Adaptation of fibers in fast-twitch muscles of rats to spaceflight and hindlimb suspension

Muscle unloading: A comparison between spaceflight and ground-based models

Prolonged unloading of skeletal muscle, a common outcome of events such as spaceflight, bed rest and hindlimb unloading, can result in extensive metabolic, structural and functional changes in muscle fibres. With advancement in investigations of cellular and molecular mechanisms, understanding of disuse muscle atrophy has significantly increased. However, substantial gaps exist in our understanding of the processes dictating muscle plasticity during unloading, which prevent us from developing effective interventions to combat muscle loss. This review aims to update the status of knowledge and underlying mechanisms leading to cellular and molecular changes in skeletal muscle during unloading. We have also discussed advances in the understanding of contractile dysfunction during spaceflights and in ground-based models of muscle unloading. Additionally, we have elaborated on potential therapeutic interventions that show promising results in boosting muscle mass and strength during mechanical unloading. Finally, we have identified key gaps in our knowledge as well as possible research direction for the future.

Simulated microgravity inhibits C2C12 myogenesis via phospholipase D2-induced Akt/FOXO1 regulation

The skeletal muscle system has evolved to maintain body posture against a constant gravitational load. Mammalian target of rapamycin (mTOR) regulates the mechanically induced increase in the skeletal muscle mass. In the present study, we investigated mTOR pathway in C2C12 myoblasts in a model of mechanical unloading by creating a simulated microgravity (SM) using 3 D clinorotation. SM decreased the phosphorylation of Akt at Ser 473, which was mediated by mTOR complex 2 (mTORC2), in C2C12 myoblasts, leading to a decrease in the cell growth rate. Subsequently, SM inhibited C2C12 myogenesis in an Akt-dependent manner. In addition, SM increased the phospholipase D (PLD) activity by enhancing PLD2 expression, resulting in the dissociation of mSIN1 from the mTORC2, followed by decrease in the phosphorylation of Akt at Ser 473, and FOXO1 at Ser 256 in C2C12 myoblasts. Exposure to SM decreased the autophagic flux of C2C12 myoblasts by regulation of mRNA level of autophagic genes in a PLD2 and FOXO1-dependent manner, subsequently, resulting in a decrease in the C2C12 myogenesis. In conclusion, by analyzing the molecular signature of C2C12 myogenesis using SM, we suggest that the regulatory axis of the PLD2 induced Akt/FOXO1, is critical for C2C12 myogenesis.

Divergent Anabolic Signalling responses of Murine Soleus and Tibialis Anterior Muscles to Chronic 2G Hypergravity

The purpose of the study was to assess the rate of protein synthesis (PS) and elucidate signalling pathways regulating PS in mouse soleus (Sol) and tibialis anterior (TA) muscles following chronic hypergravity (30-day centrifugation at 2G). The content of the key signalling proteins of the various anabolic signalling pathways was determined by Western-blotting. The rate of PS was assessed using in-vivo SUnSET technique. An exposure to 2G centrifugation did not induce any significant changes in the rate of PS as well as phosphorylation status of the key anabolic markers (AKT, p70s6k, 4E-BP1, GSK-3beta, eEF2) in Sol. On the contrary, a significant 55% increase in PS (p < 0.05) was found in TA. The cause of such a rise in PS could be associated with an increase in AKT (+72%, p < 0.05), GSK-3beta (+60%, p < 0.05) and p70s6k (+40%, p < 0.05) phosphorylation, as well as a decrease in eEF2 phosphorylation (−46%, p < 0.05) as compared to control values. Thus, the results of our study indicate that 30-day 2G centrifugation induces a distinct anabolic response in mouse Sol and TA muscles. The activation of the PS rate in TA could be linked to an up-regulation of both mTORC1-dependent and mTORC1-independent signalling pathways.

Stochastic effects in adaptive reconstruction of body damage: implied the creativity of natural selection

After an injury occurs, mechanical/biochemical loads on muscles influence the composition and structure of recovering muscles; this effect likely occurs in other tissues, cells and biological molecules as well owing to the similarity, interassociation and interaction among biochemical reactions and molecules. The 'damage and reconstruction' model provides an explanation for how an ideal cytoarchitecture is created by reducing components not suitable for bearing loads; in this model, adaptive changes are induced by promoting the stochasticity of biochemical reactions. Biochemical and mechanical loads can direct the stochasticity of biochemical reactions, which can in turn induce cellular changes. Thus, mechanical and biochemical loads, under natural selection pressure, modify the direction of cell- and tissue-level changes and guide the formation of new structures and traits, thereby influencing microevolution. In summary, the 'damage and reconstruction' model accounts for the role of natural selection in the formation of new organisms, helps explain punctuated equilibrium, and illustrates how macroevolution arises from microevolution.

Responses of skeletal muscles to gravitational unloading and/or reloading

Adaptation of morphological, metabolic, and contractile properties of skeletal muscles to inhibition of antigravity activities by exposure to a microgravity environment or by simulation models, such as chronic bedrest in humans or hindlimb suspension in rodents, has been well reported. Such physiological adaptations are generally detrimental in daily life on earth. Since the development of suitable countermeasure(s) is essential to prevent or inhibit these adaptations, effects of neural, mechanical, and metabolic factors on these properties in both humans and animals were reviewed. Special attention was paid to the roles of the motoneurons (both efferent and afferent neurograms) and electromyogram activities as the neural factors, force development, and/or length of sarcomeres as the mechanical factors and mitochondrial bioenergetics as the metabolic factors.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Child and family adjustment following pediatric solid organ transplantation: factors to consider during the early years post-transplant

Adjusting to life after transplant can be challenging to pediatric solid organ transplant recipients and their families. In this review, we discuss a number of important factors to consider during the first 2-3 yr after transplant (defined as the "early years"), including transitioning from hospital to home, returning to physical activity, feeding and nutrition, school reentry, potential cognitive effects of transplant, family functioning, and QOL. We highlight steps that providers can take to optimize child and family adjustment during this period.

Adaptation of mouse skeletal muscle to long-term microgravity in the MDS mission

The effect of microgravity on skeletal muscles has so far been examined in rat and mice only after short-term (5–20 day) spaceflights. The mice drawer system (MDS) program, sponsored by Italian Space Agency, for the first time aimed to investigate the consequences of long-term (91 days) exposure to microgravity in mice within the International Space Station. Muscle atrophy was present indistinctly in all fiber types of the slow-twitch soleus muscle, but was only slightly greater than that observed after 20 days of spaceflight. Myosin heavy chain analysis indicated a concomitant slow-to-fast transition of soleus. In addition, spaceflight induced translocation of sarcolemmal nitric oxide synthase-1 (NOS1) into the cytosol in soleus but not in the fast-twitch extensor digitorum longus (EDL) muscle. Most of the sarcolemmal ion channel subunits were up-regulated, more in soleus than EDL, whereas Ca²⁺-activated K⁺ channels were down-regulated, consistent with the phenotype transition. Gene expression of the atrophy-related ubiquitin-ligases was up-regulated in both spaceflown soleus and EDL muscles, whereas autophagy genes were in the control range. Muscle-specific IGF-1 and interleukin-6 were down-regulated in soleus but up-regulated in EDL. Also, various stress-related genes were up-regulated in spaceflown EDL, not in soleus. Altogether, these results suggest that EDL muscle may resist to microgravity-induced atrophy by activating compensatory and protective pathways. Our study shows the extended sensitivity of antigravity soleus muscle after prolonged exposition to microgravity, suggests possible mechanisms accounting for the resistance of EDL, and individuates some molecular targets for the development of countermeasures.

Region-specific responses of adductor longus muscle to gravitational load-dependent activity in Wistar Hannover rats

Response of adductor longus (AL) muscle to gravitational unloading and reloading was studied. Male Wistar Hannover rats (5-wk old) were hindlimb-unloaded for 16 days with or without 16-day ambulation recovery. The electromyogram (EMG) activity in AL decreased after acute unloading, but that in the rostral region was even elevated during continuous unloading. The EMG levels in the caudal region gradually increased up to 6th day, but decreased again. Approximately 97% of fibers in the caudal region were pure type I at the beginning of experiment. Mean percentage of type I fibers in the rostral region was 61% and that of type I+II and II fiber was 14 and 25%, respectively. The percent type I fibers decreased and de novo appearance of type I+II was noted after unloading. But the fiber phenotype in caudal, not rostral and middle, region was normalized after 16-day ambulation. Pronounced atrophy after unloading and re-growth following ambulation was noted in type I fibers of the caudal region. Sarcomere length in the caudal region was passively shortened during unloading, but that in the rostral region was unchanged or even stretched slightly. Growth-associated increase of myonuclear number seen in the caudal region of control rats was inhibited by unloading. Number of mitotic active satellite cells decreased after unloading only in the caudal region. It was indicated that the responses of fiber properties in AL to unloading and reloading were closely related to the region-specific neural and mechanical activities, being the caudal region more responsive.

Magnetic resonance imaging findings of fatty infiltrate in the cervical flexors in chronic whiplash

STUDY DESIGN

Retrospective investigation of muscle changes in patients suffering from chronic whiplash-associated disorders (WAD).

OBJECTIVES

To quantitatively compare the presence of muscle alterations (fatty infiltrate [MFI] and cross-sectional area [CSA]) in the anterior musculature of the cervical spine in a cohort of chronic whiplash patients (WAD II) and healthy control subjects across muscle and cervical segmental level.

SUMMARY OF BACKGROUND DATA

Magnetic resonance imaging can be regarded as the gold standard for muscle imaging. There is little knowledge about in vivo features of anterior neck muscles in patients suffering from chronic WAD and how muscle structure differs across the factors of muscle, vertebral level, age, self-reported pain and disability, body mass index, and duration of symptoms.

METHODS

Reliable magnetic resonance imaging measures for MFI and CSA were performed for the anterior cervical muscles bilaterally in 109 female subjects (78 WAD, 31 healthy control; 18-45 years, 3 months to 3 years postinjury). The measures were performed on all subjects for the longus capitis and colli and the sternocleidomastoid muscles.

RESULTS

The WAD subjects had significantly larger MFI and CSA for the anterior muscles compared to healthy control subjects (all P < 0.0001). In addition, the amount of MFI varied by both cervical level and muscle, with the longus capitis/colli having the largest amount of fatty infiltrates at the C2-C3 level (P < 0.0001). MFI was inversely related to age, self-reported pain/disability, and body mass index but directly proportional to duration of symptoms.

CONCLUSION

There is significantly greater MFI and CSA in the anterior neck muscles, especially in the deeper longus capitis/colli muscles, in subjects with chronic WAD when compared to healthy controls. Future studies are required to investigate the relationships between muscular morphometry and symptoms in patients suffering from acute and chronic WAD.

Characterization of acute and chronic whiplash-associated disorders

SYNOPSIS

The development of chronic pain and disability following whiplash injury is common and contributes substantially to personal and economic costs related with this condition. Emerging evidence demonstrates the clinical presence of alterations in the sensory and motor systems, including psychological distress in all individuals with a whiplash injury, regardless of recovery. However, individuals who transition to the chronic state present with a more complex clinical picture characterized by the presence of widespread sensory hypersensitivity, as well as significant posttraumatic stress reactions. Based on the diversity of the signs and symptoms experienced by individuals with a whiplash condition, clinicians must take into account the more readily observable/measurable differences in motor, sensory, and psychological dysfunction. The implications for the assessment and management of this condition are discussed. Further review into the pathomechanical, pathoanatomical, and pathophysiological features of the condition also will be discussed.

LEVEL OF EVIDENCE

Level 5.J

Morphological comparison of different protocols of skeletal muscle remobilization in rats after hindlimb suspension

The aim of this study was to evaluate and compare the efficacy of different remobilization protocols in different skeletal muscles considering the changes induced by hindlimb suspension of the tail. Thirty-six female Wistar rats were divided into six groups: control I, control II, suspended, suspended free, suspended trained on a declined treadmill and suspended trained on a flat treadmill. Fragments of soleus and tibialis anterior (TA) muscle were frozen and processed by different histochemical methods. The suspended soleus showed a significant increase in the proportional number of intermediate/hybrid fibers and a decrease in the number of type I fibers. Some of these changes proved to be reversible after remobilization. The three remobilization programs led to the recovery of both the proportional number of fibers and their size. The TA muscle presented a significant increase in the number and size of type I fibers and a cell size reduction of type IIB fibers, which were recovered after training on a declined treadmill and free movement. Especially regarding the soleus, the present findings indicate that, among the protocols, training on a declined treadmill was found to induce changes of a more regenerative nature, seemingly indicating a better tissue restructuring after the suspension procedure.

Rapid atrophy of the lumbar multifidus follows experimental disc or nerve root injury

STUDY DESIGN

Experimental study of muscle changes after lumbar spinal injury.

OBJECTIVES

To investigate effects of intervertebral disc and nerve root lesions on cross-sectional area, histology and chemistry of porcine lumbar multifidus.

SUMMARY OF BACKGROUND DATA

The multifidus cross-sectional area is reduced in acute and chronic low back pain. Although chronic changes are widespread, acute changes at 1 segment are identified within days of injury. It is uncertain whether changes precede or follow injury, or what is the mechanism.

METHODS

The multifidus cross-sectional area was measured in 21 pigs from L1 to S1 with ultrasound before and 3 or 6 days after lesions: incision into L3-L4 disc, medial branch transection of the L3 dorsal ramus, and a sham procedure. Samples from L3 to L5 were studied histologically and chemically.

RESULTS

The multifidus cross-sectional area was reduced at L4 ipsilateral to disc lesion but at L4-L6 after nerve lesion. There was no change after sham or on the opposite side. Water and lactate were reduced bilaterally after disc lesion and ipsilateral to nerve lesion. Histology revealed enlargement of adipocytes and clustering of myofibers at multiple levels after disc and nerve lesions.

CONCLUSIONS

These data resolve the controversy that the multifidus cross-sectional area reduces rapidly after lumbar injury. Changes after disc lesion affect 1 level with a different distribution to denervation. Such changes may be due to disuse following reflex inhibitory mechanisms.

Fatty infiltration in the cervical extensor muscles in persistent whiplash-associated disorders: a magnetic resonance imaging analysis

STUDY DESIGN

Cross-sectional investigation of muscle changes in patients suffering from persistent whiplash-associated disorders (WAD).

OBJECTIVES

To quantitatively compare the presence of fatty infiltrate in the cervical extensor musculature in a cohort of chronic whiplash patients (WAD II) and healthy control subjects across muscle and cervical segmental level.

SUMMARY OF BACKGROUND DATA

Magnetic resonance imaging (MRI) can be regarded as the gold standard for muscle imaging; however, there is little knowledge about in vivo features of neck extensor muscles in patients suffering from persistent WAD and how fat content alters across the factors of muscle, vertebral segments, age, self-reported pain and disability, compensation status, body mass index, and duration of symptoms.

METHODS

A reliable MRI measure for fatty infiltrate was performed of the cervical extensor muscles bilaterally in 113 female subjects (79 WAD, 34 healthy control; 18-45 years, 3 months to 3 years post injury). The measure was performed on all subjects for the rectus capitis posterior minor and major, multifidus, semispinalis cervicis and capitis, splenius capitis, and upper trapezius.

RESULTS

The WAD subjects had significantly larger amounts of fatty infiltrate for all of the cervical extensor muscles compared with healthy control subjects (all P < 0.0001). In addition, the amount of fatty infiltrate varied by both cervical level and muscle, with the rectus capitis minor/major and multifidi at C3 having the largest amount of fatty infiltrate (P < 0.0001). Intramuscular fat was independent of age, self-reported pain/disability, compensation status, body mass index, and duration of symptoms.

CONCLUSION

There is significantly greater fatty infiltration in the neck extensor muscles, especially in the deeper muscles in the upper cervical spine, in subjects with persistent WAD when compared with healthy controls. Future studies are required to investigate the relationships between muscular alterations and symptoms in patients suffering from persistent WAD.

Skeletal muscle HIF-1alpha expression is dependent on muscle fiber type

Oxygen homeostasis is an essential regulation system for cell energy production and survival. The oxygen-sensitive subunit α of the hypoxia inducible factor-1 (HIF-1) complex is a key protein of this system. In this work, we analyzed mouse and rat HIF-1α protein and mRNA expression in parallel to energetic metabolism variations within skeletal muscle. Two physiological situations were studied using HIF-1α–specific Western blotting and semiquantitative RT-PCR. First, we compared HIF-1α expression between the predominantly oxidative soleus muscle and three predominantly glycolytic muscles. Second, HIF-1α expression was assessed in an energy metabolism switch model that was based on muscle disuse. These two in vivo situations were compared with the in vitro HIF-1α induction by CoCl₂ treatment on C₂C₁₂ mouse muscle cells. HIF-1α mRNA and protein levels were found to be constitutively higher in the more glycolytic muscles compared with the more oxidative muscles. Our results gave rise to the hypothesis that the oxygen homeostasis regulation system depends on the fiber type.

The neuromuscular junction: anatomical features and adaptations to various forms of increased, or decreased neuromuscular activity

The neuromuscular junction (NMJ) allows communication between motor neurons and muscle fibers. During development, marked morphological changes occur as the functional NMJ is formed. During the postnatal period of rapid growth and muscle enlargement, endplate size concurrently increases. Even beyond this period of pronounced plasticity, the NMJ undergoes subtle morphological remodeling--expansion and retraction--although its overall dimensions remain stable. This natural, continual NMJ remodeling is amplified with alterations in neuromuscular activity. Increased activity, presented by exercise training, typically results in expansion of NMJ size. Disuse, brought about by neurotoxins, denervation, or spaceflight, also elicits substantial reconfiguring of the endplate.

International collaboration on Russian spacecraft and the case for free flyer biosatellites

Animal research has been critical to the initiation and progress of space exploration. Animals were the original explorers of "space" two centuries ago and have played a crucial role by demonstrating that the space environment, with precautions, is compatible with human survival. Studies of mammals have yielded much of our knowledge of space physiology. As spaceflights to other planets are anticipated, animal research will continue to be essential to further reveal space physiology and to enable the longer missions. Much of the physiology data collected from space was obtained from the Cosmos (Bion) spaceflights, a series of Russian (Soviet)-International collaborative flights, over a 22 year period, which employed unmanned, free flyer biosatellites. Begun as a Soviet-only program, after the second flight the Russians invited American and other foreign scientists to participate. This program filled the 10 year hiatus between the last US biosatellite and the first animal experiments on the shuttles. Of the 11 flights in the Cosmos program nine of them were international; the flights continued over the years regardless of political differences between the Soviet Union and the Western world. The science evolved from sharing tissues to joint international planning and development, and from rat postmortem tissue analysis to in vivo measurements of a host of monkey physiological parameters during flight. Many types of biological specimens were carried on the modified Vostok spacecraft, but only the mammalian studies are discussed herein. The types of studies done encompass the full range of physiology and have begun to answer "critical" questions of space physiology posed by various ad hoc committees. The studies have not only yielded a prodigious and significant body of data, they have also introduced some new perspectives in physiology. A number of the physiological insights gained are relevant to physiology on Earth. The Cosmos flights also added significantly to flight-related technology, some of which also has application on our planet. In summary, the Cosmos biosatellite flights were extremely productive and of low cost. The Bion vehicles are versatile in that they can be placed into a variety of orbits and altitudes, and can carry radiation sources or other hazardous material which cannot be carried on manned vehicles. With recent advances in sensor, robotic, and data processing technology, future free flyers will be even more productive, and will largely preclude the need to fly animal experiments on manned vehicles. Currently, mammalian researchers do not have access to space for an unknown time, seriously impeding the advancement and understanding of space physiology during long duration missions. Initiation of a new, international program of free flyer biosatellites is critical to our further understanding of space physiology, and essential to continued human exploration of space.

Insulin-independent pathways mediating glucose uptake in hindlimb-suspended skeletal muscle

Insulin resistance accompanies atrophy in slow-twitch skeletal muscles such as the soleus. Using a rat hindlimb suspension model of atrophy, we have previously shown that an upregulation of JNK occurs in atrophic muscles and correlates with the degradation of insulin receptor substrate-1 (IRS-1) (Hilder TL, Tou JC, Grindeland RF, Wade CE, and Graves LM. FEBS Lett 553: 63-67, 2003), suggesting that insulin-dependent glucose uptake may be impaired. However, during atrophy, these muscles preferentially use carbohydrates as a fuel source. To investigate this apparent dichotomy, we examined insulin-independent pathways involved in glucose uptake following a 2- to 13-wk hindlimb suspension regimen. JNK activity was elevated throughout the time course, and IRS-1 was degraded as early as 2 wk. AMP-activated protein kinase (AMPK) activity was significantly higher in atrophic soleus muscle, as were the activities of the ERK1/2 and p38 MAPKs. As a comparison, we examined the kinase activity in solei of rats exposed to hypergravity conditions (2 G). IRS-1 phosphorylation, protein, and AMPK activity were not affected by 2 G, demonstrating that these changes were only observed in soleus muscle from hindlimb-suspended animals. To further examine the effect of AMPK activation on glucose uptake, C2C12 myotubes were treated with the AMPK activator metformin and then challenged with the JNK activator anisomycin. While anisomycin reduced insulin-stimulated glucose uptake to control levels, metformin significantly increased glucose uptake in the presence of anisomycin and was independent of insulin. Taken together, these results suggest that AMPK may be an important mediator of insulin-independent glucose uptake in soleus during skeletal muscle atrophy.

Effects of spaceflight on the attachment of tendons to bone in the hindlimb of the pregnant rat

The objective of this study was to determine the effects of spaceflight on the structure of the tendon-bone junction (TBJ). Pregnant rats either flew in the space shuttle Atlantis (flight group; F) or were exposed to simulated launch and landing protocols (synchronous control group; SC) during days 9-19 of pregnancy. Following birth of their pups, maternal hindlimbs were studied using scanning electron and light microscopic histomophometric techniques. The tibial and calcaneal tuberosities, the fibular head, and the tibia-fibula junction were studied. Myofiber density and cross-sectional area of the quadratus femoris and soleus muscles and diameters of the calcaneal and patellar tendons were also evaluated. Cortical erosion was significantly greater at the tibial tuberosity and the fibular head in F animals compared to SC animals (P < 0.001). Sharpey fiber density was significantly less at the tibial tuberosity and fibular head in F animals compared to SC animals (P < 0.001). The myofiber area of both the soleus and quadratus femoris muscles and the diameters of both calcaneal and patellar tendons were significantly less in F compared to SC rats (P < 0.05). Our data illustrate that the TBJ morphology is affected by spaceflight at the attachment sites of the soleus and quadratus femoris muscles in pregnant animals, which could adversely affect their physical properties. These atrophic TBJ changes could have resulted from atrophy of the adjacent muscles and their tendons. Atrophic changes in the structure of the TBJ could predispose an animal to injury following spaceflight, when normal gravity conditions are reestablished.

Hindlimb suspension inhibits air-righting due to altered recruitment of neck and back muscles in rats

Effects of 9-week hindlimb suspension and 8-week recovery on air-righting reaction in response to drop from a supine position were studied in adult rats. The righting time in rats at the end of suspension (approximately 220 ms) was longer than the age-matched controls (approximately 120 ms, p <0.05). The unloading-related change in righting time was accompanied by lowered activities of electromyogram (EMG) and altered recruitment of both neck and back muscles at a specific stage of drop. After 8 weeks of reambulation, righting time recovered toward the control level (approximately 153 ms, p <0.05), but the EMG activity of back muscle was still less than controls. In contrast, the EMG of neck muscle during fall was even increased. The differences in the characteristics of the muscle fibers between two groups were minor. It is suggested that inhibition of recruitment, rather than the changes in the fiber characteristics, of neck and back muscles is one of the major causes of the slow air-righting.

SMHS1 is involved in oxidative/glycolytic-energy metabolism balance of muscle fibers

With the aim of finding important mediators of muscle atrophy, we cloned SMHS1, a novel gene that was found to be upregulated in rat soleus muscle atrophied by restriction of activity. The SMHS1 amino acid sequence shares 65% similarity with RTP801-which is a cellular stress response protein regulated by HIF-1-but SMHS1 expression was demonstrated to be independent of HIF-1. SMHS1 was found to be mainly expressed in skeletal muscle, and comparisons of its expression in atrophied versus hypertrophied muscles and in oxidative versus glycolytic muscles suggested that SMHS1 contributes to the muscle energy metabolism phenotypes.

The hindlimb unloading rat model: literature overview, technique update and comparison with space flight data

The hindlimb unloading rodent model is used extensively to study the response of many physiological systems to certain aspects of space flight, as well as to disuse and recovery from disuse for Earth benefits. This chapter describes the evolution of hindlimb unloading, and is divided into three sections. The first section examines the characteristics of 1064 articles using or reviewing the hindlimb unloading model, published between 1976 and April 1, 2004. The characteristics include number of publications, journals, countries, major physiological systems, method modifications, species, gender, genetic strains and ages of rodents, experiment duration, and countermeasures. The second section provides a comparison of results between space flown and hindlimb unloading animals from the 14-day Cosmos 2044 mission. The final section describes modifications to hindlimb unloading required by different experimental paradigms and a method to protect the tail harness for long duration studies. Hindlimb unloading in rodents has enabled improved understanding of the responses of the musculoskeletal, cardiovascular, immune, renal, neural, metabolic, and reproductive systems to unloading and/or to reloading on Earth with implications for both long-duration human space flight and disuse on Earth.

Alterations in enzyme histochemical characteristics of the masseter muscle caused by long-term soft diet in growing rabbits

OBJECTIVE

Recently young people have an increasing tendency to intake an easily chewable diet and spend less time on mastication. The aim of the present study was to investigate the histochemical effects of long-term soft diet on the masseter muscle in growing rabbits.

MATERIALS AND METHODS

Twelve young male Japanese white rabbits were divided into two groups (n = 6 each) at weaning (1 month after birth) and fed a solid diet (control group) or a powder diet (soft-diet group). The duration of the experimental period was 6 months. Masseter fibers from the superficial and the deep portions were histochemically defined as type 1, 2A, 2B, or 2C fibers.

RESULTS

As compared with that of the control, the deep masseter of the soft-diet group showed a significantly lower ratio of type 1 fiber cross-sectional area to total area (6.3 and 10.1% for the soft-diet and control group, respectively), significantly more type 2A fibers (74.0%vs 50.3%) and significantly fewer type 2B fibers (4.3%vs 12.5%). However, fiber size did not differ between the two groups. NADH-tetrazolium-reductase (NADH-TR) of the masseter was less reactive in the soft-diet group, reflecting a lower oxidative capacity.

CONCLUSIONS

These findings indicate that the alteration of the functional activities contributed to selective disuse influences on the type 1 and type 2B fibers, and a resultant increase in type 2A fibers. This study suggests that long-term alteration of jaw function induced by a soft diet can lead to adaptations of the masseter muscle.

Metabolic modulation of muscle fiber properties unrelated to mechanical stimuli

The effects of chronically increasing (creatine-fed) or decreasing (beta-guanidinopropionic acid [beta-GPA]-fed) high-energy phosphates for up to 8 weeks on daily voluntary activity levels, swimming endurance capacity, electromyogram (EMG) activity, and the morphological and metabolic properties of single fibers in the soleus and extensor digitorum longus (EDL) muscles in young rats were determined. High-energy phosphate, voluntary activity, and soleus-integrated EMG levels were lower in beta-GPA-fed rats than in control rats. Endurance capacity was higher at a relatively low intensity of swimming and lower at a relatively high intensity in beta-GPA-fed rats than in control rats. Muscle mass and fiber size were smaller, and the percentage of slow fibers was higher in the soleus and EDL of beta-GPA-fed rats than in control rats. Succinate dehydrogenase activity was higher in both the fast and slow fibers of the EDL of beta-GPA-fed rats than in control rats. Thus, a reduction in high-energy phosphates transformed some fast fibers toward a slow phenotype. Creatine supplementation had minimal effects: The only significant change was an increase in alpha-glycerophosphate dehydrogenase activity in the fast fibers of the EDL. These results indicate that the metabolic environment of a muscle fiber can influence the prominence of a given muscle fiber independent of the activity level of muscle.

Electrophysiological, histochemical, and hormonal adaptation of rat muscle after prolonged hindlimb suspension

The perspective of long-duration flights for future exploration, imply more research in the field of human adaptation. Previous studies in rat muscles hindlimb suspension (HLS), indicated muscle atrophy and a change of fibre composition from slow-to-fast twitch types. However, the contractile responses to long-term unloading is still unclear. Fifteen adult Wistar rats were studied in 45 and 70 days of muscle unweighting and soleus (SOL) muscle as well as extensor digitorum longus (EDL) were prepared for electrophysiological recordings (single, twitch, tetanic contraction and fatigue) and histochemical stainings. The loss of muscle mass observed was greater in the soleus muscle. The analysis of electrophysiological properties of both EDL and SOL showed significant main effects of group, of number of unweighting days and fatigue properties. Single contraction for soleus muscle remained unchanged but there was statistically significant difference for tetanic contraction and fatigue. Fatigue index showed a decrease for the control rats, but increase for the HLS rats. According to the histochemical findings there was a shift from oxidative to glycolytic metabolism during HLS. The data suggested that muscles atrophied, but they presented an adaptation pattern, while their endurance in fatigue was decreased.

Phosphorylation of insulin receptor substrate-1 serine 307 correlates with JNK activity in atrophic skeletal muscle

c-Jun NH(2)-terminal kinase (JNK) has been shown to negatively regulate insulin signaling through serine phosphorylation of residue 307 within the insulin receptor substrate-1 (IRS-1) in adipose and liver tissue. Using a rat hindlimb suspension model for muscle disuse atrophy, we found that JNK activity was significantly elevated in atrophic soleus muscle and that IRS-1 was phosphorylated on Ser(307) prior to the degradation of the IRS-1 protein. Moreover, we observed a corresponding reduction in Akt activity, providing biochemical evidence for the development of insulin resistance in atrophic skeletal muscle.

Strength training counteracts motor performance losses during bed rest

The purpose of the study was to determine the effect of bed rest with or without strength training on torque fluctuations and activation strategy of the muscles. Twelve young men participated in a 20-day bed rest study. Subjects were divided into a non-training group (BRCon) and a strength-training group (BRTr). The training comprised dynamic calf-raise and leg-press exercises. Before and after bed rest, subjects performed maximal contractions and steady submaximal isometric contractions of the ankle extensor muscles and of the knee extensor muscles (2.5-10% of maximal torque). Maximal torque decreased for both the ankle extensors (9%, P < 0.05) and knee extensors (16%, P < 0.05) in BRCon but not in BRTr. For the ankle extensors, the coefficient of variation (CV) for torque increased in both groups (P < 0.05), with a greater amount (P < 0.05) in BRCon (88%) compared with BRTr (41%). For the knee extensors, an increase in the CV for torque was observed only in BRCon (22%). The increase in the CV for torque in BRCon accompanied the greater changes in electromyogram amplitude of medial gastrocnemius (122%) and vastus lateralis (59%) compared with BRTr (P < 0.05). The results indicate that fluctuations in torque during submaximal contractions of the extensor muscles in the leg increase after bed rest and that strength training counteracted the decline in performance. The response varied across muscle groups. Alterations in muscle activation may lead to an increase in fluctuations in motor output after bed rest.

Muscles in microgravity: from fibres to human motion

In simulated or actual microgravity, human and animal postural muscles undergo substantial atrophy: after about 270 days, the muscle mass attains a constant value of about 70% of the initial one. Most animal studies reported preferential atrophy of slow twitch fibres whose mechanical properties change towards the fast type. However, in humans, at the end of a 42-days bed rest study, a similar atrophy of slow and fast fibres was observed. After microgravity, the maximal force of several muscle groups showed a substantial decrease (6-25% of pre-flight values). The maximal power during very short "explosive" efforts of 0.25-0.30s showed an even greater fall, being reduced to 65% after 1 month and to 45% (of pre-flight values) after 6 months. The maximal power developed during 6-7s "all-out" bouts on an isokinetic cycloergometer was reduced to a lesser extent, attaining about 75% of pre-flight values, regardless of the flight duration. In these same subjects, the muscle mass of the lower limbs declined by only 9-13%. Thus, a substantial fraction of the observed decreases of maximal power is probably due to a deterioration of the motor co-ordination brought about by the absence of gravity. To prevent this substantial decay of maximal absolute power, we propose that explosive exercise be added to the daily in-flight training schedule. We also describe a system aimed at reducing cardiovascular deconditioning wherein gravity is simulated by the centrifugal acceleration generated by the motion of two counter rotating bicycles ridden by the astronauts on the inner wall of a cylindrical space module. Finally, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning in permanently manned lunar bases. Indeed, on the curved parts of the path, a cyclist generates an outward acceleration vector (ac). To counterbalance ac, the cyclist must lean inwards, so that the vectorial sum of ac plus the lunar gravity tends to the acceleration of gravity prevailing on Earth.

Effects of microgravity on myogenic factor expressions during postnatal development of rat skeletal muscle

To clarify the role of gravity in the postnatal development of skeletal muscle, we exposed neonatal rats at 7 days of age to microgravity. After 16 days of spaceflight, tibialis anterior, plantaris, medial gastrocnemius, and soleus muscles were removed from the hindlimb musculature and examined for the expression of MyoD-family transcription factors such as MyoD, myogenin, and MRF4. For this purpose, we established a unique semiquantitative method, based on RT-PCR, using specific primers tagged with infrared fluorescence. The relative expression of MyoD in the tibialis anterior and plantaris muscles and that of myogenin in the plantaris and soleus muscles were significantly reduced (P < 0.001) in the flight animals. In contrast, MRF4 expression was not changed in any muscle. These results suggest that MyoD and myogenin, but not MRF4, are sensitive to gravity-related stimuli in some skeletal muscles during postnatal development.

Skeletal muscle adaptations with age, inactivity, and therapeutic exercise

One of the remarkable features of skeletal muscle is its adaptability. Skeletal muscle adaptations are characterized by modifications of morphological, biochemical, and molecular variables that alter the functional attributes of specific skeletal muscle fiber types. Skeletal muscle adaptation is diverse and the magnitude of change is dependent on many factors, such as activity pattern, age, and muscle fiber type composition. The adaptation of skeletal muscle in the adult population is well described. In contrast, the adaptation of skeletal muscle in the older population is less documented, especially in the area of inactivity-induced alterations. Age-related changes in skeletal muscle may play a significant role in the magnitude of change with inactivity and influence the rehabilitation process for the older adult. A consistent feature of age and inactivity is limb muscle atrophy and the loss of peak force and power. Differences exist in the rate and mechanisms of muscle wasting and in the susceptibility of a given fiber type to atrophy. Most likely, the rapid muscle wasting might be in part due to a decrease in protein synthesis coupled with an increased degradation. Besides the quantitative change in muscle mass, age and inactivity induce important qualitative changes in the structure of key skeletal muscle proteins that are manifested in alterations in contractile properties. Therefore, the purpose of this clinical commentary is to identify the major effects of age and inactivity on skeletal muscle structure and function, and discuss potential therapeutic interventions. Special emphasis will be placed on how alterations in muscle structure affect function and on the cellular and molecular mechanisms of the age-related and inactivity-induced muscle changes.

Gravitational unloading effects on muscle fiber size, phenotype and myonuclear number

The effects of gravitational unloading with or without intact neural activity and/or tension development on myosin heavy chain (MHC) composition, cross-sectional area (CSA), number of myonuclei, and myonuclear domain (cytoplasmic volume per myonucleus ratio) in single fibers of both slow and fast muscles of rat hindlimbs are reviewed briefly. The atrophic response to unloading is generally graded as follows: slow extensors > fast extensors > fast flexors. Reduction of CSA is usually greater in the most predominant fiber type of that muscle. The percentage of fibers expressing fast MHC isoforms increases in unloaded slow but not fast muscles. Myonuclear number per mm of fiber length and myonuclear domain is decreased in the fibers of the unloaded predominantly slow soleus muscle, but not in the predominantly fast plantaris. Decreases in myonuclear number and domain, however, are observed in plantaris fibers when tenotomy, denervation, or both are combined with hindlimb unloading. All of these results are consistent with the view that a major factor for fiber atrophy is an inhibition or reduction of loading of the hindlimbs. These data also indicate that predominantly slow muscles are more responsive to unloading than predominantly fast muscles.

Functional and structural adaptations of skeletal muscle to microgravity

Our purpose is to summarize the major effects of space travel on skeletal muscle with particular emphasis on factors that alter function. The primary deleterious changes are muscle atrophy and the associated decline in peak force and power. Studies on both rats and humans demonstrate a rapid loss of cell mass with microgravity. In rats, a reduction in muscle mass of up to 37% was observed within 1 week. For both species, the antigravity soleus muscle showed greater atrophy than the fast-twitch gastrocnemius. However, in the rat, the slow type I fibers atrophied more than the fast type II fibers, while in humans, the fast type II fibers were at least as susceptible to space-induced atrophy as the slow fiber type. Space flight also resulted in a significant decline in peak force. For example, the maximal voluntary contraction of the human plantar flexor muscles declined by 20–48% following 6 months in space, while a 21% decline in the peak force of the soleus type I fibers was observed after a 17-day shuttle flight. The reduced force can be attributed both to muscle atrophy and to a selective loss of contractile protein. The former was the primary cause because, when force was expressed per cross-sectional area (kNm−2), the human fast type II and slow type I fibers of the soleus showed no change and a 4% decrease in force, respectively. Microgravity has been shown to increase the shortening velocity of the plantar flexors. This increase can be attributed both to an elevated maximal shortening velocity (V0) of the individual slow and fast fibers and to an increased expression of fibers containing fast myosin. Although the cause of the former is unknown, it might result from the selective loss of the thin filament actin and an associated decline in the internal drag during cross-bridge cycling. Despite the increase in fiber V0, peak power of the slow type I fiber was reduced following space flight. The decreased power was a direct result of the reduced force caused by the fiber atrophy. In addition to fiber atrophy and the loss of force and power, weightlessness reduces the ability of the slow soleus to oxidize fats and increases the utilization of muscle glycogen, at least in rats. This substrate change leads to an increased rate of fatigue. Finally, with return to the 1g environment of earth, rat studies have shown an increased occurrence of eccentric contraction-induced fiber damage. The damage occurs with re-loading and not in-flight, but the etiology has not been established.

Physiology of a microgravity environment invited review: microgravity and skeletal muscle

Spaceflight (SF) has been shown to cause skeletal muscle atrophy; a loss in force and power; and, in the first few weeks, a preferential atrophy of extensors over flexors. The atrophy primarily results from a reduced protein synthesis that is likely triggered by the removal of the antigravity load. Contractile proteins are lost out of proportion to other cellular proteins, and the actin thin filament is lost disproportionately to the myosin thick filament. The decline in contractile protein explains the decrease in force per cross-sectional area, whereas the thin-filament loss may explain the observed postflight increase in the maximal velocity of shortening in the type I and IIa fiber types. Importantly, the microgravity-induced decline in peak power is partially offset by the increased fiber velocity. Muscle velocity is further increased by the microgravity-induced expression of fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by the increased expression of fast type II fiber types. SF increases the susceptibility of skeletal muscle to damage, with the actual damage elicited during postflight reloading. Evidence in rats indicates that SF increases fatigability and reduces the capacity for fat oxidation in skeletal muscles. Future studies will be required to establish the cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.

How the science and engineering of spaceflight contribute to understanding the plasticity of spinal cord injury

Space programs support experimental investigations related to the unique environment of space and to the technological developments from many disciplines of both science and engineering that contribute to space studies. Furthermore, interactions between scientists, engineers and administrators, that are necessary for the success of any science mission in space, promote interdiscipline communication, understanding and interests which extend well beyond a specific mission. NASA-catalyzed collaborations have benefited the spinal cord rehabilitation program at UCLA in fundamental science and in the application of expertise and technologies originally developed for the space program. Examples of these benefits include: (1) better understanding of the role of load in maintaining healthy muscle and motor function, resulting in a spinal cord injury (SCI) rehabilitation program based on muscle/limb loading; (2) investigation of a potentially novel growth factor affected by spaceflight which may help regulate muscle mass; (3) development of implantable sensors, electronics and software to monitor and analyze long-term muscle activity in unrestrained subjects; (4) development of hardware to assist therapies applied to SCI patients; and (5) development of computer models to simulate stepping which will be used to investigate the effects of neurological deficits (muscle weakness or inappropriate activation) and to evaluate therapies to correct these deficiencies.

Survey of studies on how spaceflight affects rodent skeletal muscle

Rodent muscles have been examined in more than 89 spaceflight studies over the last 25 years with much variation in the procedures and results. Mission duration ranged from four days to three weeks, postflight data collection ranged from a few hours to two days after landing, and there is great diversity in the number, size, and age of the rats that have flown. Several different types and sizes of animal enclosures have also been used--a significant factor because cage design affects animal activity and muscle loading. Only a small percentage (approximately 16%) of the total number of striated muscles in the rat have been examined. We have identified both substantial redundancy and inconsistencies in the results from studies to date. However, many of these appear unavoidable due to the great variation in experimental protocol of the different missions. Nevertheless these studies repeatedly confirm that exposure to spaceflight decreases the mass of limb muscles and leads to muscle atrophy. The majority of missions were flown by the former Soviet Union, but the majority of papers have been published by U.S. researchers. A relatively small number of investigators (about 50) clustered into fewer than 15 identifiable research groups worldwide account for most of the results to date. These groups have had access to rodent muscle tissue from two to seven spaceflights each. International cooperation in the post-cold war era and the publication of future work in peer-reviewed international journals should help greatly in reducing redundancy and enriching our knowledge of how gravity affects biological systems.

Effects of laparotomy, cage type, gestation period and spaceflight on abdominal muscles of pregnant rodents

We studied the effects of four variables on the histological properties of three body wall muscles-rectus abdominis (RA), transversus abdominis (TA), and external oblique (EO)-from pregnant rats. The variables examined were (1) gestation period; (2) cage design; (3) the effect of a midline laparotomy, performed to determine fetus numbers; and (4) exposure to a nine-day spaceflight. We measured fiber cross-sectional area (CSA), metabolic enzyme levels (succinate dehydrogenase, glycerophosphate dehydrogenase), and myosin heavy chain (MHC) immunoreactivity in samples from each muscle. A major effect of spaceflight was an increase of 42-171% in fibers double-labeled for MHC in all three muscles. Based on fiber CSA, the TA and RA muscles showed signs of stretching with increased gestation; i.e., the CSA decreased 11-12% over a nine-day period. The EO, a torso rotator, hypertrophied by 9% in rats group-housed in cages with a complex 3-D structure, compared to controls housed singly in standard flat-bottom cages. The TA and EO, whose contractions would pull on the suture line, showed signs of atrophy in laparotomized animals, exhibiting a 12% decrease in muscle fiber CSA. Exposure to weightlessness is known to induce atrophy in most skeletal muscles. Surprisingly, the EO actually hypertrophied 11% in our flight animals; however, this can be explained by the fact that those rats actively rotated their torsos seven times more often than ground controls. The flight rats also had twice as many contractions as controls. However, they were still able to give birth on time postflight.

Effects of pre- and perinatal exposure to hypergravity on muscular structure development in rat

This study evaluated the influence of precocious exposure to hypergravity on the expression of myosin heavy chain (MHC) protein isoforms in nape, masticatory and respiratory developmental rat muscles. Pregnant females were maintained at 1.8 g from the 11th day of gestation to the 7th day after birth. The 7-day-old rats were used for muscle sampling. Hypergravity induced a marked decrease in the weight and protein content of all six muscles. Three MHC isoforms were detected in the young rats' muscles: embryonic (E), perinatal (P) and slow type 1 MHC. In centrifuged nape and masticatory muscles, there was a decrease in MHC E and an increase in P without reduction (indeed, even an increase) in MHC 1, whereas in the respiratory muscle MHC E was increased and MHC 1 decreased. These results indicate that hypergravity produces important changes in the contractile properties not only of antigravity muscles but also masticatory and respiratory muscles. MHC P has a higher shortening velocity than MHC E, which has a higher one than MHC 1. The hypergravity-induced transformations of MHC isoforms would thus lead to increased velocity of all muscles studied. In spite of the observation of a hypergravity-induced muscle hypotrophy, the results of this study reflect the adaptational properties of developing muscles to increased gravitational forces.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Cytoplasm-to-myonucleus ratios following microgravity

The cytoplasmic volume-to-myonucleus ratio in the tibialis anterior and gastrocnemius muscles of juvenile rats after 5.4 days of microgravity was studied. Three groups of rats (n = 8 each) were used. The experimental group (space rats) was flown aboard the space shuttle Discovery (NASA, STS-48), while two ground-based groups, one hindlimb suspended (suspended rats), one non-suspended (control), served as controls. Single fibre analysis revealed a significant decrease in cross-sectional area (microns2) in the gastrocnemius for both the space and the suspended rats; in the tibialis anterior only the suspended rats showed a significant decrease. Myonuclei counts (myonuclei per mm) in both the tibialis anterior and gastrocnemius were significantly increased in the space rats but not in the suspended rats. The mean myonuclear volume (individual nuclei: microns3) in tibialis anterior fibres from the space rats, and in gastrocnemius fibres from both the space and the suspended rats, was significantly lower than that in the respective control group. Estimation of the total myonuclear volume (microns3 per.mm), however, revealed no significant differences between the three groups in either the tibialis anterior or gastrocnemius. The described changes in the cross-sectional area and myonuclei numbers resulted in significant decreases in the cytoplasmic volume-to-myonucleus ratio (microns3 x 10(3)) in both muscles and for both space and suspended rats (tibialis anterior; 15.6 +/- 0.6 (space), 17.2 +/- 1.0 (suspended), 20.8 +/- 0.9 (control): gastrocnemius; 13.4 +/- 0.4 (space) and 14.9 +/- 1.1 (suspended) versus 18.1 +/- 1.1 (control)). These results indicate that even short periods of unweighting due to microgravity or limb suspension result in changes in skeletal muscle fibres which lead to significant decreases in the cytoplasmic volume-to-myonucleus ratio.

Neuromuscular adaptation to actual and simulated weightlessness

The chronic "unloading" of the neuromuscular system during spaceflight has detrimental functional and morphological effects. Changes in the metabolic and mechanical properties of the musculature can be attributed largely to the loss of muscle protein and the alteration in the relative proportion of the proteins in skeletal muscle, particularly in the muscles that have an antigravity function under normal loading conditions. These adaptations could result in decrements in the performance of routine or specialized motor tasks, both of which may be critical for survival in an altered gravitational field, i.e., during spaceflight and during return to 1 G. For example, the loss in extensor muscle mass requires a higher percentage of recruitment of the motor pools for any specific motor task. Thus, a faster rate of fatigue will occur in the activated muscles. These consequences emphasize the importance of developing techniques for minimizing muscle loss during spaceflight, at least in preparation for the return to 1 G after spaceflight. New insights into the complexity and the interactive elements that contribute to the neuromuscular adaptations to space have been gained from studies of the role of exercise and/or growth factors as countermeasures of atrophy. The present chapter illustrates the inevitable interactive effects of neural and muscular systems in adapting to space. It also describes the considerable progress that has been made toward the goal of minimizing the functional impact of the stimuli that induce the neuromuscular adaptations to space.