A physical activity programme for community-dwelling, chronically ill, and mobility-restricted patients has been developed, which: (a) on the individual level, contains multidimensional exercise (according to state-of-the-art guidelines) and consultations (including strategies fostering behavioural change), and (b) on the institutional level, establishes a cooperation between GPs and exercise therapists. After successful completion of a "development" and a "feasibility" phase, the present protocol describes the first study of the "evaluation" phase .
For this study, a measure of physical performance (namely functional lower body strength) has been chosen as primary endpoint. Using a physical performance test as the primary efficacy endpoint of an RCT makes it worthwhile to contemplate the position of physical functioning in different theoretical ability/disability frameworks. A traditional model developed by Nagi in 1964  suggests that all disability originates from pathology or disease: disease leads to impairment, impairment to functional limitation and functional limitation to disability. Considering the evidence that physical inactivity or muscle disuse can be just as responsible for physical decline leading to disability [e.g., [6, 118]], Rikli & Jones [105, 119] recommended revising the traditional Nagi model by including "lifestyle/inactivity" as another possible origin of a disabling process. This revised model not only supports the role of physical inactivity in the loss of function (independent of the disease process), but also strengthens the role of physical activity/lifestyle interventions in the prevention of disability. WHO's International Classification of Functioning, Disability and Health (ICF) (which is the successor to the International Classification of Impairment, Disability and Handicap [ICIDH, ]) provides another, much more complex conceptual framework of functioning and disability . The ICF defines "functioning" as a multidimensional (bio-psycho-social) concept relating to body structures and functions, activities, and participation. In addition, a new factor in comparison to former ability/disability models is that a person's functioning is always viewed in relation to health conditions and contextual factors (environmental and personal). "Disability", which is complementary to "functioning", encompasses any or all of the following: an impairment of body structure or function, a limitation in activities, or a restriction in participation. In contrast to earlier ability/disability frameworks, disability is not considered as an ultimate endpoint of a (mainly unidirectional) process. Rather than categorising people with disabilities as a separate group, disability is conceived as a continuum .
Referring to the ICF model and to a "conceptual description of the rehabilitation strategy" by Stucki et al. , which is based on this model, the primary target (and therefore a key outcome) of our intervention is optimal (physical) functioning. To achieve this target, "approaches to optimise a person's capacity" are applied and integrated: "approaches which build on and strengthen the resources of the person, which provide a facilitating environment, and which develop performance in the interaction with the environment" [, page 280]. The complexity of our intervention means that it can be expected to have diverse effects. This necessitates the use of several secondary outcome measures , not only on the physical, but also on the mental and behavioural level. Furthermore, quantitative and/or qualitative data concerning the appraisal of the programme will be obtained from all stakeholders.
The ultimate goal of our research is to guide policy-makers in planning health care services. In turn, one of their high priority goals is to optimally allocate limited resources. This will necessitate an economic evaluation of our programme, including cost-effectiveness and cost-benefit analyses [124–126]. Therefore, future RCTs will have to show the efficacy on endpoints such as hospital admissions, nursing home use, or utilisation of health care services. For the most cost-effective results, recruitment for the programme should then be focused on patients who will most likely benefit from it [127, 128]. Consequently, GPs will have to be outfitted with simple screening tools to identify those patients and to be motivated and trained to use them.
In conclusion, the object of research is a home-based exercise programme that approaches and supports community-dwelling but mobility-restricted older adults via their GP and an exercise therapist with regard to effects on physical function, physical activity, health-related quality of life, fall-related self-efficacy, and exercise self-efficacy. If the project is successful, long-term effects on further endpoints (e.g. hospitalisation rate) and cost-effectiveness of the programme should be evaluated. The programme's high level of flexibility could facilitate future implementation as part of primary health care.