Transmission electron micrograph (TEM) of muscle fibres of an astronaut before (top) and after (bottom) a spaceflight. The stimulus of gravity affects RNA (ribonucleic acid) production, which helps maintain the strength of human muscles on Earth. Astronauts in orbit and patients on Earth fighting muscle-wasting diseases need countermeasures to prevent muscle atrophy, indicated here by white lipid (fat) droplets in the muscle sample after spaceflight. Muscle proteins are what give muscles their strength, so when the RNA blueprints aren't available for producing new proteins to replace old ones, as occurs in microgravity, the muscles atrophy. When the skeletal muscle system is exposed to microgravity during spaceflight, the muscles undergo a reduction in mass that translates to a reduction in strength. When this happens, muscle endurance decreases and the muscles are more prone to injury. This could cause problems for astronauts performing space walks or emergency egress because their bodies are functionally compromised.