Effects of Microgravity on Muscle and Bone
Writer’s Comment: This literature review for UWP 104E (Writing in Science) was our first research-intensive assignment. Having always been simultaneously fascinated and skeptical about how accurate depictions of space travel are in science fiction, this was the perfect opportunity for me to build my knowledge base on something I truly enjoy. Upon early investigation I became intrigued by the fact that not only muscles, but bones also weaken while astronauts are in space. It’s logical that heavy, strong bones aren’t needed in an environment without gravity, but the proposed mechanisms behind the bone weakening grabbed my interest. My passion for the subject and Aliki Dragona’s fun-loving attitude fostered the creativity and confidence I needed to pull the ideas together and create a cohesive, understandable, and interesting literature review.
Instructor’s Comment: “What do you know about the amount of muscle and bone astronauts lose due to the microgravity environment in space?” Dennis asked me. In UWP 104E, I request that my students think of a single topic to pursue for their four papers, all written on that topic but for different audiences and purposes. “Absolutely nothing,” I replied. And then Dennis proceeded to write four elegant papers as well as to give a clear oral presentation. His literature review manifests his passion for the research, his ability to synthesize complex information clearly while always keeping his readers in mind, and his elegance as a writer. If, like me, you knew nothing about bone degradation in space and about ongoing research meant to help astronauts maintain their muscle mass and bone density, here is your chance by reading Dennis’ wonderful literature review.
—Aliki Dragona, University Writing Program
Muscle and bone atrophy is an alarming threat to astronauts on lengthy missions in space. In low gravity environments muscles become less active, impairing protein synthesis and the breakdown of thin filament actin. These effects lead to degraded muscle performance. Exercise regimens have been shown to lessen the amount of muscle lost due to the microgravity environment in space, but they aren’t entirely effective. Further study is needed to understand the cellular mechanisms underlying this muscle loss. Bone loss in microgravity is attributed to a cephalic shift in fluid and to slowed bone marrow mesenchymal stem cell differentiation into bone-forming osteoblast cells. Mechanisms for these changes have been suggested, but further studies are needed so tools can be developed to combat these changes and protect the bones from degradation.
Astronauts face enormous obstacles and dangers while in space. Keeping astronauts alive in the vast nothingness requires the most advanced technology available. They are protected from the obvious physical harshness of the environment, but an invisible danger awakens during extended space flights. When astronauts return from a six-month or longer voyage, their bone densities have dropped and their muscles have atrophied. These effects of microgravity in space have become a topic of great interest. Unfortunately there is no way to accurately replicate microgravity on earth and long trials in space are extremely expensive. These obstacles have made progress in this field slow and difficult. By fully understanding the mechanisms behind the degradation of muscle and bone in space, scientists can develop new tools to keep astronauts healthy during long space missions.
Microgravity and Muscle
Astronauts returning from space often reported a delayed onset of muscle soreness (Blaber et al. 2010). This led to studies in which muscles were observed before and after spaceflight. Without gravity and constant use, muscles experience a decline in protein synthesis and a degradation of actin thin filaments (Fitts et al. 2000). These filaments are important contractile proteins in the cell, and when they are degraded, muscle stamina, contractile velocity, and contractile strength suffer (Fitts et al. 2000). Studies on rats in low-gravity environments led to the conclusion that slow type I muscle fibers experienced more atrophy than fast type II muscle fibers. However, in humans slow type I muscle fibers and fast type II muscle fibers experience similar atrophy (Fitts et al. 2000, Riley et al. 2000). The cellular mechanisms behind all these changes are unknown (Fitts et al. 2000, Riley et al. 2000).
Studies showed that weight-bearing muscles and those that help with posture experienced more mass loss than other muscles (Fitts et al. 2000; Trappe et al. 2009). The weight-bearing calf muscle is easily measured and typically used for studies of muscle loss in space. To combat the muscle loss, astronauts followed an exercise program. After 6 months in space, astronauts who exercised consistently lost 13% of their calf muscle mass. Different exercise regimens showed drastically different results in limiting calf muscle loss. Astronauts using the treadmill more than 200 minutes a week lost only 7% of their calf muscle mass, while those using the treadmill fewer than 100 minutes a week lost 17% (Trappe et al. 2009). Future exercise regimen studies can determine which exercises are most beneficial in preventing muscle atrophy in space. Further cellular studies on microgravity effects on muscle are needed and may lead to hormonal or steroid measures to prevent muscle atrophy (Fitts et al. 2000).
Microgravity and Bone
Studies on Humans
As with muscles, bones that bear weight are affected most by microgravity. The tibial site of the leg showed bone loss within the first month of spaceflight while the radius bones of the arm showed no major changes (Vico et al. 2000). Early reports suggested that this is due to astronauts using their arms more than their legs in space. More recent studies have focused on the cellular mechanism behind this bone loss. Osteoblasts are the cells responsible for bone formation, while osteoclast cells break the bone down. The Bone Marrow Mesenchymal Stem Cells (BMSCs) that differentiate into osteoblast cells react differently in microgravity, causing osteoblast cell populations to fall. Within a BMSC, changes in protein transcription alter the receptor proteins in the plasma membrane, making it less receptive to growth factors that signal differentiation into osteoblast cells (Dai et al. 2007). Since osteoblast populations decline and osteoclast populations remain stable, the bone is broken down more quickly than it can be rebuilt (Dai et al. 2007; Blaber et al. 2010). Upon returning to gravity after a space mission, astronauts’ bones showed various rates of healing or no healing at all (Vico et al. 2000). This data suggests that the bone loss experienced in microgravity may be permanent.
The body is built to remain in homeostasis in gravity’s presence. Without gravity, the fluid dynamics of plasma flow in the circulatory systems change. A microgravity environment causes a cephalic fluid shift that effectively pumps more blood to the upper region of the body and less blood to the legs (Blaber et al. 2010). This explains the pattern of major bone loss in the legs.
Studies on Mice
Studies of bone loss are most commonly done on mice; rather than send mice into space, researchers use an effective and cheaper way to simulate microgravity on Earth. A method known as hind-limb unloading (HU) simulates the conditions of space flight by lifting mice by the tail with traction tape, leaving only the front legs on the ground. This takes the weight off the back legs while keeping weight on the front legs, allowing the mice to reach their food more normally. Also, this causes the cephalic fluid shift, as gravity pulls the plasma toward the head of the mice (Morey-Holton et al. 1998; Colleran et al. 2000). HU mice have a drastic increase in vascular resistance in the leg (+30mmHg/mL/min/100g), which leads to a major decrease in blood flow to the bone marrow (-50mL/min/100g) (Colleran et al. 2000). The fall in blood flow leads to an increased concentration of metabolites, which, in turn, leads to lower numbers of osteoblasts and decreased osteoclast inhibition. In 28 days the femur lost 11% of its mass while the mandible gained 10% in mass (Colleran et al. 2000). Clearly blood flow directly affects osteoblast populations and bone mass.
The effects of HU are more dramatic in young, developing mice. In 6-week old mice, osteoblast density drops 77% after 14 days in a microgravity environment as opposed to a 17% drop in 6-month old mice (Basso et al. 2004). This could be a critical area of research in the future if spaceflight becomes accessible to the general public. The risk to a child’s bones must be better understood before children and pregnant women can fly into space.
Protecting astronaut health has been a significant consideration in lengthening space missions. The lack of exercise against gravity leads to muscle atrophy. The cephalic fluid shift and BMSC characteristics in microgravity lead to weaker bones and possibly permanent osteoporosis. While these changes may be bearable in space, an extended space flight may leave the astronaut debilitated when returned to gravity. More research can further disclose the mechanisms behind bone and muscle loss and lead to a better understanding of how to minimize or even eliminate the damage done in space.
Basso, N., Bellows, C.G., and Heersche, J.N. 2005. Effect of simulated weightlessness on osteoprogenitor cell number and proliferation in young and adult rats. Bone 36:173–183.
Blaber, E., Marcal, H., Burns, B.P. 2010. Bioastronautics: The influence of Microgravity on Astronaut Health. Astrobiology. 10(5):463-73.
Colleran, P.N., Wilkerson, M.K.., Bloomsfield, S.A., Suva, L.J., Turner, R.T., and Delp, M.D. 2000. Alterations in skeletal perfusion with stimulated microgravity: a possible mechanism for bone remodeling. J. Appl. Physiol. 89:1046-1054.
Dai, Z.Q., Wang, R., Ling, S.K., Wan, Y.M., and Li, Y.H. 2007. Simulated microgravity inhibits the proliferation and osteogenesis of rat bone marrow mesenchymal stem cells. Cell Prolif. 40:671–684.
Fitts, R.H., Riley, D.R., Widrick, J.J. 2000. Physiology of a Microgravity Environment: Microgravity and Skeletal Muscle. J. Appl. Physiol. 89:823-839.
Morey-Holton, E.R. and Globus, R.K. 1998. Hindlimb unloading of growing rats: a model for predicting skeletal changes during space flight. Bone 22:83S–88S.
Riley DA, Bain JLW, Thompson JL, Fitts RH, Widrick JJ, Trappe SW, Trappe TA, and Costill DL. 2000. Decreased thin filament density and length in human atrophic soleus muscle fibers after spaceflight. J Appl Physiol 88: 567–572.
Trappe, S., Costill, D., Gallagher, P., Creer, A., Peters, J.R., Evans, H., Riley, D.A., and Fitts, R.H. 2009. Exercise in space: human skeletal muscle after 6 months aboard the International Space Station. J. Appl. Physiol. 106:1159–1168.
Vico, L., Collet, P., Guignandon, A., Lafage-Proust, M.H., Thomas, T., Rehaillia, M., and Alexandre, C. 2000. Effects of longterm microgravity exposure on cancellous and cortical weightbearing bones of cosmonauts. Lancet 355:1607–1611.