Gravity Hurts (so Good)
Gravity hurts: you can feel it hoisting a loaded backpack or pushing a bike up a hill. But lack of gravity hurts, too: when astronauts return from long-term stints in space, they sometimes need to be carried away in stretchers.
The question is, do such losses matter?
Perhaps not if you plan to stay in space forever. But eventually astronauts return to Earth -- and the human body has to readjust to the relentless pull of gravity. Most space adaptations appear to be reversible, but the rebuilding process is not necessarily an easy one.
Muscle, too, can be recouped. Most comes back "within a month or so, "although it might take longer to recover completely. "We normally say that it takes a day [of recovery on Earth] for each day that somebody's in space," says Schneider.
Bone recovery, though, has proven problematic. For a three to six month space flight, says Schneider, it might require two to three years to regain lost bone -- if it's going to come back, and some studies have suggested that it doesn't. "You really have to exercise a lot," says Schneider. "You really have to work at it."
According to Dr. Alan Hargens, recently of NASA Ames and now a professor of orthopedics at the University of California San Diego medical school, it is important to keep astronauts in good physical condition. "You want the crew members to function normally when they come back to Earth ... and not have to lie around for long periods of rehabilitation," he says.
Exercise is the key. But exercising in space differs from exercising on Earth. Here, gravity's pull automatically provides a resistive force that maintains muscles and bones. "[In space] even if you do the same amount of work that you were doing down here on Earth, you miss that gravity component," says Schneider.
Various devices have been developed to mimic the help that gravity provides. One Russian experiment provides resistance by strapping jogging cosmonauts to a treadmill with bungee cords. But that particular combination has not yet proven effective in preventing bone loss -- perhaps because it cannot provide sufficient loads. "The straps are so uncomfortable that the cosmonauts can only exercise at 60 to 70 per cent of their body weight," says Hargens.
There's also IRED, a NASA-developed Interim Resistive Exercise Device. IRED consists of canisters that can provide more than 300 pounds of resistance for a variety of exercises. IRED's effectiveness is still being monitored, says Schneider.
The device, explains Hargens, prevents much of the loss of cardiovascular function and of muscle. It also seems to be effective in reducing some indices of bone loss. One reason is that the LBNP allows astronauts to exercise with an effective body weight between 100% and 120% of what they would feel on Earth. Another is that -- unlike any previous exercise device -- it restores the blood pressure gradient, increasing blood pressure to the legs.
There's growing evidence, Hargens says, that the body's systems interact with each other. For example, "you can't just put high loads on the bone and then expect it to recover if you're not taking care of the blood flow to that bone as well."
Solving these problems, says Schneider, could lead to better therapies for people who aren't using gravity properly here on Earth. Aging is the perfect example. Zero-G living mimics closely the effects of old age. Like astronauts, the elderly fight gravity less. They're more sedentary, which triggers the loop of muscle atrophy, bone atrophy, and lower blood volume.
If researchers can identify the signals that generate strong muscles and bones, it might be possible "to get new pills and do exercises" that would trigger those signals here on Earth.
"We've just begun to do research ... looking at the changes that can happen to humans," says Schneider. "There are so many wonderful questions."
And the answers? They're waiting for us ... up there in space, where the absence of weight reminds us that gravitation isn't all bad. Sometimes it's a struggle, our daily contest with gravity, but now we know the struggle is good!