Human Body – Further Structural Insights by Kotur Raghavan
An earlier article (https://www.fembestpractices.com/2020/10/load-paths-in-human-body-by-kotur.html
) focused on the load paths in human body for two different cases of loading
conditions. The load paths were described with the help of a figure which is
reproduced below.
Fig. 3.1 , Human
Skeleton Load paths.
The load paths are indicated by
red and black coloured circles. The colour code (red=40, Black=20) clearly
indicate bifurcation and union of loads during their passage from the
application points to reaction points. The body parts which are in the load
path are very important from analysis point of view. They are indeed the “candidates
for structural analysis”. When the load is placed on the head the hands and
shoulders are merely functional (non-load-bearing) appendages. When the load is
carried in hands the skull and the neck joint become functional appendages.
Notice also that the ribs are not occupying the load path in either of the
loading conditions. The same is true of a large number of the parts of anatomy
like sensory organs, digestive system, blood circulation system and so on. Identification
of the load path is the first step in any structural analysis. There is no room
for compromise in this regard and the all the load path components need to be
included in the simulation model. Non-load-carrying members can be included but
they will have zero impact on the results of analysis.
The load-bearing components “respond’
to the applied loads. The response is in terms of deformations and stresses.
Evaluation of these deformations and stresses is the main purpose of structural
analysis. From design point of view it is necessary to ensure that these
response parameters, more often than not the stress, do not exceed certain
limits so that structural integrity is ensured.
We now come back to the human
body. All our actions evoke some type of response from our body. The response
is generally felt in the form of ‘pain’. When we carry loads some parts feel
more pain, some parts feel less pain while a few others zero pain. When the
load is placed on the head, the hands feel no pain at all. But pain will be
felt in the skull, neck joint, spinal cord and lower limbs. The pain is of
varying degrees and generally the pain is highest in the neck joint
In order to understand varying
pain let us look into the case of suitcases being lifted in hands. The load
application points are folded fingers and the fingers experience considerable
pain. In general we use all four fingers for carrying the load. If, instead, we
use only three fingers the pain will increase. If only two fingers are in use
the pain will be even more. The reason is that lesser amount of material is
available for supporting and transferring the load when fewer fingers are used.
Here we see the analogy with stress in structural members. We know that for a
given load the stress is inversely proportional to the amount of material
available to support the load (say area of cross section).
Let us view the situation from a
different perspective. An average adult male can lift about ten to fifteen
kilograms in each hand. What it implies is that he can endure the pain induced
by that activity. A slenderly built person will be able to lift relatively much
less. On the other hand, a well-built muscular person will be able to lift much
more than ten to fifteen kilograms each. In other words, the stronger man has
capacity to endure pain induced by greater amount of load. Thus different
persons have different load carrying capacities. This is the limit load. This
limit will be dependent on the manner in which the load is carried. The limit
is defined by the endurable pain. Beyond this amount of pain the body ‘protests’
and something adverse will happen.
It is not difficult to recognise
identical situations in structures. Any given structure has a certain load
carrying capacity. This will be different for different loading conditions. For
example a given beam has higher capacity when loaded axially as compared to when
loaded laterally. The load capacity is increased by increasing the volume of
material. When the limit is exceeded, the structure will have either permanent
deformation or will break. This is loss of structural integrity.
Different structural materials
will have different stress limits. Strengths of materials can be enhanced by way
of alloying and heat treatment. Human beings can build stronger muscles through
nutritious food and proper exercise.
A structure of higher load capacity is called a stronger structure. In any weightlifting competitions the winner gets the title ‘strongest person’.
Is the article complete?
ReplyDeleteAnything needs to be added?
DeleteI think, I remember a similar write-up from you earlier. But now, I have a different idea also in my mind.
ReplyDeleteThe muscle system forms a load path parallel to the bones and may support some of the load, thus relieving the bones to some extent. This is much more so, when the loads are consciously handled by the body i lifting of loads. This is particularly true of limbs,(both arms and legs). A large difference may exist between the muscle shared load y a well built muscular man and a lanky man. What does the pain sensory system sense? The stress in the muscles or in the bones? I am not very clear. More information on this aspect will be interesting.
U.M.Chaudhari
My article is not likely to be technically precise from a medical viewpoint. I am also confused about bones or softer tissues or both that support the load. I will think about it.
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