Developing an integrated biomedical and behavioural theory of functioning and disability: adding models of behaviour to the ICF framework

The International Classification of Functioning Disability and Health (ICF) offers an agreed language on which a scientific model of functional outcomes can be built. The ICF defines functional outcomes as activity and activity limitations (AL) and defines both in behavioural terms. The ICF, therefore, appears to invite explanations of AL as behaviours. Studies of AL find that psychological variables, especially perceptions of control, add to biomedical variables in predicting AL. Therefore, two improved models are proposed, which integrate the ICF with two psychological theories, the theory of planned behaviour (TPB) and social cognitive theory (SCT). These models have a sound evidence base as good predictors of behaviour, include perceived control constructs and are compatible with existing evidence about AL. When directly tested in studies of community and clinic-based populations, both integrated models (ICF/TPB and ICF/SCT) outperform each of the three basic models (ICF, TPB and SCT). However, when predicting activity rather than AL, the biomedical model of the ICF does not improve prediction of activity by TPB and SCT on their own. It is concluded that these models offer a better explanation of functional outcomes than the ICF alone and could form the basis for the development of improved models.