Adonis Diaries

Human factors graphs: Summary of long good stories

Posted on: October 26, 2008

 “How to tell long and good stories from human factors graphs?”

Article #33, (Feb. 28, 2006)

If we concentrate on a graph we might generate a long story that span many disciplines and furnish us with a wealth of information and knowledge that pages of words barely can convey. A graph might open the gate for dozen of questions that are the foundation of scientific, experimental, and critical thinking.

Suppose that we are comparing the efficiency in energy consumption between walking bare feet or wearing shoes that weight 1.3 Kg.  Considering the walking speed as the other independent variable, along with the type and weight of shoes, we observe that the curves show that we are consuming less energy at low speed, then both curves decreasing to a minimum consumption of 0.2 KJ/Nm and intersecting at around 80 meter/min and then increasing as walking speed increases.

This graph is telling us that casual walking consumes less energy per unit walking effort than fast walking and that, at a cut off speed of 80 meter/min, the energy consumption is equal for both foot wares.  Some people might jump to the conclusion that this cut-off speed can be generalized to all foot wears, but more experiments are necessarily needed to verify this initial hypothesis.

Another piece of information is that after the cut-off speed, it is more economical in energy to walk barefoot. Basically, this graph is saying that the more weight you add to your lower limbs the more energy you should expect to spend, a fact that is not an earth shattering observation.

Biomechanics tells us that the structure of our body is not geared toward saving on our muscular effort, but to increasing our range and speed of movements.  Most of our muscles are connected to the bones of our limbs and their respective joints in manners they have to exert great effort and many fold the weight of our body members to overcome any of our limb’s mass.

Usually, the tendons of our muscles are inserted to the limb bones, close to the joints, and thus the muscles have to exert a huge effort to overcome the moment of the bone and flesh weight in order to effect a movement. Any extra mass to our limbs will tax our muscles to produce many folds the additional weight.

There is a caveat however; if you wrapped a weight of 1.3 kg around your ankles and walked bare feet you would consume more energy than without the added weight, but the curve would be parallel to the previous curve and not increasing more steepily than walking with shoes weighting 1.3 Kg.  Consequently, the variation in the behavior of the graphs result from a combination of added weight and lesser static coefficient of friction exerted by the shoes on the walking surface than the bare foot..

Thus, what this graph does not mention is the static coefficient of friction between the footwear and the ground, and which is the most important variable and in this case, can concatenate many independent and control variables such as the materials of the footwear and the type of ground into a unique independent variable of coefficient of friction.

The higher the coefficient of friction the easier it is to move and progress and thus walking faster for the same amount of effort invested.  It is not that important to generate muscle force if the reaction force on the surface cannot be produced to move a person in the right direction.  For example, it is extremely difficult to move on slippery surfaces no matter how much muscular effort we generate.  Apparently, the shape and skin texture of our foot provide a better and more efficient coefficient of friction than most foot wears.

However, the most important fact of this simple experiment is showing us the behavior of the curves and offering additional hypotheses for other studies.

What this graph is not telling us is the best story of all, and which can excite the mind into further investigation. For example, what kind of earth materials are we walking on; sands, asphalt, rough terrains, slippery roads or grassy fields?  Does the sample of bare feet walkers include aboriginals used in walking bare feet, city dwellers, and people from the province?  Does the sample includes groups of  people according to the softness of their feet skins or the size of feet?

May be the shape of the curves are the same for females as well, but it would be curious to find out the magnitude of variations compared to males.  It is clear that a simple and lousy graph delved us into the problems of experimentation and raised enough questions to attend to various fields of knowledge.

In the final analysis, the question is how relevant is this experiment practically?  How far can a modern man walk bare feet?  Does any economy in energy compensate for the ache, pain and injuries suffered by walking bare feet?  Would athletes be allowed to compete bare feet if it is proven to increase performance and break new records?  Does anyone care of walking barefoot in order to save a few kilo Joules?

The theme of this article is that you can venture into many fields of knowledge, just by focusing your attention on graphs and tables and permitting your mind to navigate into uncharted waters through queries and critical thinking.

 

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adonis49

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