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 1 g 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

Peak Force and Maximal Shortening Velocity of Soleus Fibers after Non-Weight-bearing and Resistance Exercise

Widrick, Jeffrey J., and Robert H. Fitts. Peak force and maximal shortening velocity of soleus fibers after non-weight-bearing and resistance exercise. J. Appl. Physiol. 82(1): 189–195, 1997.—This study examined the effectiveness of resistance exercise as a countermeasure to non-weight-bearing-induced alterations in the absolute peak force, normalized peak force (force/fiber cross-sectional area), peak stiffness, and maximal shortening velocity (V o) of single permeabilized type I soleus muscle fibers. Adult rats were subjected to one of the following treatments: normal weight bearing (WB), non-weight bearing (NWB), or NWB with exercise treatments (NWB+Ex). The hindlimbs of the NWB and NWB+Ex rats were suspended for 14 days via tail harnesses. Four times each day, the NWB+Ex rats were removed from suspension and performed 10 climbs (∼15 cm each) up a steep grid with a 500-g mass (∼1.5 times body mass) attached to their tail harness. NWB was associated with significant reductions in type I fiber diameter, absolute force, normalized force, and stiffness. Exercise treatments during NWB attenuated the decline in fiber diameter and absolute force by almost 60% while maintaining normalized force and stiffness at WB levels. Type I fiberV oincreased by 33% with NWB and remained at this elevated level despite the exercise treatments. We conclude that in comparison to intermittent weight bearing only (J. J. Widrick, J. J. Bangart, M. Karhanek, and R. H. Fitts. J. Appl. Physiol. 80: 981–987, 1996), resistance exercise was more effective in attenuating alterations in type I soleus fiber absolute force, normalized force, and stiffness but was less effective in restoring type I fiberV oto WB levels

Effect of Intermittent Weight Bearing on Soleus Fiber Force-velocity-power and Force-pCa Relationships

Rat permeabilized type I soleus fibers displayed a 33% reduction in peak power output and a 36% increase in the free Ca2+ concentration required for one-half maximal activation after 14 days of hindlimb non-weight bearing (NWB). We examined the effectiveness of intermittent weight bearing (IWB; consisting of four 10-min periods of weight bearing/day) as a countermeasure to these functional changes. At peak power output, type I fibers from NWB animals produced 54% less force and shortened at a 56% greater velocity than did type I fibers from control weight-bearing animals while type I fibers from the IWB rats produced 26% more absolute force than did fibers from the NWB group and shortened at a velocity that was only 80% of the NWB group mean. As a result, no difference was observed in the average peak power of fibers from the IWB and NWB animals. Hill plot analysis of force-pCa relationships indicated that fibers from the IWB group required similar levels of free Ca2+ to reach half-maximal activation in comparison to fibers from the weight-bearing group. However, at forces 1) attenuated the NWB-induced reduction in fiber Ca2+ sensitivity but 2) failed to prevent the decline in peak power that occurs during NWB because of opposing effects on fiber force (an increase vs. NWB) and shortening velocity (a decrease vs. NWB)

Soleus Fiber Force and Maximal Shortening Velocity After Non-Weight Bearing with Intermittent Activity

This study examined the effectiveness of intermittent weight bearing (IWB) as a countermeasure to non-weight-bearing (NWB)-induced alterations in soleus type 1 fiber force (in mN), tension (P(sub o); force per fiber cross-sectional area in kN/sq m), and maximal unloaded shortening velocity (V(sub o), in fiber lengths/s). Adult rats were assigned to one of the following groups: normal weight bearing (WB), 14 days of hindlimb NWB (NWB group), and 14 days of hindlimb NWB with IWB treatments (IWB group). The IWB treatment consisted of four 10-min periods of standing WB each day. Single, chemically permeabilized soleus fiber segments were mounted between a force transducer and position motor and were studied at maximal Ca(2+) activation, after which type 1 fiber myosin heavy-chain composition was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. NWB resulted in a loss in relative soleus mass (-45%), with type 1 fibers displaying reductions in diameter (-28%) and peak isometric force (-55%) and an increase in V(sub o) (+33%). In addition, NWB induced a 16% reduction in type 1 fiber P., a 41% reduction in type 1 fiber peak elastic modulus [E(sub o), defined as ((delta)force/(delta)length x (fiber length/fiber cross-sectional area] and a significant increase in the P(sub o)/E(sub o) ratio. In contrast to NWB, IWB reduced the loss of relative soleus mass (by 22%) and attenuated alterations in type 1 fiber diameter (by 36%), peak force (by 29%), and V(sub o)(by 48%) but had no significant effect on P(sub o), E(sub o) or P(sub o)/E(sub o). These results indicate that a modest restoration of WB activity during 14 days of NWB is sufficient to attenuate type 1 fiber atrophy and to partially restore type 1 peak isometric force and V(sub o) to WB levels. However, the NWB-induced reductions in P(sub o) and E(sub o) which we hypothesize to be due to a decline in the number and stiffness of cross bridges, respectively, are considerably less responsive to this countermeasure treatment

Space Flight Human System Standards (SFHSS)

This viewgraph presentation reviews the standards for space flight hardware based on human capabilities and limitations. The contents include: 1) Scope; 2) Applicable documents; 3) General; 4) Human Physical Characteristics and Capabilities; 5) Human Performance and Cognition; 6) Natural and Induced Environments; 7) Habitability Functions; 8) Architecture; 9) Hardware and Equipment; 10) Crew Interfaces; 11) Spacesuits; 12) Operatons: Reserved; 13) Ground Maintenance and Assembly: Reserved; 14) Appendix A-Reference Documents; 15) Appendix N-Acronyms and 16) Appendix C-Definition. Volume 2 is supported by the Human Integration Design Handbook (HIDH)s

Developing EnviroSuite Resources at the National Synchrotron Light Source

The objective of Brookhaven National Laboratory's EnviroSuite Initiative is to develop the facilities, user support infrastructure, and techniques necessary to conduct world-class molecular environmental science research at the NSLS. This is intended to benefit the research of ERSD-supported scientists, both through direct access and assistance and through the indirect benefits of a broader network of environmental scientists as collaborators and users. Much of the EnviroSuite research involves close collaboration with members of the Center for Environmental Molecular Science (CEMS), an EMSI based at BNL and nearby Stony Brook University and jointly supported by ERSD (Project 1023761, P. Kalb) and NSF. This offers unique opportunities to benefit from both national laboratory facilities and university resources. Other collaborators, from around the US and the world, investigate various aspects of the underlying molecular-scale processes in complex natural systems. In general, synchrotron techniques are ideal for studying the molecular-scale structures, chemical/physical interactions, and transformations that govern the macroscopic properties and processes (e.g. transport, bioavailability) of contaminants in the environment. These techniques are element-specific, non-destructive, and sensitive to the very low concentrations found in real-world samples