If you're like most people, you don't think much about your back--at least not until it hurts. What does your back actually do for you? The answer is a matter of balance. Cars have four wheels; chairs have four legs. And fellow mammals from mice to monkeys walk on four feet. Four points make balance easy. But humans stand and move about on two legs--a remarkable piece of engineering. Muscles contract and relax to enable you to stand and move. Tendons fasten muscles to the bones in your back (vertebrae). Ligaments--tough, fibrous bands--hold your vertebrae together. Your backbone also protects the spinal cord, the main pathway for your central nervous system. It takes this delicately balanced interworking of bones, muscles, ligaments, tendons and nerves to balance and bear the weight of your body and the loads you carry.