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’.