Why does a person carry more weight when sitting down?

Question

Today, while shopping for a chair at a shopping mall, I saw this poster regarding the amount of weight a person is carrying when doing various activities. When standing upright, one carries 100% of his weight, while when lying down, one carries 25% of his weight. However, what confused me was that when sitting down, one carries 125% of his weight, and when leaning forward while sitting down, one carries 325% of his weight.

From a rational layman perspective, this seems contradictory. When we stand up, we feel tired. Sitting down is definitely much more comfortable.

From a physics student perspective, I tried rationalising this. When standing upright, we feel tired because weight is exerted all on our legs and feet, making it difficult to stand for long. However, when we sit down, by the principle of moments, we would be carrying more weight. On a side note (slightly deviating from the principles of physics), I believe the greater comfort when sitting down is because most weight is carried by the backbone.

When we lean forward while sitting down, again by the principle of moemnts the force carried by our backbone will be greater.

Now, I am not definitely sure if this theory is correct, so if possible, could someone provide a more convincing explanation?

Answer 1

Assuming by “carrying” we mean stress on the backbone, we can think in terms of three types of stress on the backbone: tension, compression, and bending.

Insofar as pure tensile and compressive stresses are concerned, they are a function of axial loading.

Bending stress is a combination of tension and compression. For concave upward bending (smile) you have compression at the top and tension at the bottom.

I would think that all vertical positions would involve compressive stress. But since the backbone is not perfectly vertical (see diagram below) vertical positions can also produce tensile stress and bending.

All of this makes it very complicated because you need to isolate what type of stress is under consideration. However, we can make some general observations.

Any vertical position will maximize compressive stress on the backbone. So standing and sitting straight up should maximize compressive stress. Why sitting down is more than standing up is not clear. Perhaps it has to do with concentrated stress that the reaction force of the seat imposes on the bottom of the spine (tail bone). Or perhaps sitting down causes more curvature of the spine, though I’m not sure.

Any horizontal position will minimize compressive, tensile and bending stress. So lying down should be less “stressful” on the backbone than all the other positions.

Sitting and leaning over would probably be the most stressful since bending of the backbone is maximized. This results in both tensile and compressive stress on the backbone.

Hope this helps.

Answer 2

We have to start with defining the word "carry" in this context. I believe they meant support. In that regard, we'll have to borrow a bit of knowledge from anatomy of the human body. The mass distribution across our body is different, most of the mass is at the upper body and this determines the comfort part of the question. When humans stand, all the entire body weight is on our heel bone, and the stance is an unstable equilibrium, meaning the entire army of muscle along our back from the calf muscle to the neck muscles does most of the work of keep us upright because we tend to fall forward most times, and overtime they go into muscle fatigue, hence discomfort and pain. When sitting and leaning back on something at an angle, the back muscles are doing only the job of keeping the back and pelvic rigid so you don't slide off the chair, and how much work they're doing depends on the leaning angle. When leaning forward without the limb support, the back muscles are at the max passive tensioning to keep your entire upper body from falling. In that sense, the back muscles are supporting more than half of the body weight against most of gravity influence, so I'll say it's some work. If we're allowed to replace all the "carries" with "supports", then the numbers are utterly wrong, but the pattern is a bit right. If I'll estimate base on support, then:

1. laying on a even, horizontal, and relatively flat surface, you're supporting ~0% of your weight.

2. Standing, you support ~100 % of your weight.

3. Sitting upright, you support ~20% of your weight.

4. Sitting and leaning forward at 45°, you support ~65% of your weight.