Physics and Sitting Posture

Author: Leon Zhou

Editors: Galiba Anjum and Ethan Liu

Artist: Aurora Chen

If you ever were to observe the sitting posture of the people around you, it wouldn’t be difficult to notice that almost everyone has a unique way of sitting. These various sitting postures, closely related to the health conditions of our back (especially the spine bone), are the results of several factors that correlate to back support, position, and even chair type. This article will present several factors affecting our sitting posture and discuss the optimal sitting posture for our health condition.

1. Hard chair/ Soft chair

Soft chairs are generally preferred by office workers mainly because they are more comfortable to sit on. However, compared to hard chairs (made of wood for example), soft chairs are not helpful for users to maintain their correct sitting posture. It would be hard for users to sit straight over a long time sitting on soft chairs, and thus leading to back pain and potential severe back diseases.





2. With/ Without back support

Whether a chair has a back support or not has a significant impact on users’ postures. Without a back support, people would easily hunch while sitting; In contrast, a chair with back support not only increases the comfortableness but also helps users to keep sitting straight without the need to stay aware of their sitting postures.

3. Sitting position

The two factors mentioned above focus on the chairs’ characteristics; for “sitting position”, we pay attention to people’s habits of sitting.

1) When sitting on a chair, different individuals tend to cover different proportions of the area of the chair surface. For example, some students may sit on half of the entire surface of the chair, while others choose to sit on the entire surface of the chair. How will this factor make any difference? Some simple analysis of the forces exerted on people’s back is needed to demonstrate the effects:


In this diagram, the red arrows indicate the forces exerted on our back, Fn, and our body’s gravitational force, Mg, respectively. The angle θ indicates how much of the surface area individuals sit on. (The more surface area a person sits on, the greater is the value of θ indicated by the diagram. )

To achieve the balance in forces, some equations can be drawn:

Fny = Fn ✕ cosθ Fnx = Fn ✕ Sinθ

Mgy = Mg ✕ cosθ Mgx = Mg ✕ Sinθ

Since the value of our body’s gravitational force is constant, a changing value of θ will lead to different values of forces exerted on our back (Fn).

Through this analysis, it can be recognized that varied sitting positions can bring different effects on our backs. Based on the body condition of every individual, a universal optimal sitting position regarding the amount of chair surface sat on cannot be concluded. But it is plausible for every individual to improve their sitting posture by adjusting how much of the chair surface is sat on.

2) Another minor factor that influences sitting posture is crossed-legs. Sitting with crossed-legs does harm to people’s health, causing people’s pelvis to rotate and tilt, inflicting pain in the lower back, leading to deformation of the spine over time. As a non-controversially bad habit of sitting, sitting with cross-legs should be prevented in order to achieve an optimal sitting posture.

Citations:

Asian Spine Journal- “The Effect of Standing and Different Sitting

Positions on Lumbar Lordosis: Radiographic Study of 30 Healthy Volunteers”

By Il Youp Cho, Si Young Park, Jong Hoon Park, Tae Kwon Kim, Tae Wan Jung, Hyun Min Lee

Asian Spine J 2015;9(5):762-769•http://dx.doi.org/10.4184/asj.2015.9.5.762

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