 Adult patients with complex congenital heart disease will be recruited based on defined inclusion and exclusion criteria. Muscle function, muscle strength, muscle metabolism and body composition will be evaluated before randomisation and at follow-up after twelve weeks. The patients will be randomised to twelve weeks of home based resistance training or to a control group. The randomisation ratio will be 1:1 intervention:control. The control group will be instructed to continue with their habitual physical activities. The resistance training protocol will be individualised based on the results of the muscle function and muscle strength tests. 