I was just watching CNN today, and Sanjay Gupta was interviewing some scientists at NASA on the affects of weightlessness on the astronauts bodies.

Here's the article:? Making Gravity Work For You by Dr Sanjay Gupta

Some hightlights;

I learned how beneficial gravity is for us on Earth. Doctors call it axial load. You can think of it as a slight tension on our bodies, more importantly our ligaments, tendons and bones. Turns out, this slight pulling of gravity slows down the loss of bone mass, which in space accelerates 10 times faster than a post-menopausal woman.

and more importantly…

The message for the rest of us is to embrace gravity. In addition to your aerobic exercise, which you should do most days of the week, add some axial load to your routine. And, this is a message for everyone, especially women in their 40s and 50s who will have to deal with menopause. Pick up some dumbbells, park yourself under a bench press or learn how to use a cable system. It?s good for your health and for your bones.

For the rest of us into bodyweight training, all the different types of pushups, handstands, pullups etc all count as weightbearing exercise too.