A woman sits in the 135-degree position on the special MRI scanner. ((RSNA))

People who like to recline at the office have a new excuse for kicking back: it may be better for your back.

Researchersused a magnetic resonance imaging machine, or MRI, to study sitting postures and determine which is best for back health.

They concluded that sitting upright for hours places unnecessary strain on the back, leading to potentially chronic pain problems.

"A 135-degree body-thigh sitting posture was demonstrated to be the best biomechanical sitting position, as opposed to a 90-degree posture, which most people consider normal," said WaseemBashir, author of the study and a clinical fellow in radiology at the University of Alberta Hospital.

"Sitting in a sound anatomic position is essential, since the strain put on the spine and its associated ligaments over time can lead to pain, deformity and chronic illness."

Bashir presented the study on Monday at the annual meeting of the Radiological Society of North America in Chicago. The team suggested people may prevent back problems by finding a chair that allows them to sit at 135 degrees.

Preventioninstead oftreatment

"This may be all that is necessary to prevent back pain, rather than trying to cure pain that has occurred over the long term due to bad postures," Bashir said. "Employers could also reduce problems by providing their staff with more appropriate seating, thereby saving on the cost of lost work hours."

Musculoskeletal disorders, including back pain, cost Canadians more than $16 billion in treatment, rehabilitation and lost productivity, according to Health Canada estimates.

In a herniated disc, the doughnut-shaped soft tissue cushioning the vertebrae in the back protrudes, putting pressure on the spinal nerves, a condition that can be painful. A herniated disc is most common in the lower back.

People show a natural hollow in their back, but it disappears when seated. That can lead to the pain of a herniated disc.

Seated MRI

By showing disc deformity in people, the MRI study points to a convincing injury mechanism, said Prof. Stuart McGill, a professor of spine biomechanics at the University of Waterloo.

To identify poor seating postures, Bashir and his colleagues used a type of MRI machine that allows people to sit or stand while the images are taken. In traditional scanners, people lie flat, which may mask causes of pain that stem from different movements or postures.

In the experiment, 22 healthy volunteers in Scotland sat in three different positions:

  • A slouching position with the body hunched forward like in front of a video game console.
  • An upright 90-degree position.
  • A relaxed position reclining 135 degrees with feet on the floor.

Seating strategies

Researchers measured spinal angles and spinal disc movement from the different positions. Spinal disc movement occurs when weight-bearing strain is placed on the spine, causing a misalignment in the internal disc material.

The most disc movement was seen in the 90-degree sitting posture, and the least in the 135-degree posture, which suggests less strain is placed on the spinal discs, related muscles and tendons in the more relaxed position.

To prevent the back pain, McGill suggests finding a "sweet spot" that minimizes stress on the disc. Strategies include:

  • Varying positions, such as standing up when the phone rings.
  • Tilting the seat pan forward with the seat under the thigh and buttocks and the pelvis at the backrest.
  • Leaning back in the backrest the way bucket seats in cars tilt back for passengers.
  • Using a lumbar support.
  • Following a tailored exercise program.

Sitting at 135 degrees may help.But given the natural variation in the population, some people feel more stress when standing and others from sitting, meaning there is no one-size-fits-all solution, McGill said.