Background
Physical fitness is one of the most powerful health indicators and is considered a strong predictor of risk for morbidity and mortality. There is increasing evidence that patients with inflammatory bowel disease (IBD) suffer from an impaired physical fitness. Consequently, the effect of exercise programs has been evaluated in patients with IBD, which revealed promising results. However, there is still insufficient routine screening for physical fitness in clinical practice to distinguish patients with adequate physical fitness from patients who might benefit from exercise interventions. More insight into physical fitness in patients with IBD is needed to elucidate in which specific subgroups of patients these interventions are most beneficial and which domains of physical fitness these interventions should target. Since existing gold standard measurement methods are not feasible to be implemented on a large scale due to complexity and costs, an accurate screening instrument and assessment method, validated for IBD patients, is needed to (pre)select patients with an impaired physical fitness. This study, therefore, aims to objectively assess different components of physical fitness in an outpatient IBD-population, to describe predictors associated with impaired physical fitness, and to validate dedicated and easily applicable assessment methods for clinical practice.

Methods
150 adult patients with IBD and 150 control subjects (ASA I and II) will be invited to perform physical fitness tests. Patients will perform gold standard tests as well as the dedicated alternative tests and complete questionnaires to assess the criterion validity of these tests within the IBD population. The results of the fitness tests performed by control subjects will serve as reference values.

Discussion
Physical fitness screening and assessment within the IBD population will ultimately lead to the application of targeted and person-based interventions to improve physical fitness and treatment outcomes.