Standing is a seemingly simple activity that the human body doesn’t do very well. Our bodies are designed to handle much more ambitious activities (climbing hills, hunting elk, and so on). Keep us stuck in one place, and we suffer—our backs ache, our posture slips, and we end up with more spinal distress than a performer in Cirque du Soleil.
During short periods of standing, you can reduce strain by avoiding the worst mistakes. Keep your head, shoulders, and feet lined up (imagine a straight line that links the top of your head to the middle of your feet). The biggest backaches start when you pull your head forward or lean your upper body backward.
If you must stand for a long time, shift your weight from one foot to the other or rock your feet from heel to toe. Putting one foot on a very low rail, like the sort you find at some bars, can also reduce the pressure on your back. Posture experts say leaning is acceptable, but only if you put your lower back, shoulders, and the back of your head against a wall—a position that might draw a few curious stares.
Source of Information : Oreilly - Your Body Missing Manual