Mechanics of Sitting


Unfortunately, many people are now sedentary and obesity levels are dramatically on the increase. A major concern is the amount of time that people are spending sitting on their butt, watching TV, at a work desk or driving. Therefore, it is very important that you know how to sit down properly, as there are a whole host of issues attached to prolonged sitting.

Poor seating positions or postures are linked to many major problems within the body. Specifically, the muscles of the back pick up the addition stress, as it attempts to balance the upper body. These issues start when the stress caused by poor mechanics of sitting are passed onto the discs and the joints of the spine. Let’s discuss this further…

Prolonged sitting can have a negative impact on the development and function of the major muscle groups. When you change your sitting position, it can have a negative effect on the natural curvature of the back. The muscles that support the back are activated to maintain a proper posture. This scenario overburdens the major muscle groups, and this can lead to overuse injuries of the back. We must take into consideration the shape and structure of the spine, and how this can eventually change with poor mechanics of sitting e.g. kyphosis= outwards curve of the spine and lordosis= inwards curve of the spine.

As we spend so much time sitting down, it is very important that we discuss the risks, the consequences and the solutions of prolonged sitting….

The Risks of Prolonged Sitting

Research has indicated that individuals with sedentary lifestyles and jobs have a higher incidence rate of back pain, when compared to people who have active lifestyles and jobs (1). When comparing postures, sitting down does have greater pressure on the back discs than standing up. Studies have shown that sitting actually places twice the pressure on the intervertebral discs than standing (1). Why is this case?

When you sit down the lumbar (lower back) region of the spine loses its protective/natural curve, and it becomes flattened. The total elasticity that is formed by the natural curvature of the spine is lost along with its protective nature. This leads to the discs losing their protection against uneven loads or too much pressure. Essentially, this will determine the extent of the outward and inward curves of the back.

Stopping kyphosis (outward curve) in the lower region of the back and maintaining a lumbar lordosis (inward curve) is extremely important in stopping back injuries. Many studies tend to only concentrate on the military style of sitting i.e. vertical spine, knees and hips flexing at 90 degree (a right angle) (3).

The majority of people in the normal population, don’t have the control to maintain the body in this ‘military type’ posture when seating for a prolonged periods of time. Try sitting in this ‘military’ position for prolonged periods of time, you will soon start to feel fatigued, and you will probably change your sitting position to get more comfortable.

The Angle of the Hips

Proper function of the hip muscles do have an important role to play in changing the positions from a standing to sitting down. ‘Do we really flex our hips 90 degrees when changing the stance between these 2 positions?’

Probably not, as numerous studies reveal that the hips flex to about 60 degrees because the lower part of the back is flattened out when we sit down. This changes the position of the pelvis. When we are in a relaxed position (lying down) the muscles at the front and the back of the hips are relaxed and in balance. However, when the hips are flexed at 90 or 60 degrees the hamstring (back of the leg) and the glutes (butt muscles) are constantly engaged.

These muscles are attached to the rear of the hip and engaging them leads to an outwards curve of the lower back. In addition, the quads (thigh muscles) are in a relaxed state when you are sitting down. If you change your stance into a standing position (from a sitting position), your hips extend; this engages the quads and increases the curve of the lower back. A study of school children revealed that in a relaxed posture, not one child could sustain the physical demands of sitting up straight. When asked to sit up properly only 31% could maintain the sitting up straight position by engaging the appropriate muscle groups (3).

Loading within the Discs

The outer structure of the discs in the back has no defense mechanism against prolonged bouts of sitting. This can lead to overuse injuries in the back. The overuse of the outer parts of the discs is only a part of the problem. The stress on the back can lead to cartilage damage of the discs, especially in the lower back. This is further worsened within the lower back when we go from a standing to a sitting position. Around 95% of long term back pain comes from the lower back, and sufferers can’t remain in the correct sitting position for lengthy periods of time (3). Therefore, lower back pain, poor seating and overuse injuries have a clear link between them.

The Cost to Health of a Poor Sitting Position

It is well documented that the loss of the natural curve within the lower back and too much load bearing of the discs are major causes of lower back pain. An uneven load on the discs, can lead to bulging and/or slipped discs. There has been major increase in the reported incidents of overuse injuries to the back discs over the past 20 years, especially within the younger generation (1). The health issues don’t stop there with prolonged sitting…

Back issues can lead to poor alignment of the knees and hips and this is a major factor in increased wear and tear at the joints. Sitting in the right sitting position and the correction of the curve of lower back, improves joint alignment and reduces pain caused by lower back injuries (3).

The Importance of the Right Posture

Granted we have focused on the risk and issues of prolonged sitting. It is now time to concentrate on some of the solutions in preventing overuse injuries to the lower back. Here are the 3 main strategies to avoid the flattening of the lower back when seating down on an office chair. The office chair is probably the best mode of sitting to focus on, as this goes hand in hand with the increase in usage of computers at home and in the work place.

Stop Sitting in an Incorrect Position

The office chair should be designed so that the angle between the upper body and the upper legs is in the range of 120 to 140 degrees. This will keep the back in a neutral position, and this can be broken down further into 4 sub-categories:

  1. Backwards tilt of the back support- whilst working at a desk, there should be short distances between your eyes and hands to the keyboard or book etc. This prevents the back from moving forwards and stops you from moving forwards and the back bending.
  2. The eye level should be straight forwards- looking down can lead to neck injuries because the head is not in a neutral position. This can exhaust the extending muscles in the neck.
  3. Forward tilt of the seat- this helps to keep the thighs in a lower position, it increases the hip angle and this reduces the pressure on the lower back.
  4. 4) Lower the position of the knees - this increases the angle of the hips.

Active Sitting

Active sitting relates to the muscles being stimulated to balance around a point where the upper body rests on the sitting bones. When sitting in this position the optimal curvature of the back is sustained, as the majority of the weight is on the sitting bones. Specifically, these bones are designed absorb the force and to protect the back. Many seats are now double curved, as they do take into consideration the notion of active sitting. Active sitting does lead to a better posture, as the balance is focused on the sitting bones.

  1. Donald D. Harrison, DC, PhD, Sanghak O. Harrison, DC, Arthur C. Croft, DC, Deed E. Harrison, DC, and Stephan J. Troyanovich, DC; Sitting Biomechanics Part I: Review of the Literature; Journal of Manipulative and Physiological Therapeutics Volume 22 • Number 9 • November/December 1999
  2. Waseem Bashir et al (2006); The Way You Sit Will Never Be the Same! Alterations of Lumbosacral Curvature and Intervertebral Disc Morphology in Normal Subjects in Variable Sitting Positions Using Whole-body Positional MRI
  3. Andrew P. Claus , Julie A. Hides , G. Lorimer Moseley , Paul W. Hodges ; Is ‘ideal’ sitting posture real?: Measurement of spinal curves in four sitting postures; Manual Therapy 14 (2009) 404–408