The concept of stress is used in a variety of disciplines such as physiology, engineering and sociology. The concept originated in physics where stress is defined as external force applied to a system, and strain is the change in the system that is due to the applied force. In psychology and behavioural medicine the study of stress has moved beyond simple definitions towards identifying the processes of stress and mediating factors. Thus in psychology and medicine the term ‘stress’ covers a wide range of research that concentrates on different aspects of the stress process. For example, much medical stress research does not consider psychological factors and concentrates on physical stress responses, such as the impact of physical exercise on blood volume depletion. There are therefore conflicting views on how useful the concept of stress is. On the one hand some believe the concept is so widely misused and poorly defined that it is no longer useful and should be abandoned (e.g. Kasl, 1983). On the other hand, stress can be seen as an important construct that has the potential to unify different disciplines and help us understand the relationship between mind and body.

Stress is therefore a complex and multifaceted construct with many component parts. At a basic level, it is useful to distinguish between stressors, which are internal or external factors that cause ‘stress responses’, which can be physical, behavioural, cognitive or affective; and chronic strain, which is the negative impact of the stress process on the person.