Kevin Fong in Wired:
Take gravity away, and our bodies become virtual strangers to us.
In our daily lives, gravity is that pedestrian physical force that keeps us glued to the ground. You have to go out of your way — climb a cliff face or jump out of a plane — before it starts demanding your attention.
But we are constantly sensing the effects of gravity and working against them, largely unconsciously.
Without the quadriceps, buttocks, calves, and erector spinae that surround the spinal column and keep it standing tall, the pull of gravity would collapse the human body into a fetal ball and leave it curled close to the floor. These muscle groups are sculpted by the force of gravity, in a state of constant exercise, perpetually loaded and unloaded as we go about our daily lives. That’s why the mass of flesh that constitutes the bulk of our thighs and works to extend and straighten the knee are the fastest-wasting group in the body.
In experiments that charted the changes in the quadriceps of rats flown in space, more than a third of the total muscle bulk was lost within nine days.
Our bones, too, are shaped by the force of gravity. We tend to think of our skeleton as pretty inert — little more than a scaffold on which to hang the flesh or a system of biological armor. But at the microscopic level, it is far more dynamic: constantly altering its structure to contend with the gravitational forces it experiences, weaving itself an architecture that best protects the bone from strain. Deprived of gravitational load, bones fall prey to a kind of space-flight-induced osteoporosis. And because 99 percent of our body’s calcium is stored in the skeleton, as it wastes away, that calcium finds its way into the bloodstream, causing yet more problems from constipation to renal stones to psychotic depression.
Medical students remember this list as: “bones, stones, abdominal groans, and psychic moans”.