The human body is designed to live in Earth's gravity. But in space, the body begins to adapt to the micro gravity environment. In the zero gravity of space, astronauts face many challenges. Astronauts have experienced significant muscle and bone changes even after spending as little as four or five days in space. More than half the muscles in the human body are designed primarily to combat gravity. Those muscles are not used in a weightless environment. As a result, muscles quickly weaken and atrophy or waste away. Exposure to long-term reduced gravity leads to reduction in muscle mass and strength, especially in the lower extremities-the legs.Less load on the leg muscles and on the back muscles used for posture are results of extended spaceflight. Muscles can begin to weaken or atrophy, as a result, and could lead to injuries and accidents during exploration missions.In flight durations of less than 14 days on the shuttle, astronauts experienced up to 1/3 reduction in muscle fiber size. Space flight and ground-based flight analog research indicates that muscle size and function could be reduced as much as 20-40% during long duration exploration missing ig effective countermeasures are not if place. When a muscle fiber contracts, it does not completely and always It’s produces the same amount of tension. This is known as the all-or-nothing principle of muscle contraction. WHile this principle applies to individual muscle fibers, it does not apply to entire muscles. If a muscle could only contract completely or not at all, it would be useless. The amount of tension or pull in a muscle can vary depending on how many muscle fibers in that muscle are stimulated to contract.
