How much work can the back do without strain?

About 80 percent of American adults complain of back pain and it is one of the most common reasons for calling in sick to work. Some $50 billion is spent on back-pain care in the U.S. each year. Still, it is difficult to know when we’re putting too much strain on the complicated mass of bones, nerves, muscles, joints and ligaments. One expert, Kee Kim, chief of spinal neurosurgery at University of California, Davis, explains what the back is capable of and how posture and core strength play a part in its job.

Giving full motion

The back provides structure and support for body weight and protects the spinal cord and nerves running from the brain to the rest of the body, says Dr. Kim. It also allows the body to move. “Even when you are sitting, your back is working,” he says. And there is pressure on the back even during sleep, he says.

The back probably does more work than the arms or legs, says Dr. Kim, but there are no hard data to prove it and the amount of strain a back can take is very individualized. Good posture lowers the amount of work the back must do. “It’s not just that standing and sitting upright looks better. When you slouch forward at your desk, you are putting almost twice as much strain on the disks than if you were sitting straight up,” he says. Obesity also makes the back exert more energy. “We physicians encourage overweight patients to lose weight to help reduce stress on the spine and improve efficiency of the back,” he says.

Manual labor can create problems over time. “One study found that the prevalence of back pain was 40 percent among manual laborers as opposed to 18 percent among those who had sedentary jobs,” he says.

The core of the issue

The back doesn’t work independently, and strong core muscles can help it be more efficient. “If you have strong muscles surrounding the spine, both in the front and the back of the body, that helps lower the stress on your back,” says Dr. Kim. Weak muscles cannot support the spine as well, which may lead to degenerative problems, an abnormal curve and pain, he says.

Dr. Kim doesn’t recommend wearing a brace for more than a short job. “If you’re just moving house and it’s temporary, then a brace is a good idea,” he says. “But using it all the time is counterproductive because you are allowing the muscles to weaken.” A healthy adult male lifting correctly should be able to carry a 50-pound box without much difficulty and without a brace, he says. The key is to use the back as part of a system with the legs and core muscles, Dr. Kim says.

How much is too much

The general rule of thumb is that a fit person should be able to comfortably carry one third of his body weight, as long as it is balanced evenly. If a person isn’t in good shape, he should carry less than one quarter of his weight. An overweight person could be at risk for injury by even carrying a small percentage of his weight. “The length of time hasn’t really been worked out,” Dr. Kim says.