Physiology of a Microgravity Environment Invited Review: Microgravity and skeletal muscle

Abstract

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.

Spaceflight results in increase of thick filament but not thin filament proteins in the paramyosin mutant of Caenorhabditis elegans

“…Although we are not normally aware of this force, major biological changes in a number of organ systems occur in microgravity (Fitts et al, 2000;Turner, 2000). One of the most affected is the neuromuscular system.…”

“…Weightlessness induces atrophy of skeletal muscle (Riley et al, 1987;Widrick et al, 1999) and cardiovascular deconditioning (Philpott et al, 1990;Antonutto and Prampero, 2003). Weightlessness also reduces functional capacity in limb skeletal muscle of animals including humans (Ilyina-Kakueva et al, 1976;Widrick et al, 1999;Fitts et al, 2000). In vertebrate skeletal muscle, the greatest changes are observed in the limb antigravity muscles, such as soleus (Riley et al, 1987;Widrick et al, 1999;Harrison et al, 2003).…”

Parallels between astronauts and terrestrial patients – Taking physiotherapy rehabilitation “To infinity and beyond”

“…Progressive muscular atrophy in microgravity has been documented by several researchers (diPrampero & Narici 2003;Fitts et al, 2000;Akima et al, 2000;Riley, 1999Vandenberg, 1999;Leblanc et al, 1995). Without countermeasures, muscle mass has been shown to plateau at two thirds of the initial mass after about 270 days (diPrampero & Narici 2003).…”

“…Without countermeasures, muscle mass has been shown to plateau at two thirds of the initial mass after about 270 days (diPrampero & Narici 2003). Atrophy has been shown to be greatest in postural antigravity muscles (diPrampero & Narici 2003;Fitts et al, 2000;Tesch et al 2005).…”

Subcellular proteomics of mice gastrocnemius and soleus muscles

“…Human skeletal muscle is very heterogeneous in composition, being constituted by different types of muscle fibers showing significant differences in their contractile speed and metabolic profile that result from specific protein expression [1][2][3][4]. In rodents, especially mice and rats, muscle fibers present a more uniform distribution among the different muscles, allowing the use of the entire muscles to study specific phenotypes.…”