Ergonomics is an analysis based on fatigue of a system, be it neuromusculoskeletal, cardiac, or cognitive. Ergonomic analysis can focus on a body part or a whole body analysis, but the focus is always identifying or preventing fatigue of the system.
The human being works best, and longest if there is some balance designed in the work tools, layout and performance requirements. Designing work in such a fashion will prevent or delay fatigue of systems and injuries. However, many modern work systems are designed to be static or dynamic work systems and are out of balance with a healthy, sustaining physical human beings.
Static work is usually seated work with a computer and one or more screens. The large muscles of posture are held still while the small fine motor muscles in the eyes and the hands are working overtime. Unattainable mental workloads may be associated with static jobs, so your mind is always sifting the list of what needs to get done, while the body is held still at the workstation. The classic tools in this domain, chair, worksurfaces, computers, keyboards, pointing devices and monitors may not fit or be adjusted to support the individual.
Dynamic work is work that allows movement but often only repetitive movement that encourages fatigue of body parts, or heavy work, moving or manipulating heavy loads.
In both cases fatigue is related to the loads moved or held and the repetition or maintenance of a single posture. In both cases either body parts or whole body fatigue is the result.
The primary way to reduce fatigue is to improve the ergonomic situation in the workplace. Once its in place the second issue is to get the whole body moving, stretching areas that are compressed from the work, and relaxing or reliving internal strain or cognitive strain by reducing stimulation.
Work may not be designed to prevent fatigue and in some cases it seems designed to fatigue the end user.