I know from taking yoga classes, where we stretch our muscles to get better flexibility, that some people are able to do the yoga exercises more easily than others. I think this is partly because people have slightly different attachment sites of muscle to tendon to bone. Have you ever seen someone whose feet turn out more than others when they walk, or someone whose toes point inward when they walk more than others? This is caused by different attachment sites, like I mentioned above. These people will be able to stretch their muscles and bend in slightly different ways than other people. I can sit "Indian-style" (legs crossed while sitting on the floor) more easily than some of my friends, but I have trouble sitting with my knees underneath me. Check out a picture of the human skeletonand see if you can visualize where some of your muscles attach to bone. (This is what I mean by "muscle to bone attachment sites")
(http://www.imcpl.lib.in.us/nov_skel.htm)

Here is a cool site that might begin to help answer your question. Search for others!
http://www.aomc.org/HOD2/fitness/ConditioningForHealth.html

Also, excercise helps stretch muscles, and here's my best guess as to how this occurs. Muscles that are stretched receive more blood to them, and more blood circulation means more oxygen to those hardworking cells. Waste products (lactic acid and carbon dioxide and oxygen radicals) are removed more rapidly from these stretched muscles making them more efficient organs. They become more supple as opposed to taut, stiff muscles. They are healthier because they are receiving increased supplies of oxygen (ultimately for get energy from food), and live in a cleaner environment (less cellular waste products from respiration). Happy, stretched out muscles, or at least as much as your skeletal design will allow!

As you know, bones give us most of our shape.Bones are held together by ligaments. Muscles are held onto bones by tendons. If the ligaments and tendons stretch well, a person is more flexible. Next time you have chicken for dinner, look at the joints of the chicken (where it bends). You will see the ligaments and tendons if you look for them. Slow, gentle stretching will also make a person more flexible, just like exercise will make them strong. What might be some advantages to being flexible? What might some of the disadvantages be? (Hint: what happens if they're too loose to do their job?)

Thanks for asking,

Those are good questions. I think we're often born with greater or lesser flexibility, because I know some people can do lots of yoga and still not bend very well. And of course when we stretch we get more flexible than if we don't do any stretching exercises for a long time. I looked up 'osteoporosis' on the web with metacrawler.com to find out about bones getting weak with age, but I don't remember what the best web sites were. You could try looking for 'bones' to start with.

Good luck with your science investigations.