, both groups have losses

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Special Edition on Long Duration Spaceflight of less than the 20% standard. However, an examination of the individual data shows that many individuals have lost more than the targeted 20% threshold. It is also important to keep in mind that the medical requirement testing is conducted approximately one week after landing and therefore may not reflect a crewmember’s performance ability in the immediate post-landing time frame. While it would be ideal for all crewmembers to actually return with no loss of strength at all, it is important to note that this would not necessarily be reflective of crew ability to complete mission objectives. The Human Research Program aims to develop more performance-based strength standards that can better be used as benchmarks for mission success. Doing so will not only aid in designing better exercise countermeasure strategies, but will ultimately lead to greater assurance of crewmember safety.


While the delivery of ARED to the ISS already appears to be eliciting better strength maintenance, Human Research Program-funded research is beginning to examine how to better utilize ARED to not only improve strength and muscle mass outcomes, but also how to do so with a reduced weekly training volume. The Integrated Resistance and Aerobic Training Study (also known as SPRINT) is implementing a resistance training protocol based on greater intensity and reduced volume. Subjects are performing high load resistance training three days versus six days per week. In addition to pre- and post-flight muscle performance measures such as muscle mass, strength, power, and neuromuscular recruitment, in-flight measures of muscle mass will be tracked for the first time ever using ultrasound technology. A recent investigation examined the effects of iRED versus ARED use onboard the ISS on body composition. Eight iRED users and five ARED users were subjected to DXA analysis prior to flight and again anywhere from 5 to 45 days post-flight (mean 12-11 days post-flight). Total body mass was unchanged in both groups; however, lean body mass was increased in ARED users and fat mass was reduced. These data are consistent with the view that ARED use is better for musculoskeletal outcomes following ISS missions; however, the effect of space flight on postural skeletal muscles following space flight is difficult to assess via wholebody lean mass, as the target tissues do not likely represent a large enough portion of the total lean mass pool to detect changes with sufficient accuracy. Authors attempted to determine whether the changes in muscle strength shown in Figure 11 correlated with pre- to post-flight changes in lean body mass and found that changes in strength correlated poorly with changes in total body lean mass. This result may be due to the aforementioned delays in obtaining these measurements post-flight or to the generic nature of total body lean mass changes as opposed to the greater specificity of leg lean mass or, optimally, regional changes in the quadriceps and calf muscle. It appears that MRI and potentially ultrasound imaging technologies are required to adequately detect morphological changes associated with loss of muscle strength. Functional fitness test results for long-duration ISS crewmembers are presented in Table 3. Generally, iRED crewmembers experienced small to moderate decreases in performance of practical exercise tests such as pushups, pullups, bench press, and leg press. For all but one measure, ARED crewmembers fared better than their iRED counterparts, with most outcomes actually showing improvements after spaceflight.

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Nutritional regulation of protein metabolism as it pertains to maintenance of muscle mass is a growing research topic with implications for aging populations and those undergoing unloading such as the ISS crew. Numerous investigations have addressed the roles of protein and amino acid intake in bed rest analogs for long-duration spaceflight, whereas spaceflight data are much more limited. Aboard the ISS, protein intake has well-exceeded the U.S. Recommended Dietary Allowance (0.8 g/kg/d) both in the past (1.1 g/kg/d) and more recently (1.4 g/kg/d) . Total caloric intake has historically been a problem; Stein et al. reported significant decreases in body mass and protein synthesis after long-duration spaceflight on Mir. The reduction in protein synthesis was positively correlated with a decrease in energy intake during flight (r2 =0.86). These findings demonstrate the synergistic, deleterious effect of reduced energy intake on skeletal muscle metabolism and mass during mechanical unloading.


Muscle Volume, Strength, Endurance, and Exercise Loads During 6-Month Missions in Space In this study , authors report changes in the volume of skeletal muscle and strength in four crewmembers following missions of 161 - 194 days to the ISS . Authors also provide information on typical loads from ISS exercise countermeasures, including iRED, the Treadmill with Vibration Isolation System (TVIS), and the Cycle Ergometer with Vibration Isolation System (CEVIS), that these crewmembers completed together with data from exercise logs and daily monitoring so that effects of a given " dose " of the exercise countermeasure on the musculoskeletal system can be evaluated. Four healthy male astronauts (49.5 ± 4.7 yr, 179.3 ± 7.1 cm, 85.2 ± 10.4 kg) volunteered to participate in this study and completed long-duration missions aboard ISS (181 ± 15 d). The study protocol was approved by the Committee for the Protection of Human Subjects at NASA’s Johnson Space Center, Houston, TX, and by the Institutional Review Boards at the Pennsylvania State University, State College, PA, and the Cleveland Clinic, Cleveland, OH.

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Results Muscle Volume The largest volume losses occurred in the plantar flexors (soleus: -19 ± 7% and medial gastrocnemius: -10 ± 5%) and ankle dorsiflexors (anterior calf: -10 ± 3%), while smaller decrements were observed in the muscle groups of the thigh (knee extensors: -6 ± 3%, knee flexors: 7 ± 4%, and adductors: -4 ± 3%, Fig. 2 , Table I ). There was little or no loss (0 - 1%) in the muscles of the upper extremities for the two crewmembers for whom data was available. Results from the pre- and postflight STIR images revealed the absence of edema in the three subjects for whom this sequence was available.

Muscle Strength Fig. 3 (mean ± SD) and Table I (individual crewmembers) show the postflight changes in the peak isokinetic and isometric strength from the five-repetition protocol. Isokinetic strength decreased in both the knee flexor ( -24 ± 8%) and knee extensor (-10% ± 11%) groups. Ankle plantarflexor strength decreased almost three times more ( - 22% ± 6%) than dorsiflexor strength ( 8% ± 16%). The mean hip extensor isokinetic torque showed a slight mean gain (2% ± 16%), while mean strength decreased in the hip flexors (-8% ± 17%). This large variation includes one subject who showed a gain in strength postflight. Isometric strength data showed considerably greater loss in the plantar flexors ( - 20% ± 16%) compared to dorsiflexors ( -4% ± 22%), but variability in the latter test was large. Both knee extensors and flexors lost isometric strength ( - 15% ± 13% and - 20% ± 17%, respectively). The hip extensors and flexors isometric strength decreased ( -15% ± 26% and - 28% ± 9%, respectively). Isometric elbow extensor and flexor strength measured in two subjects exhibited losses ( -11% ± 4% and -8% ± 13%, respectively), as did isokinetic strength ( - 8% ± 1% and -17% ± 3%).

Endurance Total work performed during the knee endurance test decreased pre- to postflight ( - 14 ± 4%). There was also a loss in total knee flexion work postflight ( - 14 ± 8%). Fig. 4 shows the peak torque and work done during each effort in the endurance test where the data points represent the percentage change from the eighth repetition. The initial postflight decrements in both peak torque and work performed are apparent in all tests, but there is a lower postflight rate of fatigue in both muscle groups (knee extensor work preflight vs. postflight decline: 1.55% vs. - 1.14% per contraction and knee flexor work preflight vs. postflight: - 1.0% vs. - 0.62% per contraction). The rate of fatigue (slopes of the linear fit) of the torque data shows similar trends (knee extensor peak torque decline: - 1.68% vs. - 1.16% postflight per contraction and knee flexors peak torque decline: - 1.04% vs. - 0.64% postflight per contraction).

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Foot Forces and Logs from Exercise Activities Fig. 5 shows typical in-shoe forces from one foot during iRED exercise recorded at the maximum resistance setting used during the days when measurements were made. Peak single leg in-shoe forces were approximately 0.65 x bodyweight (BW), 0.55 x BW, 0.37 x BW, 1.30 x BW, and 0.92 x BW for heel raises, squats, dead lifts, single leg heel raises, and single leg squats, respectively. The peak in-shoe forces during running and cycling on ISS were approximately 1.28 x BW and 0.10 x BW ( Fig. 6 ). No preflight data using iRED were available. Although authors only collected between 4-8 days of foot force data, the exercise logs provided information on exercise patterns for the entire mission. Fig. 1 indicates that these crewmembers were highly reliable in carrying out their specified exercise prescriptions throughout the mission, but that they emphasized different preferred exercises. For example, Subject A accumulated more than twice as many CEVIS sessions as subject D.

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Subjects D and B accumulated 110 and 121 TVIS sessions, respectively (approximately one every other day), while Subjects A and C only used TVIS for 39 and 68 days , respectively. Most notably, Subject B performed almost three times more iRED exercise than other crewmembers.

The mean changes in isometric strength ranged from about - 2% for the hip abductors to about - 35% for the hip adductors. Isokinetic concentric changes ranged from + 2% (an increase) in hip extensor strength to 24% (loss) in the knee flexors. There was about a 2.3 times greater loss in postflight peak isokinetic torque in the knee flexors compared to the knee extensors.

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All subjects engaged in regular prescribed exercise and it is notable that losses in muscle volume and strength were in all cases less than the values previously reported for 16 - 28-week Mir missions. The decrements in endurance expressed per week of spaceflight were also less than those seen during the longer Mir missions of 129 - 145 day. This suggests that the "dose" of countermeasures in these four ISS crewmembers was higher as a result of more exercise repetitions, higher intensity, and/or greater adherence to exercise protocols. The protocols performed varied from one subject to another, depending on such factors as availability of the device, scheduling constraints due to mission-related activities, and individual preferences. Data from exercise logs indicate that approximately 55%, 46%, and 85% of exercise sessions included the use of the TVIS, the CEVIS, and the iRED, respectively, over the course of a mission. The ankle dorsiflexor group is worthy of special discussion since the foot is often placed in a foot loop during work on the ISS. Under such circumstances, the ankle dorsiflexors are used to align and move the body into a new position against the resistance of inertial forces. While the volume losses in the ankle dorsiflexors were close to the mean loss of all lower extremity muscles, the isometric and isokinetic strength losses were well below the mean of all muscles tested (particularly the isometric strength loss of - 4%), suggesting that foot loop use could have provided an additional form of "resistance training." Variability was, however, high in this muscle as in all the strength results and this limits the conclusions that can be drawn. Despite these regular exercise sessions, the "dose" of exercise was probably insufficient to preserve the musculoskeletal system, as pointed out by both Trappe et al. and Lee et al. The measured loads from all exercise countermeasures were considerably less than the loads measured from similar exercises on Earth. For example, during resistance exercise on the ISS the maximum single leg load during a squat was 0.6 x BW. This contrasts with similar exercise on Earth, where a load of 1.4 x BW on the shoulders adds to the weight of the body to generate a quasi-static force under 1 foot of (1.4 + 1.0)/2 = 1.2 x BW. The maximum peak in-shoe forces from a single-leg squat (0.92 x BW) is just slightly less than that while performing the same exercise on Earth with no added load. The measured foot loads do not agree with the "nominal" load settings for the exercises as specified in the exercise prescription, which apparently overestimated the load capacity of the iRED. Similarly, the in-shoe forces during walking and running were approximately 25% less than walking on Earth (0.89 vs. 1.18 BW) and 46% less than typical running (1.28 vs. 2.36 BW) as measured from the same subjects using the same instrumentation on Earth. The forces during cycling were extremely small - 0.1 x BW and 50% less than what was experienced on Earth. The reason for the different responses seen in walking and running is likely due to a combination of speed and externally applied load. It should be noted that a speed limitation of 6 mph was placed on the treadmill during most of the time period studied, but it is likely that all crewmembers ran faster than 6 mph (10-min miles) during their exercise on Earth, which was the basis for comparison. Also, the load applied to the harness worn by the crewmembers during exercise on the TVIS was typically generated by either a combination of bungee

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Special Edition on Long Duration Spaceflight cords and clips or a SLD. These observations provide some additional context regarding magnitude of muscular force during countermeasure exercise to the information on the number of contractions and duration of muscle activity reported by Trappe et al.. Human skeletal muscle after 6 months aboard the International Space Station The aim of the study was to document the exercise program used by crewmembers aboard the ISS and examine its effectiveness for preserving skeletal muscle size and function. The focus was on the calf muscles, since they have been shown to atrophy more than other leg and upper body muscles with unloading.


Ten crewmembers participated in this study . For the analysis presented, one crewmember had incomplete data sets and was not included. The subject population consisted of American astronauts and Russian cosmonauts. The subjects’ ( n=9 ) age, height, weight, and days in space were 45 ± 2 yr, 176±2 cm, 81 ± 3 kg, and 177 ± 4 days (range = 161-192 days), respectively. An overview of each crewmember’s exercise history in the weeks preceding their launch is shown in Table 1. Before volunteering to participate in this skeletal muscle research, all crewmembers were briefed on the project objectives and testing procedures by a member of the investigative team. Crewmembers were informed of the risks and benefits with the research and gave their written consent to protocols approved by the Human Subjects Institutional Review Boards at Ball State University, Marquette University, and the National Aeronautics and Space Administration (NASA; Johnson Space Center). This study was conducted in accordance with the Declaration of Helsinki.

Exercise in space During the 6 month, the crewmembers were on the ISS, they had access to a treadmill (treadmill with vibration isolation system), two bicycle ergometers (cycle ergometer with vibration isolation system and a Velosiped, i.e., Russian bicycle exercise device), and an interim resistive exercise device (iRED). The crewmembers also had access to bungee cords, which they could use to provide resistance-type exercise for various muscle groups. The treadmill device could be used in a passive (subject driven) or active (motorized) mode of operation, which was selected by the crewmember during each exercise session. Crewmembers used a subject-loading device to fix themselves to the treadmill, which provided varying levels of loading relative to body weight (typical load was ≈70% of body weight) during use. In this way, the crewmembers could complete running or walking exercise while partially loaded. The bicycle ergometers provided typical loading in 1-W increments up to 350 W and had clipless pedals for securing their feet. The iRED is an elastomer-based resistance exercise device consisting of two canisters capable of producing up to ≈ 68 kg of force per canister. Additional bungee cords can also be attached to increase the load characteristics. A known limitation of the iRED is the inability to precisely set and quantify workloads. The operational guidelines prescribed that crewmembers exercise while in space with up to 2.5 h allocated per day for 6 of 7 days of the week. The 2.5-h period included time needed for hardware setup, stowage, and personal hygiene. The exercise prescription was not fixed or targeted to a specific level of performance for a given physiological system. The exercise program was structured to allow for personal preference from the crewmembers along with guidance from trainers and staff within NASA and the Russian Space Agency. To track the exercise profile while in space, crewmembers kept logbooks of their physical activity. In addition, analog data from the devices (treadmill and cycle ergometer) were downloaded (when the downlink was

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Special Edition on Long Duration Spaceflight operational) at various times while on-orbit and accounted for ≈ 65% of the treadmill and cycle ergometer data. Members of our investigative team personally interviewed each crewmember after their mission. The combination of these three elements (logbook, downloaded data, and personal interviews) comprised the database that enabled us to profile the exercise program conducted by each crewmember while in space.


Results Exercise training profile A summary of the aerobic exercise performed while in space is shown in Table 2. There was a wide range of aerobic training performed by the crewmembers. Aerobic exercise (cycle + treadmill) approached ≈ 5 h/wk or ≈ 50 min/day. On average, subjects completed 138 ± 26 min/wk of cycle exercise that generally ranged between 100 and 150 W. While in orbit, crewmembers used the cycle ergometer ≈60% of mission days. Four of the crewmembers (subjects A, B, C, and G) used the cycle ergometer ≈ 81% (range=70 - 90%) or more of mission days, whereas four others (subjects D, F, H, and I) averaged ≈ 37% (range= 28 - 41%) of mission days, and one crewmember (subject E) did minimal cycling. For the treadmill exercise, subjects averaged 146 ± 34 min/wk on a level grade at a speed ranging from 2.1 to 5.5 miles/h. The treadmill appeared to be used less frequently compared with the cycle ergometer, accounting for 200 min/wk) of walking/running activities. The other five crewmembers (subjects A, C, D, F, and I) used the treadmill much less (≤85 min/wk).

A summary of the resistance exercise performed while in space is shown in Table 3. Generally, all crewmembers performed a suite of leg exercise routines consisting of squats, heel raises, and dead lifts. The program varied for each crewmember, with everyone performing resistance exercise at least 3 days/wk and several conducting resistance training 5 - 6 days/wk. During the resistance training sessions, crewmembers averaged 3 - 6 sets of 12-20 repetitions for each leg exercise. Authors were unable to get an accurate account of the time involved with the resistance exercise. Based on the number of contractions and assuming a 2-s count for concentric and eccentric contractions (from video), authors estimate crewmembers spent more than 1 h/wk with the leg muscles under tension during resistance exercise. Based on the exercise database and available videos of crewmembers while exercising (cy-

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cling, running, and lifting), authors were able to estimate the number of muscle contractions performed while in space (Table 4). On average, crewmembers performed more than 435,000 muscle contractions per leg during scheduled exercise while in space. As noted earlier, the exercise program was highly variable with a low of ≈ 200,000 (subject C) and a high approaching 1 million (subject B) muscle contractions for each leg. Generally, crewmembers performed exercise 6 of 7 days/wk. Of the total time in orbit (≈ 4,248 h), the exercise program presented (minus setup, stowage, and hygiene) constituted ≈ 3.4% of the time. When sleep, workday schedule, and leisure time were considered, the estimate for exercise time increased to 7-10% of the available time for the crewmembers while in space.

Muscle volume A summary of calf muscle volume before and after space flight is shown in Table 5. The gastrocnemius (medial and lateral) and soleus muscle were smaller (P < 0.05) after 6 mo in space. Combined, the gastrocnemius and soleus atrophied (P < 0.05) -13±2% pre- to postflight. The soleus (-15±2%) atrophied more (P < 0.05) than the gastrocnemius (-10±2%) pre- to postflight (Fig. 1). One crewmember (subject E) had insignificant (-1%) atrophy after the flight. Two of the crewmembers (subjects A and F) lost more than 20% of their calf muscle mass. Of the remaining six crewmembers, five lost more than 10% calf muscle mass. At R+19 after landing, the gastrocnemius was still 5-6% atrophied, but this was not significant. Conversely, the soleus was still reduced (P < 0.05) compared with preflight, averaging -9 ±1% for all crewmembers. Although calf muscle volume was

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still reduced -8±2%, it represented a partial recovery from the more immediate (R+4) flight measurement.

Muscle performance A summary of each crewmember’s calf muscle performance for MVC at one angle (neutral position) and a slow (60◦ /s) and fast (180◦ /s) isokinetic speed are shown in Table 6. MVC was reduced (P < 0.05) -14±2% at R+7 and remained lower (-13±5%; P < 0.05) at R+13. All nine crewmembers had a decline in MVC (range=7 to -22%) with flight. At R+13, seven of the nine crewmembers were lower (range=-9 to -33%), with two crewmembers (subjects A and H) having a 5-10% increase compared with preflight. At the slow isokinetic speed (60◦ /s) there was a -20±3% loss (P < 0.05), which was sustained (-19±4; P < 0.05) at R+13. This pattern was also evident at the faster (180◦ /s) isokinetic speed with a 25±10% reduction at R+7 and R+13. For both the slow and fast speeds, eight of the nine subjects had a decrease in muscle performance. A force-velocity curve for all subjects from pre- to postflight (R+7) is shown in Fig. 2. On average, force-velocity characteristics were reduced -20 to -29% across the velocity spectrum (P < 0.05). Peak power was 134±11, 91±10, and 94±13 W preflight, R+7, and R+13, respectively. On average, peak power declined 32% with spaceflight (P < 0.05). Muscle fiber type Authors isolated and analyzed the MHC profile on a total of 4,328 single muscle fibers from the gastrocnemius and soleus muscles before and after flight. The breakdown was 1,960 muscle fibers (1,109 preflight, 851 postflight) for the gastrocnemius and 2,368 muscle fibers (1,277 preflight, 1,091 postflight) for the soleus. The average MHC profile of the gastrocnemius and soleus muscles from the crewmembers before and after space flight is shown in Fig. 3. Individual data from the gastrocnemius and soleus of each crewmember are shown in Tables 7 and 8, respectively. One individual (subject B) had a small muscle biopsy sample and therefore was not

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Special Edition on Long Duration Spaceflight included in these analyses. The gastrocnemius had a 12% decrease (P < 0.05) in MHC I fibers and an increase (P < 0.05) in MHC I/IIa (+4%) hybrid fibers and MHC IIa fibers (+9%). Seven of the eight subjects had a decrease in MHC I fibers (range=-6 to -31%). There were minimal MHC IIx and MHC I/IIa/IIx fibers detected in the pre- and postflight muscle samples. The 4% increase in hybrid muscle fibers appears to be the result of the MHC I/IIa hybrid fiber type.


On average, the soleus had a 17% decrease (P < 0.05) in MHC I fibers. The shift away from MHC I fibers was distributed among the other fiber types (MHC I/IIa, IIa, IIa/IIx), with a nonsignificant increase of 4-5% within each MHC phenotype. Combined, the soleus had a 12% increase (P < 0.05) in hybrid MHC isoforms. Three of the crewmembers (subjects D, E, and I) did not have any major alterations in fiber type of the soleus. Four of the crewmembers (subjects A, C, F, and G) had a decrease in soleus MHC I fibers that ranged from -20 to -44%.

The main finding from this study was that the exercise program did not completely protect the calf muscles. Authors observed a substantial decrease in calf muscle mass and performance along with a slow-to-fast fiber-type transition in the gastrocnemius and soleus muscles, which are all traits associated with unloading in humans. These data suggest that changes to the exercise countermeasure program are required to more fully protect human skeletal muscle while crewmembers are in space for extended periods.

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The average amount of muscle mass lost (13%) with spaceflight was slightly less than with previous long-duration stays on the Russian Space Station Mir (-17%). The current ISS and previous Mir calf muscle volume loss is about one-half that of long-duration (60- to 120day) bed rest studies showing a ≈29% decrease among the control subjects without countermeasures . These data imply that the exercise in space is having a beneficial effect but is not complete, with the soleus being more difficult to protect than the gastrocnemius.

The data suggest that the crewmembers with larger calf muscles had a greater degree of atrophy with long-duration spaceflight. Second, the volume of treadmill exercise may have provided a level of protection for calf muscle mass. The three individuals (subjects B, E, and H) who performed >200 min/wk on the treadmill lost about onethird of the calf muscle volume (-43 ± 19 cm3 ) compared with crewmembers who used the treadmill 4 months). Women demonstrate greater impairment in neural activation of muscle after short-term unloading; future studies should determine if this leads to greater fatigue susceptibility in women in the first 2 weeks of unloading. There is one study suggesting that recovery of strength after unloading may be slower for women thanmen. Taken together these data suggest that the time course of unloading-induced muscle loss may be sex specific. There are also areas where sex differences appear quite unlikely. For both men and women, whole muscle and single muscle fiber atrophy does not fully account for the strength and power loss; the reduction in the force and cross-sectional area of type 1 fibers appears to be very similar in both genders. A significant gap in knowledge is whether sex differences in strength loss/neural activation translate to differences in functional performance (e.g., mission-related tasks). Negative Energy Balance Some bed rest studies have restricted energy intake and allowed weight loss by design or allowed subjects to consume food at their discretion, so as to not coerce intake. The 60-day Women’s International Simulation for Space Exploration study was one of these studies, and as a result, these female subjects did lose body weight (lean tissue more than fat) during bed rest at a rate of 0.06 kg/day. In a similar 90-day study with male subjects conducted earlier at the same institution, men also lost weight at 0.04 kg/day (calculated from the published average weight loss). Due to the many differences in study design, it cannot be concluded with any certainty if this slight difference in rate of weight loss between men and women is of any significance. While "weightlessness" is a key aspect of space travel, an unexpected analog comes in the form of studies related to weight loss. Though there is a fair amount of literature on weight loss and effects on bone similar to space-related research, few studies have examined the effects of negative energy balance on bone with regard to gender, and those that have attempted are plagued bymany confounding factors (age, body size, diet- and/or exercise-induced weight loss, rate of weight loss, etc.), making drawing conclusions difficult. Hence, there is a paucity of literature evaluating sex related differences relative to the effects of energy deficit on bone and muscle metabolism. Making comparisons across separate studies evaluating male and female re-

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Special Edition on Long Duration Spaceflight sponses is fraught with confounding factors. If one were to speculate, there do not appear to be major sex differences in the bone or muscle responses to energy deficit between men and women.


Joint Injury Sex-based differences have been identified in the incidence of osteoarthritis (OA), with OA of the knee, in particular, significantly more common in women. Sex-based risk factors explaining this include the loss of estrogen’s anabolic effect on cartilage after menopause, a higher incidence of predisposing knee injuries-such as anterior cruciate ligament tears-in women, and increased joint laxity. There is clear evidence from animal studies that regular mechanical loading is essential to cartilage health. In humans, 6 or more weeks of nonweight- bearing can produce changes in magnetic resonance imaging images of knee cartilage that resemble OA. However, sexbased differences in the response to joint unloading have not been elucidated. Because articular cartilage health is impacted by the quality of the underlying bone as well as the strength of muscles around the joint, assessment of the potential risk for articular cartilage injury imposed by unloading needs to include evaluation of all three tissues: bone, muscle, and cartilage. There is some evidence to suggest that osteopenia of subchondral bone underlying articular cartilage contributes to cartilage degeneration. Conversely, damaged cartilage releases receptor activator of nuclear factor kappa-B ligand (RANKL) and other inflammatory components, which can lead to the loss of adjacent bone. Since muscles serve to stabilize and dampen forces across joints, loss of muscle mass and strength after a prolonged unloading can contribute to joint injury risk and early degenerative joint changes, especially in the knee. However, sex-based differences in the relative impact of bone and muscle loss on joint health have not been defined. Specific interventions to increase loadbearing or strengthening activities in space will be indicated.They may also identify the need for progressive strengthening and joint loading upon arrival on a planetary surface after extended microgravity exposure, after return from space or after prolonged period of non-weight-bearing on Earth. Musculoskeletal injuries have been reported in-flight at a rate of 0.021 per flight day for men and 0.015 per flight day for women; hand injuries are the most common, with abrasions and small lacerations the most common manifestations. There are few data on the recovery of the musculoskeletal system following spaceflight and even less data on sex differences in recovery rates. Generally, international space station crew have substantial recovery of muscle strength within a month following flight. The time course of recovery of bone mineral density has been evaluated but not specifically for sex differences. In general, half-lives for recovery of bone mineral density are ≈150-200 days depending on site. Effects of prolonged space flight on human skeletal muscle enzyme and substrate profiles The purpose of this study was to determine the effects of a 6-month spaceflight on the ISS on selected anaerobic and aerobic enzymes, and the content of glycogen and lipids in slow and fast fibers of the soleus and gastrocnemius. The effects of countermeasure exercise were evaluated by relating pre- and postflight enzyme patterns to the extent of in-flight treadmill exercise. The crew members participating in this study flew aboard the ISS from increments 5 to 11 (2002-2005). In the overall study group there were 10 crew members: 5 American astronauts and 5 Russian cosmonauts; however, for the metabolic studies described here there were nine subjects (5 astronauts and 4 cosmonauts). Due to small sample size and/or problems in shipment from Russia to the United States, the histochemical and biochemical assays were performed on biopsy tissue from six or seven and eight crew members, respectively. RESULTS Authors purpose was to relate the atrophy of a given fiber type to specific enzyme and substrate changes. As reported previously , the degree of fiber atrophy varied greatly between crew members, and considerable variability was also found in muscle enzymes. The LT crew members, running