Load Paths in Human Body by Kotur Raghavan

 By definition, a structure is an assembly of materials which is devised to support and sustain loads. Four typical structures, a suspension bridge, a multi-storey building framework, a bicycle and dining set, are shown in Fig. 1.1. They all are designed to sustain and support some type of the load or other to meet “safely” their functional needs.

Fig. 2.1 Examples of Structures

Any assembly generally consists of three major categories of components or sub-assemblies. They are structural members, functional members and components intended for safety and aesthetics. The main concern of a structural engineer or a structural analyst is the assemblage of structural members or load-carrying members. The load carrying members form what is known as Load Path. In any active structure we have load application locations or regions. We also have support or reaction points. In general, the reaction points are the locations where the assembly interfaces with the ground. Sometimes load transfer points can also be considered as supports as in the case of a table placed on a column-supported slab. The simplification is valid as the slab is much stiffer than the table.

The first step in any structural analysis is the identification of load path. For a given structure there can be different load paths for different types of loads. For example in a building structure, slabs, beams and columns form the load path for gravity loads. But if we consider wind load also then the outside walls. For the purpose of illustration we will now consider the human body.  

By definition, our bodies are also structures. Skeleton in combination with muscles, tendons, hamstrings and joints support the body’s self-weight. In addition they will also support additional weight carried by the person.  The additional loads can be carried by persons in in different ways depending on their weight, shape and size and also personal preference. Let us consider the case of a porter who has to carry two suitcases weighing twenty kilograms each. He may place both the suitcases on his head or he may carry one suitcase in each of his hands.

The two cases are shown together in Fig. 2.2. In the figure the arrows correspond to load application points and reaction points. Circles are used to indicate the load path. Red colored arrows and circles correspond to 40 kilograms. Black colored arrows and circles correspond to 20 kilograms. We see that the spinal column, pelvic joint, thighs, knee joints and legs are common to both the load paths. 

Fig. 2.2. Human Skeleton Load paths.

When the load is placed on the head, the skull and the neck joints are loaded and the hands are not. On the other hand, when the load is carried in hands the skull and the neck are free of load. Each hand and shoulder joint is loaded to the extent of twenty kilograms. In both the cases there will be 20 kilogram reaction at each feet.


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