Nota : This page contains documents for which the access may require a particular software. If the software is not installed, you can download it and follow the instructions for installation.
The human body is an extraordinary machine, an unending source of wonder. In order to unlock its mysteries, scientists sometimes go out of this world. In 2001 and 2002,
Dr. Douglas Watt of McGill University in Montreal studied how the human body adapts to weightlessness through his H-Reflex experiment. H-reflex was the first medical experiment completed on the International Space Station.
But how does zero gravity affect an astronaut's body? In the weightless space environment, astronauts simply float rather than walk or sit as we do on Earth, and they stop using their body's structure to support themselves. After a few weeks, their muscles and bones undergo changes that affect strength, posture and balance.
Dr. Watt and other experts think that one way the human spinal cord adapts to microgravity is by responding less and less to stimulation. To compensate for this, more signals from the brain are then needed to stimulate muscles to exert the same effort as they would on Earth. If this hypothesis is correct, the longer the astronaut remains in space, the less effective his or her exercise routines will be. The experiment H-Reflex was designed to test this theory.
It's very easy to do the H-Reflex experiment: the astronaut assumes a seated position, being held in place by straps so that he or she doesn't move or float away. Slight electroshocks are given to stimulate nerves in the leg. Muscle contractions are recorded to determine the level of sensitivity of the spinal cord which, in turn, transmits the signals sent and received by the H-Reflex equipment. Astronauts test themselves several times in space to see if alterations in stimulation response can be observed and correlated over time.
The last run of the H-Reflex experiment was in April 2002 by the crew of Expedition 4 aboard the International Space Station. The researchers are in the process of comparing the values recorded before, during, and after the space mission.
"The results so far indicate that the excitability of the spinal cord falls off quite quickly in zero gravity, and remains reduced as long as the astronauts are in space. It then recovers within 10 days of landing," Dr. Watt says.
The results of the H-Reflex experiment will enable researchers to adapt the quantity and type of exercises done by mission astronauts in order to counter the loss of muscle and bone mass. Doctors will also be able to identify the best ways for astronauts to recover their posture and balance once they return to Earth. "It could be very useful for astronauts to be able to walk immediately upon their return to Earth, in particular in the event of an emergency landing," Dr. Watt adds.
It is probable that the results of H-Reflex will also make it possible to better understand certain disorders of the inner ear and the spinal cord, and so, open the way for new treatments.