Thigh-body angle

The 90-degree angle between the thighs and the body is bad for the back

The traditional sitting position strains the lower back too much. The 90-degree angle between the thighs and the body causes pains and other symptoms. The least straining position for the back is the one with a 135-degree thigh-body angle.

This was determined in a Canadian-Scottish research project of 2006, in which the strain resting on the spine in different sitting positions was studied. The research received wide international interest, and its results differ remarkably from the traditional concept of the good sitting position. The research was carried out in Woodend Hospital, Aberdeen, Scotland, and it was led by Waseem Amir Bashir, MBChB, from the University of Alberta Hospital, Canada.

MRI in sitting position

The research was carried out with 20 test persons, all of whom had a healthy and symptomless back. A new kind of MRI machine was used in which the person can move, sit or stand during the shoot. The spines of the test persons were shot in the sitting position. This method differs greatly from the commonly used MRI method in which the person is lying still in the tube. However, the traditional lying position is not the best way to examine back pains because those pains that only arise in specific positions or movements may not be identified.

The test persons had their MRI images taken in three different sitting positions

  • in a slouched position, the upper body hunched forward as when leaning over a table or playing video games
  • in an upright position, with a 90-degree angle in the pelvis
  • in a relaxed position, the upper body leaning back in a 135-degree angle

The researchers measured the angles of the spine and the height of the disks and their movements in all the different positions.

Least stress on the disks

The disks move when the weight resting on the spine forces them to bulge out from their places. The research showed that the disks were moving most clearly when the test persons were sitting in the 90-degree angle. In the 135-degree angle, the disks moved the least. In this position, the strain and pressure on the disks and the surrounding muscles was the smallest.

In the slouched position, the disks were pressed most. The largest part of the strain rested on the two lowest vertebrae. The slouched position was noted to strain the back even more than the upright position.

The researchers reached the conclusion that the best sitting position is the one in which the back is leaning backwards and the thigh-body angle is 135 degrees. In this angle, there is the smallest amount of harmful pressure on the disks.
– Sitting in an anatomically correct position is important, because over time the pressure on the spine and its ligaments may result in pain, distortion of the spine and chronic illnesses, says the leader of the research, Waseem Amir Bashir.
– Widening the angle between the thighs and the upper body while sitting is a good idea. It improves the position of the spine and makes it more S-shaped, which is the natural shape of the spine while standing, says Levent Caglar from the British BackCare Organisation.

Millions lost because of back pains

Back pains are a universal problem. In America, eight out of ten people suffer from back pains at some point in their lives. According to National Institute of Health, back pains are the second largest reason for going to a doctor, immediately after flu symptoms. It is estimated that in America 93 million working days are lost every year because of back pains – that is 11 trillion dollars yearly.

The British Chiropractic Association estimates that 32 percent of the British population sits more than ten hours every day. Half of them do not even leave their desks for lunch. Two thirds of the people continue sitting at home after the working day.
– One third of people suffer from pains in the lower back. Sitting for prolonged periods of time causes pain because our bodies are not designed to be sedentary, according to Rishi Loatey from the British Chiropractic Association.

Sources, e.g.
Technesworld: Sitting Up Straight Hurts Your Back
BBC: Sitting Straight 'bad for backs'