Microgravity-Induced Fiber Type Shift in Human Skeletal Muscle

Prolonged microgravity exposure alters human skeletal muscle by markedly reducing size, function, and metabolic capacity. Preserving skeletal muscle health presents a major challenge to space exploration beyond low Earth orbit. Humans express three distinct pure myosin heavy chain (MHC) muscle fiber types (slow fast: MHC I, IIa, and IIx), along with hybrids (MHC I/IIa, IIa/IIx, and I/IIa/IIx). After reviewing current research, this paper presents evidence for a “slow to fast” microgravity-induced skeletal muscle fiber type shift in humans. Spaceflight and bed rest induce decreased MHC I fiber proportion while increasing fast hybrid types (particularly MHC IIa/IIx fibers). This alteration in muscle cell phenotype negatively impacts performance and induces undesirable metabolic adaptations. While exercise has been postulated to minimize the negative effects of microgravity on human muscle, past spaceflight countermeasures have insufficiently prevented fiber type shifts in humans. However, a new high-intensity, low volume resistance and aerobic exercise regimen has recently been implemented aboard the International Space Station (ISS). This paper aims to reveal that 1) a slow to fast microgravityinduced fiber type shift occurs in humans and 2) the new high-intensity, low volume exercise countermeasures program onboard the ISS has promise to mitigate this fiber type transition and preserve skeletal muscle health.