When we talk about trauma in terms of ergonomics or repetitive stress we are talking about physical trauma as opposed to emotional trauma. Physical trauma refers to an injury to the body and the physical and cognitive response the body has to that trauma. Physical trauma can be differentiated into two categories: cumulative and acute.
Acute trauma is where the injury occurs all at once and is significant enough that the body's response will be noticed. Cumulative trauma is where the individual instance of trauma is not significant in itself. If the trauma stopped there then the body's response would most likely be unnoticed and cause no negative effects, however if the trauma persists and/or reoccurs frequently, like through a repetitive motion, then the injury keeps occurring and the cumulative trauma becomes significant.
The Body's Response to Physical Trauma
The body's response to trauma is healing. When there is trauma the body puts a lot of things into motion. Chemical messengers are sent out and things start to happen. The injured cells send out a transmission that you notice as some sort of pain. In most anatomical features the body reacts to those injured cells by protecting/stabilizing, heating, repairing and reinforcing. While this is not true for every part of your anatomy it will serve as a general explanation for most injuries that can be addressed through ergonomics or our caused by repetitive stress and cumulative trauma.
The body first stabilizes the injured area. This is typically done through swelling. Fluid fills the adjoining anatomy to stiffen it to keep you from injuring it any further. This may be a low level inflammation or a boot full of edema in the case of a severe sprain or bone breakage. In some cases like blisters the fluid provides a protective barrier against the source of the irritant/trauma against the irritated and injured area.
Next in the body's response to trauma is the heating. Injuries are often warmer than the surrounding area and can be a good indicator of the severity of the injury. In part this is the result of increased blood flow to the area. Blood is the carrier for all the nutrients needed to heal the body so the more blood supplied to the area the faster it can heal. The local increase in heat also fights off infection in the same way that a fever helps kill off the nasty germs making you sick.
Then comes the repairing state. This is where the body rebuilds the damaged and missing cells, knitting the existing anatomy back together. Depending on how things were injured this could be clean or not so clean, such as scar tissue.
After the anatomy is repaired the body keeps going and often adds some additional layer of the anatomical material on top of the newly repaired part to reinforce it. This is how the body strengthens the areas that need it and is how you build muscle. When you use your muscles you produce micro-tears in them. Those micro-tears are knitted back together and reinforced with some additional muscle fibers. The goal is to create a strong enough anatomical feature that it does not get injured again.
This reinforcement occurs in all type of tissue. Calluses are the build-up of skin to protect areas that are constantly rubbed and irritated. Martial artists that regularly break bricks show increased bone density and bone mass (as well as calluses) in the bones they break the bricks with.
How the Body's Response to Cumulative Trauma Becomes a Disorder
Usually it is the body's response to the cumulative trauma that causes a problem in a cumulative trauma disorder. Most of the time the problem is created because of the inflammation that occurs during the protection phase or the build-up of tissue through the reinforcement phase.
Sometimes the swelling and/or tissue build up makes something too large to fit through something else. This is the case in disorders such as Carpal Tunnel Syndrome or De Quervain's Syndrome.
Other times the swelling and/or tissue build up irritates something else or is irritated by something else when, in its non-irritated state, things would be fine, such as Thoracic Outlet Syndrome.