The purpose of this study is to investigate the short- and long-term effects of a 10-week walking program, based on the self-determination theory, in people with mental disorders. The investigators expect positive effects on:

• Physical fitness
• Physical activity
• Well-being (less feelings of anxiety, less feelings of depression) The investigators also expect underlying psychosocial processes to influence these effects.

Framework: Previous research has shown beneficial health effects of a 10-week walking program in a senior organisation. Based upon the self-determination theory, similar results can be expected in people with mental disorders and could be highly relevant, given the fact that these people:

• Are often less active
• Have mental problems and often also comorbidities
• Are often socially isolated

Participants: Psylos tries to recruit a convenience sample of at least 100 participants for an intervention group (which will undertake the walking program) and 80 participants for a control group. All of them have to sign an informed consent. Due to attrition, at least 60 participants in the intervention group and 40 in the control group can be expected. A post hoc analysis, with detailed information about the variation in this population, may give additional information about the necessary power to obtain significant results.

Data analysis: First there will be examined whether the participants in both groups are equal at baseline according to relevant parameters (due to the non-randomisation). Second, to test the main effects, the difference scores will be examined between both groups. If certain assumptions are met, by an unpaired t-test; if not, by a non-parametric test. Finally (if relevant), the underlying psychosocial processes will be examined by regression analyses.

Inclusion Criteria:

• At least 18 years old
• Member of Psylos
• Medically approved by a physician