One problem facing future space travellers is the consequence of living for an extended time in zero gravity. Even in the relative weightlessness of the International Space Station astronauts experience both short and long term medical problems. (I write relative weightlessness, for at 350km above the earth, the pull of gravity on the ISS is around 90% as strong as on Earth. Astronauts experience weightlessness, however, for both they and the ISS are falling together towards the planet below, in much the same way as when you jump in a downward accelerating elevator.) The actual gravity felt by the astronauts is around 0.001% of that felt on Earth.
Bones, along with most every other body tissue, are affected by this environment. There was a reason the doctor in Star Trek was nicknamed Bones, for bones and space do not make good partners. Without the constant pull of gravity, bones begin to decalcify and weaken. Demineralisation of the bones occurs at up to 2.4% per month in the weight-bearing bones of the legs and spine. Astronauts who underwent extended stays on the Mir space station lost as much as 20% of their bone mass. It is not clear if this bone loss plateaus with time, but the implications for, example, a manned flight to Mars are obvious. A good way to loose weight, perhaps (but then, in zero gravity, your weight is also zero).
Your bones are not simply a calcified framework, but dynamic, living tissue. After a few days of exposure to weightlessness, the calcium stored in the bone is broken down and released into the blood stream. (The high level of circulating calcium can in itself cause problems, from kidney stones to psychiatric problems.) The resulting drop in bone density is similar to that of osteoporosis – indeed, this bone loss is termed disuse osteoporosis.
Consequently, when an astronaut returns to earth, their weakened bones may fracture as they struggle to suddenly support a full body weight. Newly arrived cosmonauts on Mir were forbidden to exchange traditional Russian bear hugs with crew members who had been in space a while, for fear of causing broken ribs. The Force was literally against, not with, them.
Aside from loss of bone mass, astronauts often develop back pain. Loss of gravity can cause tendons and ligaments to stretch. (Some astronauts have grown up to 6cm in height whilst in orbit.) This can put abnormal strain on facet joints in the spine, as well as changing pressures in the intervertebral discs. General musculoskeletal pain can also arises from simple moving around in a weightless environment, as the tendons and ligaments are not protected.
Astronauts regain most, but not all, of their bone mass following their return to earth. To keep bone loss to a minimum, they do regular resistance exercises as well as take vitamin D tablets. Resistance exercises include, for the upper body, curls and presses, with squats, heel raises and dead lifts for the lower body.
On a positive note, feet love space. Relieved of the burden of bearing your full body weight, those callouses which have built up with time to cushion your feet against the impact of walking are no longer needed. Just be careful when taking off your socks in space; callouses have a habit of sloughing off at unexpected times, and the sight of them floating in your cabin might be enough to turn your stomach – which you definitely don’t want in space.
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