The basic premise of applied psychology is the use of psychological principles and theories to overcome practical problems in other fields, such as business management, product design, ergonomics, nutrition or clinical medicine.

Ergonomics (from Greek ergon - work and nomoi - natural laws) is the study of optimizing the interface between human beings, and the designed objects and environments they interact with.

In strict usage the term is specific to increasingly productivity, reducing operator fatigue, and improving work environments. In common usage, though, 'ergonomics' can refer to the study of any man-machine interface, whether physiological or psychological.

Underlying science
Physical ergonomics rests on the underlying scientific field of anthropometrics (human measurement). Although anthropometrics still has unanswered questions, it's still true that human physical characteristics are predictable and objectively measurable.

Psychological ergonomics explores design issues in terms of cognitive psychology, cognitive workload, human error, the way humans perceive their surroundings and, very importantly, the tasks they choose to undertake. These issues of user experience are less predictable and less objectively measurable.

The commercial usage of the word 'ergonomics' to promote a product does not necessarily mean that any well-researched scientific design solution has been reached. In fact, such scientific design solutions might be inherently contrary to the manufacturer's commercial goals. In her book "The Chair", UC Berkeley Professor Galen Cranz explores the conventional wisdom and social expectations behind chair design, the many challenges of identifying an optimal design based on anthropometrics, and finally comes to the conclusion that the optimal chair shape from the standpoint of improving posture is a backless stool.