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This paper deals with psychological determinants of activity and with activity promotion in a sample of mostly acute low back pain patients in primary care setting. A cluster-randomised trial in two study regions served to collect the data. Using multilevel mixed-effects modelling, the first study showed that a motivational-counselling approach to enhance physical activity by practice nurses was not more effective than repeated measurements alone. In the second study constructs grounded in theory like depressive mood and fear-avoidance beliefs could not explain the readiness for behaviour change or changes in the amount of energy expenditure in the course of one year. Regression analysis demonstrated that self-efficacy was a more important predictor. In the third study structural equation modelling in a cross-lagged panel design did not support the “deconditioning–paradigm” inherent in the fear-avoidance beliefs model. Contrary to our expectation, the total amount of physical activity was high, and initial fear avoidance beliefs did not predict final physical activity. Limitations of the study are reliance on self-report instruments, and attrition bias cannot be excluded. The studies support the assumption that LBP patients in general have not to be “reconditioned”. Obviously there are other reasons that better explain the positive outcome of exercise therapies apart from an increase in physical fitness. In future studies, the underlying mechanisms of the beneficial effects of functional restoration and reconditioning will have to be studied in more detail. In health psychology, theories will have to integrate emotional state and trait-variables in order to explain the process of behavioural change.