Microgravity strips the human body of its usual workload, so astronauts often return weaker and slightly shorter despite all the machinery keeping them alive. In orbit, muscles that normally fight gravity barely engage. The body tracks that reduced demand and quietly dials down strength and structure.
Skeletal muscle atrophy sets in as protein synthesis slows and fibers shrink, especially in the legs and back. Bone tissue follows the same logic: without regular mechanical loading, osteoclast activity outpaces osteoblast activity, accelerating bone resorption and reducing bone mineral density. Exercise devices on spacecraft can only partially replace the continuous gravitational load that shaped human physiology on Earth.
The spine responds too. In the absence of compression, intervertebral discs absorb more fluid and expand, allowing the vertebrae to separate slightly. That decompression can add measurable height, even as core muscles that stabilize the spine weaken. When astronauts land, gravity abruptly restores compression, discs lose some of that extra fluid, and height drops while muscles and bones slowly rebuild under full load.
