What happens when you sit and how it affects your body
There is no doubt that we spend an enormous portion of our day sitting down. According to statistics released by BigThink.Com, people spend over 7700 days, or 21 years of their lives sitting. Typically, professional-level office workers spend about 70 percent of their time sitting at their workstation, while deskbound workers such as telemarketers spend nearly all their working hours sitting down.
THE ANATOMY OF SITTING
When you sit there is an interaction between various body elements, broadly classified as skeletal and soft-tissue elements. The spinal column, consisting of the vertebrae and intervertebral discs, together with the pelvis, are the primary skeletal structures that have a significant impact on your comfort and wellbeing when you sit.
1. Spinal Vertebrae
The 24 vertebral bones called the spine or spinal column are the most important structure in the body when it comes to sitting. At the top of the spinal column, the cervical region (neck) consists of seven vertebrae that produce a forward spinal curvature known as a lordosis. The thoracic region (mid back) consists of 12 vertebrae that provide an opposite curvature or kyphosis. Finally, at the bottom of the spinal column, the lumbar region (low back) consists of 5 vertebrae that create another forward spinal curvature or lordosis.
2. The Pelvis
The base of the spine is called the sacrum which is a triangular fusion of five vertebrae wedged between two pelvic bones that create the pelvic girdle. The two primary bones of the pelvis that we sit on are called ischial tuberosity or sitting bones.
3. Intervertebral Discs
The intervertebral discs are tough lozenges of fibrous cartilage with a thick fluid in the centre. They separate the individual vertebrae (vertebral body) from each other and give the spine flexibility and cushioning.
WHAT HAPPENS TO THE BODY WHEN WE SIT?
When we sit and our thighs are at a 900 angle to our back, there are two major skeletal movements. In the first instance, the long thigh bones (femurs) rotate in their pelvic sockets through approximately 600. Thereafter, the ligaments attaching the femurs to the pelvis, pull on the rear of the pelvis, rotating it backwards through the remaining 300. Due to the rotation of the pelvis, the natural lordosis curve in the lower back changes to a kyphosis curve.
Most of the change to a kyphosis curve occurs in the first three or four vertebrae above the pelvis. During this process, the front edges of the vertebrae squeeze closer together while the back edges spread further apart. This increases the pressure on the front portions of the intervertebral discs.
Sitting with a flattened lumbar spine or kyphosis, can put two to three times more pressure on the intervertebral discs than sitting in a way that maintains the lumbar spine's natural lordosis curve. In a worst-case scenario, the increase in disc pressure can lead to a herniated or slipped disc.
Because unsupported sitting is unstable, you move continually by making small rocking movements over the ischial tuberosities or sitting bones. At the same time, the skin and muscles under the sitting bones are compressed, and the large buttock muscles (gluteus maximi), slide to one side leaving the ischial tuberosities resting on a cushion of fat and skin. This restricts the blood flow to and from the intervertebral discs.
As we spend a greater percentage of time working on computers, we spend up to 75% of the time in a more upright or forward-leaning seated position as opposed to a reclining position. Research has shown that a chair that allows for a reclined posture that increases the angle between the seat and backrest, reduces the rotation of the pelvis which in turn reduces disc pressure and muscle activity in the lower back. An example is a chair fitted with a synchronous swivel and tilt mechanism.
Sitting for a longer period results in the muscles in the lower back becoming tired. If there is no lumbar support to maintain the lordosis curve of the lower back, you slump down or slouch, in the chair which causes an even greater outward-curving (kyphosis) shape in the lower back. This further increases the compression and pressure in the intervertebral discs. As you slump down, your head comes forward which forces the muscles at the back of the neck to keep your head in its original position. Muscle tension at the back of the neck may increase by up to 50% when you change from an upright to a slumped sitting posture.
Slouching increases kyphosis in the lower back and can increase muscular tension in the neck by up to 50%
When a person slouches, their spine goes out of alignment and muscles must work overtime to make up for the lack of skeletal support, i.e. they have to hold up the extra weight that is not supported by the skeleton. In addition to wear and tear on the muscles, this also stresses the body's other soft tissues, specifically the ligaments and tendons. Research confirms that slouching can cause backaches, headaches, fatigue and poor concentration. A healthy posture, i.e. when the spine is properly aligned, promotes greater overall comfort, including improved concentration and endurance through long hours of seated work.
Good posture balances your body weight on your skeletal structure, just as nature intended. This reduces the workload on your muscles, enhances blood circulation throughout the body, and relieves pressure on the diaphragm, thereby improving your breathing and preventing fatigue.
A chair that provides proper lumbar support ensures that the lordosis curve in the lumbar region is maintained. This allows the lower back muscles to relax and prevents slumping during sitting.
If the backrest can be reclined, the compression and pressure in the discs is reduced. Not only does this improve blood flow to the discs, but it also reduces the stress in the lower back muscles.
Sitting can place many stresses on the body. The most obvious ergonomic risk factors are the compressive force in the discs and the constant static loading of the muscles in the lower back. The more we sit, the higher our risk of herniated discs and other back troubles.
For many years, medical and ergonomics experts have understood the importance of seating that controls the rearward tilt of the pelvis. Research has found that the addition of a wedge-shaped pelvic cushion reduces the rotation of the pelvis and prevents the flattening of the lumbar spine, or kyphosis. The angle of the wedge is important. If it is too high (above 200), the pelvis will be forced to tilt too far forward, creating extreme lumbar lordosis that causes the thoracic and cervical spine to compensate with extreme curves to maintain balance. The result of such a forced posture is increased muscle activity and disc pressure leading to even greater fatigue and discomfort.
Lumbar support alone cannot naturally restore spinal curvatures in a body that is seated in an upright position, especially as people are more likely to sit in upright or forward-leaning postures than to recline while working at the computer. An effective work chair certainly must provide lumbar support. However, recent research indicates that it is as important to provide pelvic support that reduces the rotation of the pelvis and preserve the natural lordosis curvature of the lower back.