The mechanics of the body are created through the constriction of muscles and the effect that has on the skeleton. To understand body mechanics you must know a little bit about your own physiology and anatomy. But we can use a simplified model for our purpose.
The skeleton is a rigid structure connected through joints that allow for a hinging, sliding or rotational motion of the adjacent bones. Muscles are attached to the bone with a connective tissue (ligaments and tendons). The muscle has two states: relaxed and constricted. When nerve signals hit the muscle it constricts and scrunches itself up so that it is shorter than when it is relaxed. The tendon does not stretch so when the muscle constricts it pulls the tendon which is connected to the bone on the other side of the joint. That causes the bone to move in the manner the joint allows. If you constrict the bicep on your upper arm it moves the forearm by pulling on the tendon that goes across the elbow.
Muscles can only constrict (pull) and relax (stop pulling). They don't push. So muscles are balanced throughout the body so that motion caused by one muscle can be counter acted by another. The bicep on the front of the upper arm bends the forearm at the elbow where as the tricep on the back of the upper arm straightens the forearm out.
Body mechanics gives us insight into how people sit, stand, walk, jump, run, etc. Decades ago high jumpers jumped over the bar by kicking there front leg over it, kind of like the hurdle but 7 feet in the air. Then through a revolution in body mechanics the Fosbury Flop allowed jumpers to manipulate there body and center of gravity over the bar setting new world records that would not have been possible otherwise.
Body mechanics are used in Human Factors work to help represent what the human body and the population is capable of. Anthropometry will tell us if the task can be reached. Body mechanics will tell us what position the body will need to be in to reach it and if it is even capable of doing that. And that will lead into Human Performance.
Since Human Factors work is typically performed for a large population it is useful to be able to model body mechanics and apply the population's anthropometry to it. Sometimes this is done physically and other times it is virtually. There are pros and cons to both environments but the resulting information is always a benefit to the work being performed.
There is a simplified model of the body that uses simplified joints to evaluate body mechanics. It is called Body Links and can be quite useful at a gross level. With body links you can do a paper analysis with measurements and calculations determining the body mechanics. You can also build a stick figure model, like a simplified skeleton, than you can manipulate in a real world environment.
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However the body is quite complicated and most joints allow for more than one type of motion so in modern Human Factors analysis computer generated manikins that can mimic the body's complex mechanics are used within a CAD environment to represent the body mechanics aspect of Human Factors.
The issue that arises when using a computer manikin for body mechanics is that the manikin does exactly what you program it to do and nothing else. It does not lose balance or complain of an uncomfortable position or get a cramp. The most useful representation of a human's body mechanics is still a human with the right anthropometry.