The latest NASA shuttle mission STS-131 to the International Space Station (ISS) is coming to an end, with the Discovery shuttle undocking from the ISS on Saturday, and due to make a weather delayed landing back at the Kennedy Space Centre, Florida on Tuesday 20th April 2010.

One part of the mission this time was the delivery of a piece of exercise/research equipment - the Muscle Atrophy Research and Exercise System (MARES), a European Space Agency sponsored project. MARES is able to "assess the strength of isolated muscle groups around joints, or complete limbs by controlling and measuring the interrelationship between position/velocity and torque/force as a function of time" (ESA).

This piece of equipment, along with the treadmill, stationary bikes and a device to lift weights that are already on board the ISS are all vitally necessary for helping keep astronauts healthy in a gravity free environment, and also for continued research into the effect of prolonged periods of time in microg on human physiology. A manned space flight to Mars for example, would mean astronauts being effectively 'weightless' for a period of nearly 18 months for the round trip, something that potentially can cause many physiological as well as psychological issues.

One major cause of concern is the loss of bone density recorded in astronauts returning from a period of time in Space. In fact, this is also an Earth bound problem for anyone subject to spending a long period of time off their feet perhaps due to ill health, accident or disability.

Our bodies are constantly losing old bone material (resorption), whilst at the same time producing new bone mass (growth) in a balanced process. Many factors can affect this balance including diet, physical activity and gravity. When we sit or lie down, quite literally, we take the weight of our feet. Likewise, in the microgravity environment of Space the loading weight caused by Earth's gravity is removed and this causes a disruption in the process of bone maintenance.

Studies have shown that the load bearing bone structures, particularly spine, neck and pelvis are those most affected by this reduction in bone mineral density which, can decrease on average around 1-1.6% a month for the time an astronaut is in Space. Times this by the 18 month length of a Mars mission and the implications for health are clear.

Obviously, this topic is far more complex than this little scratch I have made on the surface . . . . . but suffice to say that when your doctor tells you next time to go for a walk every day, the advice is based on sound facts, years of research, and a knowledge that walking in Earth's 1g gravity applies load bearing to your bones and muscles, promoting bone growth and health.

Sometimes, Doctor really does know best!