Problems with depression are by no means limited to those who are clinically diagnosed as being so. Generalised, negative affect is experienced by everyone throughout the course of a lifetime. Usually, non clinical depression is linked to some kind of stressor in the environment which can be clearly identified, for example, the termination of a job. It is only when these depressive episodes go further than certain boundaries of intensity, duration and frequency can the disorder can be labelled clinical depression. Depressive symptoms, e.g. sadness, disturbed sleep and fatigue can also be symptoms of other medical conditions (for example stroke) however, in this situation depression may not require a separate diagnosis.

Clinical depression is a very prevalent problem in today’s society; 15% of people will have an episode of major depression in their lives and it is said to be the fourth most common cause of disability worldwide (NHS Website 2004). One of the earliest treatments for depression was psychotherapy, but Dishman (1986) notes than in severe cases medication will almost certainly be required. In the past 10 years or so, ‘exercise on prescription’ schemes have become popular in primary health care, many of which include depression in the referral criteria. (Biddle and Fox 1991) Strict diagnostic criteria are crucial since one of the problems with studies considering the effect of exercise and depression is a lack of agreement between researchers as to the criteria. Many reviews in the area include cases where depression could not be defined as clinical but more as negative affect. The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) contains a list of criteria for diagnosing depression (See appendix 1). A major depressive episode is categorized by either a depressed mood or loss of pleasure in all or most activities, and the presence of other symptoms for at lease 2 weeks. The gold standard for diagnosing depression and distinguishing it from negative affects is an interview that is performed by a clinician. The most common questionnaire is the Beck Depression Inventory (BDI) (Beck, Ward, Mendelson, Mock, & Erbaugh, 1961).

The Profile of Mood States (POMS) is a measure created by McNair in 1971 with the intent of measuring mood states; it is a widely used measure that has been used on psychiatric patients as well as in sport and exercise settings (Berger and Motl 2000). However, for the purpose of this investigation only studies where the participants are diagnosed as having clinical depression, rather than just scoring low in the POMS will be considered.

Traditional treatments for depression can be highly costly and ineffective, for example, psychotherapy and pharmacology (Byrne & Byrne, 1993). Similarly, drug treatments can cause unpleasant side effects such as dependence, fatigue and cardiovascular problems (Martinsen, 1990). This highlights the need for an alternative treatment and an ever growing amount of research suggests that exercise may be best suited to fill this roll. The relationship between exercise and depression has been examined since the early 1900’s when Franz and Hamilton (1905) recommended moderate to active exercise in alleviating the symptoms of depression. Since then there has been a vast amount of studies completed in this growing area. These were originally cross-sectional designs comparing the physical activity and physical capacity levels of depressed and non depressed individuals (Morgan, 1969; Morgan, 1970). Following these studies were interventions, looking at using exercise as a way of alleviating depression