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Videos of people on the ISS make it clear how little they need to bend or stretch, and how their joints stay in their neutral position most of the time as they float around - elbows, knees and hips slightly flexed, shoulders lifted towards their heads. It seems to me that if they didn't specifically stretch to ensure that their joints don't tighten up, by the time they got back to Earth they would have lost a lot of flexibility.

A quick search doesn't show anything in particular in the ISS exercise routines regarding stretching. Is it something the crew just does on their own now and then, as the urge arises? Has loss of flexibility ever been noted?

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    $\begingroup$ Seems to me reasonable that with muscle atrophy and bone density loss in microgravity, and the loss of pressure gradient, fluids could more easily build up in joints which would affect flexibility. Fair question, I doubt their exercise regime can substitute gravity for every joint in a human body. I'd be interested to see statistics on knee and elbow injuries pre and post flight. $\endgroup$
    – TildalWave
    Jan 25, 2016 at 20:45
  • $\begingroup$ Preliminary PubMed stuff: ncbi.nlm.nih.gov/pubmed/26099126 ; ncbi.nlm.nih.gov/pubmed/21265081; more on rats there. Terms used. $\endgroup$ Jan 26, 2016 at 7:37
  • $\begingroup$ Also relevant: ncbi.nlm.nih.gov/pubmed/15705722 $\endgroup$
    – called2voyage
    Apr 6, 2016 at 19:01

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There is a lot of Earth-bound experience with lack of joint motion and weight bearing due to medical immobilisation. Bad things happen, which are nicely summarized and referenced in this article: https://www.caringmedical.com/prolotherapy-news/immobilization-syndrome-damage-tendons

Joint cartilage is constantly being replaced and remodeled in response to loading. Prolonged non-weight bearing causes thinning of cartilage and replacement with bone.

Immobilisation weakens the bone-tendon complex https://journals.lww.com/jbjsjournal/Abstract/2013/05150/The_Effect_of_Immobilization_on_the_Native_and.9.aspx

Immobilised ligaments gradually shorten, resulting in decreased range of motion. Earth-bound patients often require post-immobilisation physio to restore range of motion.

The NASA site which describes ISS exercise program focuses on maintenance of muscle and bone mass, not mentioning stretching as part of the mission exercise program. Yoga, anyone?

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