Fibroblasts create the collagen fibers found in our connective tissues, but they are not the only cells that create fibers. Other cells also create the connective tissue fibers found in our bones. In this case, instead of fibroblasts, osteoblasts are laying down new bone fibers. Over ninety percent of the bone is made from Type I collagen. Other cells, called “osteoclasts” do the opposite; osteoclasts reabsorb collagen, cleaning up old bones by degrading the collagen and releasing its components into the bloodstream.
Directional stress on connective tissues
The direction of growth of the collagen fibers is key. When the osteoblasts or fibroblasts create collagen fibers, they are randomly laid down in all directions. When a stress is applied along a predominant direction, electrical fields are generated by the fibers that experience the stress.  This electric field prevents the osteoclasts from reabsorbing those fibers, but fibers that are not being stressed, and thus have not created an electric field, are reabsorbed. Over time, the body absorbs all fibers that are not supporting stress, leaving behind the fibers that are meant to do the work.
Astronauts in orbit, who are thus weightless, have no stress upon the collagen fibers in their bones. Their osteoclasts are free to reabsorb their bones everywhere. Studies  of cosmonauts and astronauts who spent many months on space station Mir revealed that space travelers will lose, on average, one to two percent of bone mass each month. In some astronauts the lack of stress has resulted in a much greater loss of bone density – up to twenty percent over a six-month stay in space! This loss of bone density generally occurred in the lower body and the lumbar vertebrae (the lower back).
Connective tissues respond to demands, to stresses upon them. Stressing the body is essential in order to keep it healthy. Bones need stress to remain strong: so too do ligaments and tendons. Simply walking is a great way to stress the bones of the legs, pelvis, and spine. Yin Yoga is another way to provide this stress, in an intelligent and safe way, to targeted areas of the body. Specifically, Yin Yoga targets those areas where the astronauts suffered the most bone loss – the legs and lower back.
- — See Chapter Five: Energy Body, for more about how this happens.
- — The Studies are at http://science.nasa.gov/headlines/y2001/ast01oct_1.htm