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Believe it or not, gravity, or G for short, is very important to your bones. Your bones are made stronger by exercising against gravity because it puts resistance on your bones and muscles. For example, when you are walking, weight is placed on the bones of your feet, legs, and back. If you are riding a bike, resistance is placed on your legs. Exercises that involve walking, running, jumping, riding a bike, and lifting are weight-bearing exercises. This is only true because we have gravity on Earth.

When you are in space, there is no gravity, so you don't weigh anything! Astronauts in space do not have their weight to put resistance on their bones and muscles. Because of this, astronauts' bones and muscles can become very weak. If your bones become too weak you develop osteoporosis. Osteoporosis is a disease where your bones become weak and break. Astronauts can develop osteoporosis in space if they don't do enough resistance exercise every day. Because of this lack of gravity, most exercises that are considered weight bearing on Earth no longer work in space. Think about what would happen if you tried to do weight lifting, jump rope, gymnastics, etc. in space. Today, this is not a problem for the astronauts. They have adapted the weight bearing exercises we do on Earth to be done on their missions. They have found ways to increase resistance and counteract the bone loss that would normally occur in space.

In order to prepare for their mission, astronauts go through intensive hours of physical training, including lots of weight bearing exercises, like running, weight lifting, and rowing while they are still on earth. They also practice the experiments they will perform in space and prepare for what life will be like there.

During the mission, astronauts must exercise 3 or 4 hours every day so that their muscles won't deteriorate during space flight. If they do not do these exercises, the astronaut's bones become soft and they can develop osteoporosis.

As you get older, your bones may deteriorate kind of like the astronauts' do when they are in space. In your bone, there are living cells that make new bone. These cells are called osteoblasts. Other cells called osteoclasts, tear down, or resorb, old bone. The osteoclasts are like "pac-men" that come along and remove old bone. This creates a space in the bone. The osteoblasts come along and fill in the space with new bone. This process of bone breaking down and reforming is called remodeling. It continues throughout our lifetime, regardless of our age.

During adolescence, more bone is made than torn down. As a result, the bone increases until peak bone mass is achieved. After peak bone mass is reached, bone formation and bone tear down occurs at equal rates. As we age, many factors can cause bone loss.

Poor nutrition and low calcium intake, a lack of weight bearing exercise, hormone deficiency, illnesses, surgery, medications, and lifestyle factors, such as smoking and alcohol, may all cause bone loss. Osteoporosis, or loss of bone mass, is a result of an imbalance in the bone remodeling process. This happens when, too much bone is being torn down and not enough new bone is being replaced. Sufficient calcium intake and weight bearing exercise are necessary for the remodeling process to work well. Your body also uses calcium for other important functions, such as muscle contraction and nerve functioning too.

The majority of our calcium is stored in our bones. A very small percentage of calcium circulates in the blood to help with those other important functions. If the blood level of calcium is too low, the body takes calcium from the bones to help with those functions such as muscle contraction and nerve functioning. Because of this, adequate daily calcium intake is necessary to replace the calcium used to support bone growth and other functions.

Adolescence is the best time in your life to build up your bones. By the time you turn 18, most of your bone mass is set. Reaching your peak bone mass during adolescence decreases your risk for osteoporosis later in life. To prevent osteoporosis, it is important to eat calcium-rich foods daily and do weight-bearing exercises everyday, just like the astronauts do in space.