In prime condition, the human body functions as a well-oiled machine, with the heart as the engine that keeps the infrastructure of muscles and bones moving properly. But when we don't take proper care of it, parts of that machine can slip up and break down with painful results. In addition, certain areas of our bodies that must endure a lot of wear and tear are more susceptible to injury than others. As a major load-bearer, our backs are particularly vulnerable regions.

Back pain affects eight out of 10 people, whether it's acute or chronic [source: National Institutes of Health]. The back is such a common trouble spot because we put a ton of stress on it whether we're lounging on our couch in front of the television or playing tackle football. Whatever activity we're engaged in, our spine is the central support for our upper bodies and holds us upright. Its network of interlocking vertebrae also encases our spinal cord, which serves as the main drag on the central nervous system's superhighway.

The spine is made up of 24 interlocking vertebrae, which are divided into three types: cervical, thoracic and lumbar. Fibrous tissues called ligaments connect those bones in our spine. To prevent our vertebrae from crunching against each other, spinal discs provide a cushion between them. Muscles then attach the spine to our skeleton, giving us upper body movement and flexibility.

When we have to lift heavy objects or do much twisting and turning, these movements put stress on our backs. Ligaments can overstretch or tear, leading to back sprains. Overworked back muscles can cause a strain, and compressed spinal discs can also wear down. But when we're sitting completely still, our backs may suffer as well. The lower back must prop up the weight of the top half of the body, and the position we sit in can make it harder for the back to do its job. Even if we aren't moving around, our backs are actively working.

So how can you sit in a way that sits well with your back?

Sitting Pretty: How to Sit in a Chair

There are four curves along the length of our spines that form an "s" shape. People with good posture maintain that natural figure, which is optimum for the back. But if you're on a long car trip or sit behind a desk all day, perfect posture can be difficult to keep up. You twist around in your seat, sprawl out with a book or hunch over a computer. After a while of not moving, your back is beat.

People with desk jobs may spend more time sitting in their office chairs every day than lying in their beds at night. For at least eight hours each day, many employees remain firmly planted in their chairs. Spending so much time in the same chair, you get to know its contours and nuances as you shift and slouch your way through the workday. You might not notice that the chair is taxing your back until you're popping a handful of aspirin to alleviate the painful crick in your spine.

The field of ergonomics examines how the work we do affects our bodies. It focuses a lot on the elements of force and motion. When sitting, the force of your upper body settles into your lower back, in the lumbar region of the spine. That's why you often hear about the importance of proper lumbar support. On top of that, if you have to move around often in your seat, it can increase the back's workload even more. Instead, when sitting, you want the chair to promote a neutral body position that keeps your vertebrae aligned. This reduces your chances of developing musculoskeletal disorders in your back.

Sitting forward in your chair increases the stress on your back. Instead, sit back and let the chair share your load.

The most important thing to remember to avoid chair-related back problems is that your chair shares the burden of supporting your body weight with your back. The more of that load you give the chair, the less work your back has to do. In a way, you should think of chairs as having a specific fit. Goldilocks was onto something when she tried out all of the three bears' chairs until she found the perfect one. Like a pair of shoes that are too small for your feet will rub and cause blisters, a chair that isn't adjusted your body will promote back problems. To know whether a chair fits, here are a few rules to sit by:

# Feet should reach the floor
# Five chair legs are better than four for added stability
# Hips should be the same level or higher than the knees
# Forearms ought to rest easily on the arm rests
# Sit completely against the back of the chair

But don't let yourself get too comfortable for too long. Holding your body in a static position for a sustained period will always stress the muscles involved at some point. Doing that day after day invites permanent physical problems. The key to reducing back pain from sitting for a long time is motion. Every 20 minutes or so, stand up, stretch and walk around if possible.

When you aren't confined to a chair, stand up and stay active. By exercising, you strengthen those muscles surrounding your spine. Maintaining a healthy weight will also lighten the load that your lower back must bear. With a healthier body overall, you'll feel better and sit a little taller.