Lesson 4 - Microgravity (Part 2)

As we learnt in Lesson 2, different gravity environments such as microgravity, Lunar gravity or Martian gravity can have drastic effects on the human body. Two additional bodily systems that are affected by a change in gravity are the skeletal and muscular systems.

Bones in Microgravity

In space, an astronauts bones do not have to fight against a persons body weight to keep them standing upright. As the human body is very efficient, it decides that there is no need to maintain the bones at this level of strength, and the bones begin to weaken. There are two main pathways through which bones weaken in microgravity:

1) Calcium loss

In a healthy human body on Earth, calcium and other important minerals are stored within bones, ready to be released in between meals so that the body has all the minerals it needs to function properly. The calcium within bones also helps them to maintain compressional strength. In just a few days of an astronaut reaching space, the level of calcium in their bloodstream and urine skyrockets compared to when they were still on Earth. This is because the body begins to absorb calcium from the bones back into the blood. When the body absorbs too much calcium from the bones, it leaves them thinner and weaker.

2) Breakdown imbalance

Human bodies have two types of cells within their bones that help manage bone growth. Osteoblasts are a type of cell that grow new bone, and osteoclasts are a type of cell that absorbs existing bone. In a healthy adult body on Earth, the osteoblasts and osteoclast are in balance, maintaining a healthy level of bone density. In children, the osteoblasts are more active than the osteoclasts, as children need to grow bigger bones as their body grows. In elderly people, osteoclasts tend to become more active, breaking down more bone than can be replaced and thus leading to a condition where bones become weak, which is called osteoporosis.

Astronauts bones begin to weaken just a few days into a space mission, and the worst weakening is seen between two months and five months into a mission. If no treatment is given during spaceflight, the astronaut can lose as much as 20% of their bone density and are at serious risk of fractures once they return to Earth. After just 6 months in space, an untreated astronauts bones are similar to an elderly woman with osteoporosis on Earth!

Muscles in Microgravity

If you have ever broken an arm, or had an injury where you cannot move a body part for a long time, you may have noticed that the muscles become much smaller as they are not being used. The same effect is seen in microgravity. The shrinking of muscles due to not being used is called muscle atrophy.

Medical data shows a typical muscle loss of around 10-20% for short duration missions, and best estimates suggest a loss of 50% would be possible on longer term missions (greater than 1 year) if no treatment is applied.

Surviving Microgravity in Space (Part 2)

The main treatment used to stop bone and muscle loss is space is exercise. Astronauts on the space station have 2 hours of mandatory exercise in their schedule each day. There are three types of exercise machines on board the International Space Station:

1) Advanced Resistive Exercise Device (ARED)

The Advanced Resistive Exercise Device is similar to a weight machine you might see at a gym. However, unlike at a gym, solid heavy lumps of metal cannot be used to provide resistance, as they would be 'weightless' in microgravity. Instead, the machine is attached to pistons that create a vacuum when the astronaut applies a force to the crossbar and foot stand.

2) Combined Operational Load Bearing External Resistance Treadmill (COLBERT)

This treadmill works in a similar way to treadmills on Earth. Astronauts strap themselves onto the COLBERT using bungee cords so that they can perform a cardio workout to help maintain their heart health in space.

3) Cycle Ergometer with Vibration Isolation and Stabilization (CEVIS)

The CEVIS system is basically an exercise bicycle that astronauts strap themselves into to provide cardio workouts.

Astronauts can also take medicine to help prevent bone loss in space. A medicine called 'alendronate' was trialed on the International Space Station to see if it helped prevent bone loss. On Earth, Alendronate is used to treat osteoporosis in the elderly by stopping osteoclasts from absorbing bone. The results of the study showed alendronate reduced bone density loss from 7% in the femur (upper leg bone) to only 1%, and it helped to lower calcium levels in the urine.

Astronauts are also given Vitamin D supplements to improve their bone health.