Without exercise, muscles begin to atrophy (shrink) due to disuse and consequently lose strength. Without stretching, muscles also lose elasticity and range of motion around joints is decreased, leading to loss of flexibility.

Little is known on the effects of a sedentary lifestyle on bone.

When the cardiovascular system isn't exercised, cardiac muscle (the type of muscle that the heart is made of) loses strength, elasticity, and decreases in its ability to pump blood efficiently. In addition, fat and cholesterol build up in the body.

Studies have suggested that lack of exercise may lead to increased chances of depression and other similar conditions.

Exercising helps to strengthen muscles and maintain and build muscle mass. Stretching assists in increasing range of motion and flexibility.

Weight-bearing activities help to strengthen bone, promoting bone formation and regeneration. This prevents the loss of bone density and essential minerals.

Regular cardiovascular exercise increases the heart's working capacity: blood is pumped through the body more efficiently, blood flow improves, and blood pressure decreases. Blood vessels maintain elasticity, the heart muscle becomes stronger, and overall endurance is improved. In addition, exercise reduces body fat and levels of harmful cholesterol (LDL), while increasing levels of beneficial cholesterol (HDL).

Exercising releases chemicals called endorphins, which help to reduce stress and anxiety. Regular physical activity also leads to better sleep, improved mood, and an overall more positive outlook.

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In addition, loss of gravity leads to fluid shift in the body. Mechanisms that prevent blood pooling in the legs due to gravity on Earth continue working in space; thus, increased quantities of blood shift towards the upper body, leading to larger chest and shoulders, congested head, and skinny legs.

At a later point in spaceflight, this fluid shifts triggers blood pressure receptors, which then act to lower the body's blood volume by around 11-22%.

Neurovestibular
The neurovestibular system is responsible for sense of balance and spatial orientation of the body. It consists of organs in the inner ear, vision, and proprioception (sense of body in space), which process data based on external stimuli to establish the body's position. One of these stimuli is the force of gravity, and its disappearance in space affects the organs that sense angle and acceleration of the body. As a result, astronauts may get space motion sickness and their neurovestibular systems will begin to depend more strongly on vision and proprioception alone.

Taste
Nasal congestion that astronauts experience in conjunction with the headward fluid shift can cause decreased taste sensitivity in some astronauts.

In space, astronauts do not have to support themselves against gravity, and this causes muscles to atrophy from disuse. As a result, muscle size decreases and force of contraction weakens. This is particularly seen in weight-bearing muscles, such as muscles in the legs and low back, which regularly work against gravity on Earth, but are unloaded in space.

By exercising daily, astronauts are able to mitigate some of the effects of microgravity on their muscles. For example, by performing load-bearing, resistive exercises on the advanced resistive exercise device (aRED), astronauts promote maintenance of skeletal muscle mass. However, even regular exercise does not completely prevent the harmful effects of microgravity on muscles.

Since bone formation is stimulated by impact activity, lack of such activity results in demineralization-the loss of calcium and other minerals. In this process, bone degrades faster than it regenerates.

Even with rigorous exercise, astronauts typically lose up to 0.4-1% of their bone density per month in space, and continue to lose bone mass for several months after returning to Earth.

Since the daily volume and pressure stimuli that the heart receives on Earth is removed in an environment of microgravity, it becomes under-stimulated. When this occurs, the heart atrophies, shrinks, and becomes less strong.

By regularly participating in cardiovascular exercise, such as running on the treadmill, astronauts aboard the ISS keep their heart as fit, strong, and efficient as possible under the effects of microgravity.

Aboard the ISS, astronauts face a variety of challenges including cramped surroundings, intense work schedules, high noise levels, general sensory deprivation, and separation from family. These factors can all cause psychological stress.

Since exercise causes the release of natural, feel-good chemicals known as endorphins in the body, regular exercise assists in reducing feelings of anxiety, depression, and stress, while promoting feelings of relaxation.