How does the virtual absence of gravity, or microgravity, immediately affect astronaut weight-bearing structures?

Answer

Weight-bearing bones atrophy and muscle mass is lost, leading to accelerated osteoporosis

The human body has evolved under the constant influence of Earth's $1g$ gravity, and its removal in microgravity causes immediate physiological disruption. Systems built to counteract this force become unnecessary, leading to systemic deconditioning. Specifically, weight-bearing bones begin to lose density as the normal mechanical stimulation that balances bone formation and resorption ceases, thus promoting accelerated osteoporosis. Similarly, significant muscle mass is lost, particularly in the postural muscles of the calf and thigh. Although exercise in Low Earth Orbit acts as a countermeasure, the full recovery from bone atrophy upon returning to a $1g$ environment can take many months or even years, presenting a significant hurdle for astronauts completing long-duration interplanetary missions.

How does the virtual absence of gravity, or microgravity, immediately affect astronaut weight-bearing structures?

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